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Found 10 results

  1. We've now reached a compact bit of code in the Dealer Demo that provides an assembler to Forth. And the assembler in Forth is a thing of beauty indeed. Written by Bill Ragsdale (as was most of the Forth kernel), it provides an assembler that can produce surprisingly readable code without the use of labels. In essence it implements high-level assembler: Recall TRAVERSE. In traditional assembler, it was written as: 154A: 4C 15 TRAV .WORD *+2154C: B4 00 LDY 0,X154E: 98 TRAV1 TYA154F: 18 CLC1550: 75 02 ADC 2,X1552: 95 02 STA 2,X1554: B5 01 LDA 1,X1556: 75 03 ADC 3,X1558: 95 03 STA 3,X155A: A1 02 LDA (2,X)155C: 10 F0 BPL TRAV1155E: 4C 4D 0E JMP POP In Forth assembler, it might be written as: CODE TRAV ,X 0 LDY, BEGIN, TYA, CLC, ,X 2 ADC, ,X 2 STA, ,X 1 LDA, ,X 3 ADC, ,X 3 STA, X) 2 LDA, 0< NOT UNTIL, POP JMP, ;CODE Notice that we didn't need a label we simply used BEGIN, to mark the beginning of a block and UNTIL, to mark the end. The Forth assembler also has IF, ELSE, THEN, which behave as expected. This is flexible enough to handle most assembly constructs without ever resorting to labels. It also uses a Forth-like syntax, with RPN ordering for the assembler codes, and the opcodes always ending in comma, since the effect is similar to the comma operator which puts data into memory in Forth. While I wouldn't want to write a large program this way, this is much more convenient than the USR function in BASIC. Another interesting implementation detail is that all opcodes are split into two sets. The first set, encoded using the word CPU, are most of the traditional one-byte 'immediate' mode opcodes, and have the opcode byte value attached to them. The remaining opcodes, encoded using the word M/CPU, use a word and a byte for each of these to select the appropriate opcode depending on the current mode and whether to emit 0, 1 or 2 additional bytes taken from the stack. This is remarkably compact, thanks to the <BUILD … DOES> construct in Forth which we discussed in an earlier post. You can read about this assembler in an article by Bill Ragsdale himself. It appeared in the September 1981 issue of Dr. Dobbs Journal (a issue dedicated to Forth), as well as the January 1982 issue of Forth Dimensions (v.3, number 5). The implementation itself was available earlier, since the Dealer Demo dates from late 1980, and the screens for the assembler date to June 1979 and earlier. It opens with: If you compare the published Ragsdale implementation with the assembler in Dealer Demo, only a few minor differences appear. The word VS was added, to give symbolic access to the overflow branches (BVS & BVC, an oversight in the original assembler), and the word END-CODE was renamed to C;. In Dealer Demo, the assembler occupies $25a0 - $2be4, or about 1.6k. So the Forth kernel and its assembler are about the same size as the Atari Assembler/Editor cartridge. Atari Forth on cartridge, that would have been an interesting product to see!
  2. From the album: CAMEL99 Forth

    Results of program to calculate day of week
  3. Hello everyone, I've made a series of 4 YouTube videos describing in detail the conceptual work, and the creation of words to implement an ANTIC disassembler tool in FORTH to compliment the ANTIC assembler that I wrote. This basically demonstrates how FORTH can quickly be used to make useful tools that can be intermingled with the development of programs in FORTH so that you don't have to load entire other programs to get stuff done, but rather just flow from one vocabulary of words to the next. Let me know what you guys, think. -Thom
  4. Local Variables for the Common Man I wrote a local variables library for TurboForth some years back. It was/is quite sophisticated; Forth words could have their own private, named variables. Nice. Just recently I find I'm interested in writing less code, not more. I find I'm more interested in what code I can do *without* rather than the code I need. This leads to very interesting thought-exercises. It is very interesting to strip code away and arrive at the simplest code you can come up with that still gets the job done. With that in mind, I recently took another look at local variables. For some folk, local variables in Forth are an anathema. I disagree. They reduce stack "juggling", or "stack traffic" where you are just juggling items on the stack to get them into the order you need them in. *Not* spending CPU cycles on juggling is getting more useful work done. Having named local variables in Forth (i.e. local variables that you can give any name to) is very nice, but we can really live without them. In assembly language on the TMS9900, we have 16 global "variables" - the registers. They are named R0 to R15. If we BLWP into a subroutine in a new workspace, we have 16 local variables, also called R0 to R15. The names are fixed, and we seem to get along with them just fine. So why not just do the same with local variables? With that in mind, I thought I would write something to be as economical as possible. I was very pleased with all the code that I *hadn't* written, so I thought I would share what I haven't written here. :-) The first problem to solve is where to put the local variables. We can't put them on the Forth data stack, because they would get in the way of other data that words are pushing/pulling to/from the stack. Imagine this word: : A ( -- ) B C D E ; Imagine that all of the words B C D and E use local variables. Furthermore, these words may internally call other words that also use local variables - maybe B calls Y and Y uses locals, and Y calls Z and Z uses locals. We need a locals stack. Well, a stack is just an area of memory with a pointer that points to the top of the stack: $ff00 value lsp \ locals stack pointer So, here's a VALUE (a type of variable) called 'lsp' (locals stack pointer). We're going to place our locals stack at >FF00 at the end of RAM. Now we need some words that can store values on the data stack. Let's implement three local variables, A, B and C. What are we going to call these words? Well, for storing data in the variables (that is, taking something off the stack and storing it in a local variable) how about >a >b and >c? The arrow before the variable name shows something "going into" the variable. It's a picto-gram. Similarly, for reading from a local variable, (reading from the variable and pushing onto the stack) how about a> b> and c>? The arrow shows something leaving the variable. Looks pretty good to me. So, each word that uses local variables can have three local variables, a,b, and c. That's 6 bytes. +-------+ lsp --> | A | 2 bytes +-------+ | B | 2 bytes +-------+ | C | 2 bytes +-------+ As can be seen, the locals stack pointer (lsp) is pointing to the top of the locals stack. So local variable A will be stored at the address in lsp, local variable B at lsp+2, and local variable C at lsp+6. Simple. Here's the code for writing to the local variables. Note the stack signatures. : >a ( n -- ) lsp ! ; : >b ( n -- ) lsp 2+ ! ; : >c ( n -- ) lsp 4 + ! ; And here's the code for reading from the local variables. Again, note stack signatures. : a> ( -- n ) lsp @ ; : b> ( -- n ) lsp 2+ @ ; : c> ( -- n ) lsp 4 + @ ; Now we need a word to make some space on the locals stack. Again, I'll use a pictogram: : lsp-> ( -- ) 6 +to lsp ; Here, the -> is pointing "upwards" on an imaginary number line, indicating that the word increases the lsp. And here's a word to decrease the local stack pointer: : <-lsp ( -- ) -6 +to lsp ; We're nearly done. All we need to do now is have some method of using the local variables in a Forth word. After some experimenting, the simplest approach I could come up with was to have a new word for : (which is used to create new words) that indicates that we want to create a new word, but with the special property of having access to local variables. For this, I chose :: (two colons). So instead of: : fred ( -- ) some clever code here ; We have: :: fred ( -- ) some clever code here ;; Both are identical, but the word created by :: has access to local variables. Here's the code: : :: : compile lsp-> ; That actually looks quite confusing, so let's break it down: The first colon means "hey, here comes a new word". The double colon is the name of the new word, so we have "hey, here comes a new word called ::" The third colon just includes the behaviour of : *in* the new word! So we have "hey, here comes a new word called :: and I want it do the same thing as : does, thanks." The "compile lsp->" part will ensure that when :: runs (when a new word with locals is being created) a reference to our lsp-> word will be compiled into *that* word (the word that is bing created). Hence, the locals stack will move down memory, and the word will get its own space for its locals at run-time. Finally, we need to terminate the definition, just like ; in a "regular" word. : ;; compile <-lsp [compile] ; ; immediate Again, we use : to create a new word called ;; and this word will compile a reference to <-lsp into the word under creation, thus re-claiming the locals stack space that the word will use at runtime. We then want the word to run the normal ; action to complete the word compilation. Well, ; is an immediate word so we use [compile] to override this behaviour. Then we terminate the ;; itself with ; and we make it immediate, so that it matches the behaviour of ; And voila. We're done. Look how much code it isn't: $ff00 value lsp \ locals stack pointer : >a ( n -- ) lsp ! ; : >b ( n -- ) lsp 2+ ! ; : >c ( n -- ) lsp 4 + ! ; : a> ( -- n ) lsp @ ; : b> ( -- n ) lsp 2+ @ ; : c> ( -- n ) lsp 4 + @ ; : lsp-> ( -- ) 6 +to lsp ; : <-lsp ( -- ) -6 +to lsp ; : :: : compile lsp-> ; : ;; compile <-lsp [compile] ; ; immediate We've just added the ability for Forth words to have true, stacked access to local variables, and it took us 188 bytes! Let's test it and see if it works: :: test2 ( n1 n2 n3 -- ) 3 * >c 3 * >b 3 * >a ." Test 2:" a> . b> . c> . cr ;; :: test1 ( n1 n2 n3 -- ) cr 2* >c 2* >b 2* >a a> b> c> test2 ." Test 1:" a> . b> . c> . cr ;; 1 2 3 test1 If you run this, you get the following output: Test 2: 6 12 18 Test 1: 2 4 6 Explanation of the code: We put 1 2 3 on the stack. Then we call test1. Test1 multiples the top of the stack (n3 in the stack signature) by 2, and stores it in local variable c. In doing so, it is removed from the stack. The next item on the stack (2) is multiplied by 2, and stored in b. Then the last item on the stack (1) is multipled by 2, and stored in a. We then get those stored values out of the local variables and back on to the stack, and call test2. Test2 does a similar thing: Takes the values off the stack, multiolying them by 3 and storing them in *its* local variables. Then it displays them. Then, control is returned to test1 just after the call to test2. Now, the local variables that were in test2 have gone, and the local variables that are in test1 are "back in scope" and we prove that by displaying them. Hence we have proved that we can nest calls to words to contain their own local variables and they work as expected and don't interfere with each other. And all in 188 bytes. Enjoy your Forth!
  5. Forth: Bouncing some ideas around (#3) In this short tutorial we'll start with something really simple: We'll get a bouncing ball moving around on the screen. As we develop our words, we'll test them as we go, rather than run the whole program in one go only to find that it doesn't work and wonder where the bugs could possibly be. When we've got the ball moving around, we'll add a bat that we can use to hit the ball. Think old style breakout. We're going to restrict ourself to characters in 32 column mode. We'll look at sprites in a future lesson; I'll just say this: Sprites are in some respects simpler than characters when it comes to moving them around, because you don't need to erase them, so there's great difficulty coming down the line regarding sprites. If your curiosity cannot be contained however, have a look at the sprite tutorial here: http://turboforth.net/tutorials/graphics.html#95 So, here's a program that bounces a ball (a zero character, actually, at the moment I'm into a rather odd "less is more" period when it comes to graphics, and am particularly fascinated with ASCII graphics; check out some of those old ZX81 games and you'll see what I mean) around the screen, inside a frame, just to keep things tidy. First in TI BASIC so that we have something as a reference: 10 CALL CLEAR 20 REM DRAW FRAME AROUND SCREEN 30 CALL HCHAR(1,2,ASC("-"),30) 40 CALL HCHAR(24,2,ASC("-"),30) 50 CALL VCHAR(2,1,ASC("|"),22) 60 CALL VCHAR(2,32,ASC("|"),22) 70 CALL HCHAR(1,1,ASC("+")) 80 CALL HCHAR(1,32,ASC("+")) 90 CALL HCHAR(24,1,ASC("+")) 100 CALL HCHAR(24,32,ASC("+")) 110 REM DEFINE BALL VARIABLES 120 BALL_COL=2+INT(RND*29) 130 BALL_ROW=2+INT(RND*21) 140 XDIR=1 150 YDIR=1 160 REM ERASE BALL 170 CALL HCHAR(BALL_ROW,BALL_COL,32) 180 REM CALCULATE NEW BALL POSITION 190 BALL_COL=BALL_COL+XDIR 200 BALL_ROW=BALL_ROW+YDIR 210 CALL HCHAR(BALL_ROW,BALL_COL,ASC("0")) 220 REM CHECK FOR EDGE OF SCREEN 230 IF (BALL_COL<3)+(BALL_COL>30)THEN 260 240 IF (BALL_ROW<3)+(BALL_ROW>22)THEN 290 250 GOTO 170 260 REM REVERSE X DIRECTION 270 XDIR=-XDIR 280 GOTO 170 290 REM REVERSE Y DIRECTION 300 YDIR=-YDIR 310 GOTO 170 We'll now look at how we could re-create this program in Forth. Note how I have separated the code above into distinct sections (using blank lines here for clarity). We'll divide (or "factor") the Forth version into pretty much the same sections, testing them as we go. Drawing the Frame Around The Screen Okay, in the program above, we need to clear the screen and then draw some lines and characters. Note that I've used the ASC function to make the code a little more "self-describing" who wants to waste time looking up ASCII codes, right? In Forth, there's an extra step, as we first need to tell the system to go into 32 column mode (TurboForth and fbForth default to 40 or 80 column text modes). The (TurboForth) command to change graphics modes is GMODE. Mode 0=40 column text mode 1=32 column graphics mode 2=80 column text mode So, lets create a word called FRAME which sets up the screen and draws the frame. : FRAME ( -- ) \ set up screen and draw frame 1 GMODE \ 32 column text mode 0 1 ASCII - 30 HCHAR 23 1 ASCII - 30 HCHAR 1 0 ASCII | 22 VCHAR 1 31 ASCII | 22 VCHAR 0 0 ASCII + 1 HCHAR 0 31 ASCII + 1 HCHAR 23 0 ASCII + 1 HCHAR 23 31 ASCII + 1 HCHAR ; Things to note in the above code: : FRAME - this part says "hey, here's a new word called FRAME; ( -- ) this is comment that tells us that word has no effect on the stack - it takes nothing from the stack and puts nothing on the stack; The \ (backslash) is a comment. When TurboForth see this is treats eveything following it as a comment; Screen coordinates are zero-based, whereas in BASIC they are one-based Read the code from left to right, top to bottom. You can have multiple instructions on the same line; The "arguments" to the function/word (in this case HCHAR and VCHAR) come BEFORE the word itself. Why? Because the words/functions take them from the stack, so they need to be on the stack BEFORE the word itself executes; The command ASCII looks at the character in front of it and pushes the appropriate ASCII value to the stack. So ASCII * is like ASC("*") in TI BASIC; Spaces are ESSENTIAL in Forth. One or more spaces MUST separate words and numbers from each other. Forth cannot recognise 99STARS if you really mean 99 STARS; The semi colon at the end says "Okay, I'm done defining my FRAME word, thank you very much, add it to the dictionary" (where all the Forth words and their code are stored). At this point, you've added a new word to the language called FRAME. Now that it exists, any other word that you create can use it. So how do we use it? We "name" it. I.e. we type its name. Forth will do the rest. If we type FRAME on the command line (i.e. when the cursor is sitting there blinking at you) then it will immediately execute it. This is the same as typing something in TI BASIC without a line number in front of it. TI BASIC will just execute it (or try to); If we type FRAME inside another definition, then it will be compiled for execution later. This is the same as putting a line number in front of something in TI BASIC. TI BASIC will store it and run it later. So, assuming you have typed the above in (the best way is to use Classic99 and just copy the text above and paste it into TurboForth) you can just type FRAME (or frame - TurboForth is not case sensitive by default (you can turn it off)) and press enter and the code will run. So, hopefully, you have a neat frame around the screen and TurboForth is saying OK at you and furiously flashing its cursor at you. We can now move on to the next bit. Note that TurboForth is still in 32 column mode. You can leave it in 32 column mode if you like, but you might be more comfortable in 40 column mode on a real TI, or, if you have an F18A or you are using classic99, 80 column mode. Type either 0 GMODE and press enter, or 2 GMODE and press enter. The screen will clear and the mode will be changed. Let's move on. We've tested FRAME, it does just one thing and has no variant behaviour, so it either works or it doesn't. The Value of VALUEs The next step of the program sets up the initial ball and row and column variables, let's remind ourselves: 110 REM DEFINE BALL VARIABLES 120 BALL_COL=2+INT(RND*29) 130 BALL_ROW=2+INT(RND*21) 140 XDIR=1 150 YDIR=1 In TI BASIC, you can go ahead and just use a variable name. Forth is not like that. In Forth everything must be created first and stored in the dictionary, then you can reference it. So, we first need to declare our variables. However, in this tutorial, as it's very early days, I'm going to suggest that we use VALUES as they work more like variables in BASIC (i.e. they work with values; whereas variables in Forth work with addresses - that's for another day!) 1 VALUE BALL_COL 1 VALUE BALL_ROW 1 VALUE XDIR 1 VALUE YDIR So, this bit of code creates some VALUEs called BALL_COL, BALL_ROW, XDIR and YDIR respectively. The number is the initial value. Here's how 0 VALUE BALL_COL is evaluated: 0 is pushed onto the stack; The word VALUE (a built-in TurboForth word) executes. VALUE is programmed to *read ahead* of itself and use the word it finds there as the *name* of the value to create, so in this case, it creates a VALUE in the dictionary called BALL_COL. The initial value (0) that is on the stack is removed, and is used to initialise BALL_COL. If we had typed 99 VALUE BALL_COL then it would be initialised with 99. Declaration of VALUES/VARIABLES should NOT be done inside a definition. They should be by themselves. Some words are special in Forth and can only be used on the command line, just like (for example) NEW in TI BASIC, which can only be used on the command line. So, that's the declaration of the VALUEs done. You can test them by simply typing their name and pressing enter: BALL_COL <enter> Note that TurboForth responds with OK:1 meaning that 1 number is on the stack. What happened? You just "executed" a VALUE. Sounds strange doesn't it? I mean, you can't execute, say, variables in BASIC. What does it mean to execute a VALUE or a variable? Well, in the case of VALUEs in Forth, they are pre-programmed (once you have created them) to push their current value to the stack when you execute them. It's as simple as that. If you're used to OOP, think of VALUE as a class, and BALL_COL as an instance of class VALUE which took 0 as the constructor. Now, type YDIR <enter> TurboForth says OK:2 meaning that 2 numbers are on the stack. Let's look at them. There are two ways: The word .S will display what's on the stack, and leave them there for us (non-destructive) The word . (a dot/period) will take what's on the top of the stack and display it, removing it from the stack as it does so. Type .S <enter> TurboForth should display: 1 1 <--TOP OK:2 So, it's told you that 1 (YDIR) is on the top of the stack, and 1 (BALL_COL) is underneath it. Now, type XDIR .S <enter> TurboForth says 1 1 1 <--TOP OK:3 As an experiment, let's change the value of XDIR. Type: -1 TO XDIR See? That wasn't so hard was it? Pretty simple. Let's display the stack again: .S<enter> 1 1 1 <--TOP OK:3 Hang on. We changed XDIR to -1. Why does it still show 1 on the top of the stack? Surely it should display -1, right? No. The stack is a separate entity all by itself. It simply stores what you push on it. If you then change the value of something, good for you. You won't see it until you push it again. So, type XDIR again. TurboForth says OK:4 Now type .S and we get 1 1 1 -1 <--TOP OK:4 Okay, let's continue, but before we do, how do we clear those four numbers off the stack? There's a number of ways: type DROP DROP DROP DROP to drop (discard) them; type . . . . (four dots, separated by spaces) to display them (they get removed as they are displayed); Or, my favourite: type some random gibberish which causes TurboForth to empty its stack and display an error message: JFKDFDJKFJ ERROR: NOT FOUND OK:0 See? 0 items on the stack! Initialisation Right, now we need to initialise random starting row and columns for BALL_ROW and BALL_COL. So, let's make a word called SET_RC ("set row and column") that does just that: Type in or paste in the following: : SET_RC ( -- ) \ set row and column 30 RND 1+ TO BALL_COL 22 RND 1+ TO BALL_ROW ; That's it. How does this work? Well, this is a new word called SET_RC so when it is executed it will: Push 30 to the stack; Call RND which takes the 30 off the stack and uses it to generate a random number between 0 and 29. We need a random number between 1 and 30, so we call 1+ ("one plus") which simply adds 1 to the number on top of the stack. The phrase "TO BALL_COL" removes whatever is on the top of the stack and stores it in our VALUE which is called BALL_COL. The exact same technique applies for BALL_ROW, but using a different random number range. Let's test it. Make sure the stack is clear by typing some gibberish. Now type: SET_RC <enter> TurboForth says OK:0 - nothing was pushed to the stack. Let's see what was stored in BALL_COL and BALL_ROW: Type BALL_COL . BALL_ROW . <enter> (note the spaces between the dots) TurboForth will respond will respond with something like 13 9 OK:0, depending on what random numbers it chose. Let's look again, but this time using .S to show us the stack: BALL_COL BALL_ROW .S 13 9 <--TOP OK:2 This time, we executed the values directly, so they were pushed to the stack. Note how it's possible to put multiple commands on the same line separated by spaces. It's not necessary to enter them one at a time on separate lines, like this: BALL_COL BALL_ROW But you could if you wanted to. Placing commands/words together on the same line is a bit like using :: in Extended Basic to separate statements, only in Forth we just use spaces because there is hardly any syntax in Forth, you're in total control. Next, we need a word to erase the ball: Erasing The Ball : ERASE_BALL ( -- ) BALL_ROW BALL_COL 32 1 HCHAR ; There's not a lot of code there - it's hardly worth making a word just for this, *however* the advantage is that it means we can test it separately from the rest of our code. Let's test it: First, fill the screen with a character, using HCHAR like this: PAGE 0 0 ASCII * 960 HCHAR ERASE_BALL PAGE clears the screen (which resets the cursor position to the top of the screen) then we fill the screen (assuming you're in 40 column mode) with asterisks using HCHAR. You should see a hole somewhere where ERASE_BALL erased an asterisk. Good. So, how does it work? Well, during execution of ERASE_BALL, BALL_COL will push its value to the stack, BALL_ROW will push its value to the stack, we then push 32 to the stack (the ASCII code for a space character), and then we push 1 to the stack (the number of repeats - this is an optional parameter in HCHAR/VCHAR in BASIC, but NOT so in Forth). HCHAR then gobbles all those values up and uses them to draw a space at the correct place on the screen. Calculate New Ball Position Next, we need to re-create the calculation of the new ball position, and display of the ball. We're going to re-create this TI BASIC code: 180 REM CALCULATE NEW BALL POSITION 190 BALL_COL=BALL_COL+XDIR 200 BALL_ROW=BALL_ROW+YDIR 210 CALL HCHAR(BALL_ROW,BALL_COL,ASC("0")) : MOVE_BALL ( -- ) XDIR +TO BALL_COL YDIR +TO BALL_ROW BALL_ROW BALL_COL ASCII 0 1 HCHAR ; And voila. We have a new word, MOVE_BALL. Let's test it. We'll first make sure that the row, columns etc. are set to realistic values: 10 TO BALL_COL 10 TO BALL_ROW 1 TO XDIR 1 TO YDIR Note how I typed all that on one line. I could have typed: 10 TO BALL_COL 10 TO BALL_ROW 1 TO XDIR 1 TO YDIR But I'm lazy and impatient. Okay, so type in PAGE MOVE_BALL <enter> The ball should be displayed. Now type MOVE_BALL again. Now type MOVE_BALL MOVE_BALL MOVE_BALL MOVE_BALL <enter>. It should leave a trail of balls (ooer!). So how does it work? Here's the breakdown: XDIR pushes its value to the stack; The word +TO removes it and *adds* it to whatever is stored in BALL_COL; YDIR pushes its value to the stack; The word +TO removes it and *adds* it to whatever is stored in BALL_ROW; We then push the values of BALL_COL, BALL_ROW, the ASCII code for 0, and the number 1 (the number of repeats) to the stack; HCHAR removes them and does it's thing. Edge Detection Okay, we're nearly there! Next, we need to check if the ball is on a screen edge, and if it is then we reverse the direction of the ball in either the X or the Y direction. TI BASIC is rather terrible at this, because IF can only target a line number, so you're forced to separate the IF from the code that should run when IF is true. Just awful. In Forth we can do much better, however, the syntax may hurt your head a little bit. Not to worry, I'll break it all down. This is the TI BASIC code that we want to re-create: 220 REM CHECK FOR EDGE OF SCREEN 230 IF (BALL_COL<3)+(BALL_COL>30)THEN 260 240 IF (BALL_ROW<3)+(BALL_ROW>22)THEN 290 250 GOTO 170 260 REM REVERSE X DIRECTION 270 XDIR=-XDIR 280 GOTO 170 290 REM REVERSE Y DIRECTION 300 YDIR=-YDIR 310 GOTO 170 I'm going to create four new words: HIT_NS? (Hit north or south?) HIT_EW? (Hit east or west?) REV_XDIR (Reverse X direction) REV_YDIR (Reverse Y direction) Now, to be clear, we could write all of the above as one word. In fact, we could write the entire program as one word, but you'd have a terrible job trying to debug it! That's the advantage of breaking our code down ("factoring it") into small chunks. We can test them, and then just string them together at the end. So, here we go: HIT_NS? first: : HIT_NS? ( -- flag ) \ check hit on top or bottom of screen BALL_ROW 2 < BALL_ROW 21 > OR ; Whoa! That is some WEIRD looking code!! What on earth does it mean? Let me break it down step by step. There's only seven instructions, so it's not difficult to understand. It just LOOKS weird (most Forth looks weird, to be honest!) First, you need to be aware of the stack signature of this word: ( -- flag ) That means that this word takes nothing from the stack, but it does *leave* something on the stack. It leaves a flag (something that is either true or false). It's also very important to realise that the stack signature is a COMMENT. It's not a function parameter declaration like in C or Java. It's a comment that tells us *humans* what this word expects and leaves on the stack. Forth itself doesn't actually *know* what the word expects or leaves on the stack. It just dumbly tears through the code, obeying what it sees, and the results are the results. If the results are NOT what you expected, well, then YOU made a mistake somewhere! So, here we go: BALL_ROW pushes its value to the stack; We push the value 2 to the stack We execute the word < which means "is less than?" Is Less Than The word < or "is less than?" takes two values off the stack and compares them. If the first value is less than the second value, it pushes a -1 (true). If the first value is NOT less than the second value it pushes a 0 (false). It's as simple as that. So... at run time, BALL_ROW will be compared to 2, and if BALL_ROW *is* less than 2, the word "<" will push a -1 to the stack, otherwise it'll push a 0. We then do the same thing but using the word ">" is "is greater than?". Here, we compare BALL_ROW to 29, and if it *is* greater than 29, ">" will give us a -1, otherwise it'll give us a 0 on the stack. So, after the execution of these two lines of code, we'll end up with *two* values on the stack. The result of the < comparison, and the result of the > comparison. Next, we execute the word OR. OR takes two values off the stack and if the first value, or the second value, or both values are true, it pushes a -1 (true) to the stack. If both values are 0, it pushes a 0 (false) to the stack. So OR pushes the flag to the stack that we refer to in the stack signature for our word. We can prove that this will work at the command line, using numbers: -1 0 OR . (result is -1 (true) because one of the inputs to OR was true 0 0 OR . (result is 0 (false) because both inputs to OR were false). Let's have a quick look at the stack comments for these words: The word < has the stack comment ( a b -- flag ) which means flag will be true if a < b. A and b are removed from the stack. The word > has the stack comment ( a b -- flag ) which means flag will be true if a > b. A and b are removed from the stack. The word OR has the stack comment ( a b -- flag ) which means flag will be true if a or b are true. A and b are removed from the stack. All quite simple and logical. Okay, I'll rattle through the next one, it uses the exact same principle. It's just check East and West (left and right) screen edges. : HIT_EW? \ Hit east or west? BALL_COL 2 < BALL_COL 29 > OR ; I'll refrain from breaking this down as the principle is identical. I will however make a VERY brief detour and discuss paragraphs: Paragraphs: When we write in C or Java or assembly language we're used to leaving blank lines between lines of code. These are paragraphs, and they separate code up into logical blobs of code. Because Forth coded horizontally, we often use multiple spaces (two or three) on a line of code to break our code up into paragraphs. Consider HIT_EW? written in a horizontal style: : HIT_EW? \ Hit east or west? BALL_COL 2 < BALL_COL 29 > OR ; It reads okay (to someone that is used to Forth) but a better way to write it is like this: : HIT_EW? \ Hit east or west? BALL_COL 2 < BALL_COL 29 > OR ; That is probably a lot more readable to you. It certainly is to me. It's now much clearer that "BALL_COL 2 <" is a separate blob of code from "BALL_COL 29 >" because we separated them using paragraphs. It also takes up less screen space, and block space if you are using blocks. REV_XDIR (Reverse X direction) and REV_YDIR (Reverse Y direction) Okay, we're nearly finished. If you're having trouble reading all this stuff, spare a thought for the guy that had to write it! Let's finish up with REV_XDIR and REV_YDIR which will reverse the direction of the ball in the horizontal and vertical directions: : REV_XDIR ( -- ) \ reverse x direction XDIR NEGATE TO XDIR ; : REV_YDIR ( -- ) \ reverse y direction YDIR NEGATE TO YDIR ; You can probably see what these do. For XDIR, XDIR goes to the stack, NEGATE then negates whatever is on the stack (1 becomes -1 and -1 becomes 1 etc.) and then TO writes it back into XDIR. Same principle for YDIR. Let's test them: -1 TO XDIR REV_XDIR XDIR . TurboForth should display 1. I'll leave you to test REV_YDIR. Now we're going to write a word to roll these four words up. We'll call it CHECK_DIR for Check Direction. : CHECK_DIR ( -- ) HIT_EW? IF REV_XDIR THEN HIT_NS? IF REV_YDIR THEN ; I reckon right about now your head just exploded. I hope you're not sat on a bus as you read this. Just what in the name of Satan's Holy Trousers is that THEN doing at the END of a line? This doesn't make sense at all! Or does it? Well, actually it does. Here's a quick detour into how IF...THEN works in Forth. First, some BASIC to compare it to: 10 INPUT A 20 IF A < 10 THEN 50 ELSE 70 30 PRINT "FINISHED!" 40 END 50 PRINT "LESS THAN 10" 60 GOTO 30 70 PRINT "NOT LESS THAN 10" 80 GOTO 30 This absolutely vile, abominable code which forces you to go searching down the code for the appropriate line numbers (what if there was 100 lines of other code between them? Just vile) can be beautifully expressed in Forth thus: : CHECK ( n -- ) 10 < IF ." LESS THAN 10" ELSE ." NOT LESS THAN 10" THEN CR ." FINISHED" CR ; Go ahead and type that in. Note the stack signature. It needs a value passed into it from the stack: 9 CHECK 11 CHECK 10 CHECK You understand how this works now: We put 9 on the stack, then call CHECK which uses the 9 we just we put there, and so on. Let's break this CHECK word down: It puts 10 on the stack. Then "less than?" executes which will compare whatever we put on the stack to 10 and leave a true or false on the stack. IF then consumes whatever "less than?" left on the stack. If it was a TRUE then the code after the IF will execute, ELSE the code after the ELSE will execute, THEN normal execution will continue to the end of the word. The following should illustrate how this works, and it's important to realise that THIS IS VALID FORTH CODE (assuming the following words existed): SUNNY? IF GET-SHADES ELSE GET-JACKET THEN GO-OUTSIDE If you read that out loud, it reads like English. And well crafted Forth code, if factored nicely (which takes experience) will often read very close to English. It's clear from the above that the THEN denotes the continuation of the rest of the code. It's an "ENDIF" in other languages. Alright, so what do the other words do? Well, the word ." just prints a string. It needs a space between it and the string, and a closing " to indicate the end of the string. The word CR means "carriage return" and moves the cursor/current print position to the next line, scrolling the screen upwards if necessary. Putting It All Together Let's review all the code we have so far: : FRAME ( -- ) \ set up screen and draw frame 1 GMODE \ 32 column text mode 0 1 ASCII - 30 HCHAR 23 1 ASCII - 30 HCHAR 1 0 ASCII | 22 VCHAR 1 31 ASCII | 22 VCHAR 0 0 ASCII + 1 HCHAR 0 31 ASCII + 1 HCHAR 23 0 ASCII + 1 HCHAR 23 31 ASCII + 1 HCHAR ; 1 VALUE BALL_COL 1 VALUE BALL_ROW 1 VALUE XDIR 1 VALUE YDIR : SET_RC ( -- ) \ set row and column 30 RND 1+ TO BALL_COL 22 RND 1+ TO BALL_ROW ; : ERASE_BALL ( -- ) BALL_ROW BALL_COL 32 1 HCHAR ; : MOVE_BALL ( -- ) XDIR +TO BALL_COL YDIR +TO BALL_ROW BALL_ROW BALL_COL ASCII 0 1 HCHAR ; : HIT_NS? ( -- flag ) \ check hit on top or bottom of screen BALL_ROW 2 < BALL_ROW 21 > OR ; : HIT_EW? \ Hit east or west? BALL_COL 2 < BALL_COL 29 > OR ; : REV_XDIR ( -- ) \ reverse x direction XDIR NEGATE TO XDIR ; : REV_YDIR ( -- ) \ reverse y direction YDIR NEGATE TO YDIR ; : CHECK_DIR ( -- ) HIT_EW? IF REV_XDIR THEN HIT_NS? IF REV_YDIR THEN ; So far, you can probably see that we don't yet have a "program" as such. We just have a collection of words that each do something, but we need to glue them together. So, let's break out the glue: : BOUNCE ( -- ) FRAME SET_RC BEGIN ERASE_BALL MOVE_BALL CHECK_DIR AGAIN ; So, BOUNCE calls FRAME which draws the screen, and then SET_RC which sets our row and column values. Then, we BEGIN a loop. The word BEGIN marks the start of the loop. Then, we call ERASE_BALL, MOVE_BALL and CHECK_DIR. Notice how at this high level the code is quite generic. It's almost like English. It's just words strung together in a sentence: "erase ball, move ball, check direction" Then, we execute AGAIN which runs everything again from the word BEGIN (in reality, it jumps back to ERASE_BALL; BEGIN is just a marker to show where it jumps back to). So we have this in our main loop: "erase ball, move ball, check direction, do it again". It's English. Well factored code at the high-level will read like English. Sure, it's not so nice at the low level with all the stack management and stuff going on, but (and this is a big one) you tested all those words "on the way up". You know they work. No need to re-visit them. Your higher level words use the lower level words, and you can keep building your code up in this way. Words stand on the shoulders of other words. It's a LOT more sophisticated than line numbers, so it takes more practice, but once you've got it you won't want to do line numbers again. If you type the complete program in as shown and type BOUNCE you will see the program run. However, there are two problems: It's too fast. You can't really see anything; There's no way to exit. Let's fix that: : DELAY ( n -- ) 0 DO LOOP ; : BOUNCE ( -- ) \ top-level code FRAME SET_RC BEGIN ERASE_BALL MOVE_BALL CHECK_DIR 100 DELAY 0 JOYST 1 = UNTIL ; So, we've introduced a delay word which uses an empty loop just to spin the wheels for a while (more on DO...LOOP in a further article - hopefully someone else will write it! - it's generic; not specific to TurboForth) and we've changed BOUNCE as follows: We now read the first joystick (unit number 0). JOYST pushes a value on the stack according to what the joystick is doing. The only value we're currently interested in is 1, which means the fire button has been pressed. So: We push 0 onto the stack. JOYST uses it to read joystick 0 and pushes the result; We push the number 1 onto the stack; The word = ("equal?") tests the value that JOYST pushed against the 1 that we pushed. If they are equal then "=" will push a true else it will push a false; UNTIL consumes the number that "=" pushed. If it is TRUE then execution is allowed to continue past the UNTIL word, otherwise it loops back to begin. So, our code will loop back to the associated BEGIN word UNTIL the fire button is pressed. There's no code after the UNTIL so everything just stops. You can see that the program is not really a program until we get to the word BOUNCE. That's where a bunch of related, but unconnected words come together to make a program, yet BOUNCE is just another word that we've added to the system. This is how programs are grown in Forth. Of course, it's possible to be more sophisticated (where words leave values on the stack for other words to consume). We haven't done that much here. There is a bit of that going on in CHECK_DIR though. Well, this turned out to be a LOT longer than I was planning. If you stuck with me to the end then I'm grateful. The whole program, in it's finished form which you can cut and paste into Classic99: : FRAME ( -- ) \ set up screen and draw frame 1 GMODE \ 32 column text mode 0 1 ASCII - 30 HCHAR 23 1 ASCII - 30 HCHAR 1 0 ASCII | 22 VCHAR 1 31 ASCII | 22 VCHAR 0 0 ASCII + 1 HCHAR 0 31 ASCII + 1 HCHAR 23 0 ASCII + 1 HCHAR 23 31 ASCII + 1 HCHAR ; 1 VALUE BALL_COL 1 VALUE BALL_ROW 1 VALUE XDIR 1 VALUE YDIR : SET_RC ( -- ) \ set row and column 30 RND 1+ TO BALL_COL 22 RND 1+ TO BALL_ROW ; : ERASE_BALL ( -- ) \ erase ball from screen BALL_ROW BALL_COL 32 1 HCHAR ; : MOVE_BALL ( -- ) \ update ball position and draw it XDIR +TO BALL_COL YDIR +TO BALL_ROW BALL_ROW BALL_COL ASCII 0 1 HCHAR ; : HIT_NS? ( -- flag ) \ hit top or bottom of screen? BALL_ROW 2 < BALL_ROW 21 > OR ; : HIT_EW? \ Hit east or west? BALL_COL 2 < BALL_COL 29 > OR ; : REV_XDIR ( -- ) \ reverse x direction XDIR NEGATE TO XDIR ; : REV_YDIR ( -- ) \ reverse y direction YDIR NEGATE TO YDIR ; : CHECK_DIR ( -- ) \ reverse direction if hit screen edge HIT_EW? IF REV_XDIR THEN HIT_NS? IF REV_YDIR THEN ; : DELAY ( n -- ) 0 DO LOOP ; : BOUNCE ( -- ) \ top-level word FRAME SET_RC BEGIN ERASE_BALL MOVE_BALL CHECK_DIR 100 DELAY 0 JOYST 1 = UNTIL ; References: HCHAR - http://turboforth.net/lang_ref/view_word.asp?ID=220 VCHAR - http://turboforth.net/lang_ref/view_word.asp?id=232 < - http://turboforth.net/lang_ref/view_word.asp?ID=50 > - http://turboforth.net/lang_ref/view_word.asp?ID=54 = - http://turboforth.net/lang_ref/view_word.asp?ID=53 BEGIN - http://turboforth.net/lang_ref/view_word.asp?ID=72 AGAIN - http://turboforth.net/lang_ref/view_word.asp?ID=71 UNTIL - http://turboforth.net/lang_ref/view_word.asp?ID=89 OR - http://turboforth.net/lang_ref/view_word.asp?ID=95 ASCII - http://turboforth.net/lang_ref/view_word.asp?ID=210 VALUE - http://turboforth.net/lang_ref/view_word.asp?ID=168 TO - http://turboforth.net/lang_ref/view_word.asp?id=167 +TO - http://turboforth.net/lang_ref/view_word.asp?id=152 I hope you enjoyed learning some Forth.
  6. We continue with decompiling Dealer Demo at $175D, seeing -TRAILING, (.") (PDOTQ), and then a handful of words leading to the word ERROR which decompiles incorrectly. Looking closely, we see that the .WORD PDOTQ precedes strings in the listing, which are represented as a count, followed by the string contents. The code to decompile such a string manually is easy to implement, since we built most of the infrastructure already to decompile Forth name fields, namely: sub cstr_buf { my ($buff, $addr, $size) = @_; my $count = unpack "C", substr($buff, 0, 1); my $string = get_string(substr($buff, 1, $count)); $string = sprintf "%s.BYTE %d,%s", get_label($addr), $count, $string; $count += 1; multi_buf($buff, $addr, $count, $string); $count;}sub cstr { my ($buff, $addr, $size) = read_img(@_); cstr_buf($buff, $addr, $size);} To decompile automatically, we can insert this snippet into forth_buf: if (get_cstrq($val)) { $i += cstr_buf(substr($buff, $i), $addr + $i, $size - $i); } where get_cstrq is: sub get_cstrq { return $_[0] == 0x179c; # PDOTQ} We can apply this to the ERROR word to test that the code works as advertised, e.g. invoking dealerdemo.pl 1a1c yields: 1A1C: 9C 17 .WORD PDOTQ1A1E: 04 20 20 .BYTE 4,' ? '1A21: 3F 20 Let's keep decompiling up through the word FORTH, which has .WORD DODOE, which we discussed last time, and then up through ABORT. Among these words, what differences do we see? EXPECT is implemented significantly differently. The Dealer Demo version is much shorter and simpler than the fig-Forth assembly version. The fig-Forth version has extra code to handle back spaces and carriage returns which are done elsewhere in the Dealer Demo kernel. The null word (literally ascii 0), is largely the same, except it uses BSCR 1 - AND instead of 0 BSCR U/ DROP. The fig-Forth screen listing uses 7 AND unconditionally and has BSCR (blocks per screen) equal to 8, so it more closely follows the Dealer Demo listing. However, in the Dealer Demo, BSCR is one, so these gymnastics to figure out where to read the character from isn't really needed. The word UPPER is dropped from Dealer Demo. This kernel (as are all Atari Forth kernels) is case sensitive so it isn't used. ERROR is modified slightly before calling QUIT. Instead of always leaving IN and BLK on the data stack, we omit IN if reading from disk. Since QUIT doesn't use this data, this presumably is left for debugging reasons. ABORT and QUIT use slightly different strings than the fig-Forth listing. ABORT ends with SEMIS, which isn't really needed since QUIT doesn't return. ABORT also calls some future word at $863A instead of CR, probably to run some Dealer Demo specific initialization. Calling Invoking -refs in our tool shows CREAT, ERROR, WORD, ABORT and QUIT forward references can now be fixed up. CREATE was the missing word used in colon and constant definitions, so we now have largely filled in all the words that implement compilation. I think that's enough for today. Our decompilation tool is now complete, we just need to keep applying it until we've cranked through the rest of the disk. The next post (or maybe two) should complete the kernel (which ends at byte $259f), and the post after that will describe the assembler (which ends at $2be4).
  7. Forth: The Cart Before the Horse (#5) This is one is going to be brief. If you've read any of the Forth primers, or any of the Forth proclamations that I and others make here on Atariage from time-to-time, you'll no doubt have read about how versatile and configurable the language is. We're going to have a very brief look at that today. But it won't be the War And Peace tome that I wrote yesterday. Brian Fox recently wrote some fascinating code for Camel99 that gives the language a more Basic-like syntax. As you probably know, in Forth, arguments and parameters to words (functions) come *before* the word/function that you want to call. This is because words/functions take their data, and put their data on the stack, so the stack has to be loaded before the word is called: TI BASIC: CALL HCHAR(ROW,COLUMN,CHAR,REPEATS) Forth: ROW COLUMN CHAR REPEATS HCHAR This tends to put people off when they look at Forth. It just looks like gobbledy-gook; at least until you understand that ROW and COLUMN etc are going on the stack, and HCHAR removes them. However, Forth is supposedly "the most flexible language of them all", "ultra malleable, "if you don't like it you can change it" blah blah blah. So, why don't we put our money where our mouth is and prove it? Well alrighty then! Inspired by Brian's look at HCHAR and VCHAR I thought it might be fun to demonstrate how the language can be changed to suit your preferences. There are some restrictions, sure, but I think you'll be impressed. We're going to change Forth's HCHAR and VCHAR into TI BASIC's HCHAR and VCHAR, where the arguments come after the word. So, again, let's recap: TI BASIC: CALL HCHAR(ROW,COLUMN,CHAR,REPEATS) Forth: ROW COLUMN CHAR REPEATS HCHAR We're going to end up with: CALL HCHAR( ROW COLUMN CHAR REPEATS ) Furthermore, we want it to be sophisticated enough such that the parameters can be numbers (called literals in Forth parlance) or calls to other words, or complex Forth expressions that compute a value etc. What might shock you is how much code is NOT required to do this. I give you: : CALL ( -- ) ; IMMEDIATE \ do absolutely nothing : HCHAR( ASCII ) WORD EVALUATE STATE @ IF COMPILE HCHAR ELSE HCHAR THEN ; IMMEDIATE : VCHAR( ASCII ) WORD EVALUATE STATE @ IF COMPILE VCHAR ELSE VCHAR THEN ; IMMEDIATE Now, you can type this directly on the command line: PAGE CALL HCHAR( 10 10 42 10 ) CALL VCHAR( 10 10 42 10 ) And behold the awesomeness. Note the spaces: The space between the open parenthesis of HCHAR( and VCHAR( is essential. The spaces between the parameters are just normal for Forth (and actually looks much nicer than using commas). The space before the final closing parenthesis is also required. You just changed Forth to be more like BASIC. You don't have to have numbers in the parameter list. They can be words or expressions: 10 CONSTANT TEN 42 CONSTANT FORTY-TWO CALL HCHAR( TEN TEN FORTY-TWO TEN TEN + ) (That last phrase: TEN TEN + puts 10 on the stack, then another 10 on the stack, then + ("add") removes them and replaces them with their sum, thus leaving the repeat count for HCHAR) So how does it work? Let's look at CALL first. CALL does nothing. It's only job is to be there to make those more familiar with BASIC happy. It has no code in it; it's empty. Furthermore, it's what is known as an IMMEDIATE word, meaning that it executes DURING COMPILATION, not during execution. An example: : FRED ( -- ) CR ." I AM FRED" CR ; Type that in. Nothing much happens. The word FRED gets stored in the dictionary ready to be used. Now type FRED and press enter. FRED executes. No big deal. Now, type this: : BOB ( -- ) CR ." BOO! BOB WAS HERE!" CR ; IMMEDIATE Okay, you typed it in. Nothing much happened. Now execute it: type BOB and press enter. Again, no big surprises. Now, try this: : TOM ( -- ) BOB CR ." HELLO! I AM TOM!" CR ; Did you see what happened? While *TOM* was being compiled, BOB got in on the act and ran, rather rudely announcing his presence. So what happens if we run TOM (type TOM and press enter)? HELLO! I AM TOM! Where's BOB? Should BOB not also say BOO!? No. And the reason why is very clever and is the secret sauce that makes Forth so very powerful. Here it is: "Immediate words execute at compile time." That is, immediate words execute when a word is being compiled, *not* when the compiled word is executed! That's possibly a brain-hurting statement. Consider this TI BASIC code: 10 CALL HCHAR(10,10,42,10) Now, when you press enter, the TI BASIC compiler switches on and compiles your code into some internal magic code that will do what you want it to do when you later RUN it. Okay. All normal stuff. But consider this: When the TI BASIC compiler is compiling that line of code, it does so entirely privately. No have no control over what compiler does. Mind your own business, it's nothing to do with you. The compiler privately compiles that code (or doesn't if there's an error), and you are just a bystander. That's not the case in Forth. In Forth, you can use "immediate" words that run when the compiler is compiling. And because they run when the compiler is compiling, you can "hijack" the compiling process, and do something: make a fart sound; say something on the speech synth; load a file; anything you want. You can even compile your own code. Think about that. Code that compiles code. And THAT is what makes Forth so powerful. So, lets get back to TOM and examine what happened. In Forth, the compiler is switched on by : (colon) and switched off by ; (semi-colon): : SOME-WORD <CODE GOES HERE> ; The compiler just walks along the line of text, and when it sees a word it looks for it in its dictionary and if it finds it, it compiles a call (like a GOSUB) to it. Now you can see why spaces are so critical in Forth. They are what separate the words so that they can be found in the dictionary. However, when the compiler is looking for a word, if it finds it, it checks to see if it is immediate or not. If it is not, it just compiles a call/GOSUB to the word. However, if it *is* immediate, it *executes* it, and does not compile it. That means you can put a reference to an immediate word in your definition, and at that point in the compilation process it will call your immediate word, and *you* can do something to the word that is *currently* being compiled, like add some more code to it. When the immediate word ends, the compiler just carries on compiling, totally oblivious to anything you may or may not have done to the word currently being compiled. It's none of its business. It's your business. You are in total control. Thus when TOM was being *compiled* the compiler saw the reference to the word BOB and saw that it was "an immediate word" and so it executed BOB, and BOB did it's thing (in this case, writing a cheeky message to the screen) and then carried on with the compiling. Now you understand why, when TOM was *executed*, there was no message from BOB. BOB did it's thing while TOM was being *compiled*. Yes. In Forth, there are two distinct excecution phases: Run-time: When a word is just plain excecuting, doing its thang; Compile time: When a word is being compiled. And you can do whatever the hell you want in either phase. You might want to go for a little lie down at this point! Now, lets look at HCHAR( and see what it does: : HCHAR( ASCII ) WORD EVALUATE STATE @ IF COMPILE HCHAR ELSE HCHAR THEN ; IMMEDIATE When the compiler sees HCHAR( it sees that it is immediate and so it executes it. The first thing is does is place the ASCII code for a ) (closed parenthesis) on the stack. Then WORD executes. WORD reads the line of text and will stop when it sees the ) character. So, if you typed CALL HCHAR( 1 2 42 4 ) WORD would capture 1 2 42 4 The output of WORD is two numbers: The address and length of the text that it found. This is fed into EVALUATE that simply evaluates the string as if it were a line of code entered at the keyboard. In this case, 1 2 42 4, or TEN TEN 42 TEN etc. are all valid code, so it executes it according to the rules of Forth: If we're compiling (i.e. the compiler was switched on with : (so we're building a word) then it will compile what it sees; If we're not compiling, it will just execute what it sees there and then, just like in BASIC when you enter something without a line number. So, we're using EVALUATE to evaluate the parameters for us between the HCHAR( word and the closing ) character. Note the cheeky use of the open parenthesis in HCHAR( which makes it look like some part of the the syntax of the word, but it isn't: It's just part of the name! And note also the closing parenthesis which again looks like syntax but is in fact nothing more than a marker for WORD to look for to isolate the parameters so that it can feed them into EVALUATE. The magic of Forth. The last bit of HCHAR( is very simple indeed. It just looks to see if we're in compile mode (the variable STATE will be 0 if we're not compiling, and >0 if we are compiling). If we ARE compiling, we compile a call to HCHAR (the original version of HCHAR built into the TurboForth EPROM). See? We're "injecting" code into the definition that is being compiled. However, if we're NOT compiling, we just execute HCHAR right there and then, which uses the parameters that EVALUATE evaluated for us. Thus we can do: CALL HCHAR( 1 2 42 99 ) (i.e. not in a definition, so it will execute immediately, like BASIC code with no line number) Or : LINE ( -- ) CALL HCHAR( 1 2 42 10 ) ; And both will work fine and do what they're supposed/expected to do. So, again, here's what happens when that LINE defintion above is compiled: The compilier sees that CALL is an immediate word, so it runs it. CALL actually does precisely nothing, it compiles nothing and runs nothing. It has 0 impact on run-time speed. It's purely "syntactic sugar" to sweeten things up for BASIC lovers. It's a total sham. You don't need to use it at all. The compiler sees that HCHAR( is immediate so it runs it. HCHAR( temporarily takes over, and reads the input up to the closing parenthesis and evaluates them. Since we're building a definition (LINE) the compiler is ON, so EVALUATE will compile them (by calling a new instance of the compiler and saying "HEY! Compile this! Thanks man!" (How's that for a mind f**k!?). HCHAR( then exits, it's done it's thing. Control now goes back to the compiler. The compiler only sees ; (semi-colon) because the parameters were consumed by WORD and EVALUATE so it completes the definition and we're done. If you were to disassemble the definition of LINE what you would see is this: 1 2 42 10 HCHAR In other words, HCHAR( re-arranged the code so that the parameters went first, then called a reference to the internal (in the EPROM) HCHAR which expects the parameters to be on the stack. The whole HCHAR( definition is nothing more than a trick which allows us to put the parameters *after* HCHAR( but internally it compiles HCHAR after the parameters. And that is the power of Forth. If you don't like: ROW COLUMN CHAR REPEATS HCHAR You can make your own word to give you: CALL HCHAR( ROW COLUMN CHAR REPEATS ) Or any other combination. And there endeth the lesson. This is without a doubt a bit of mind melter when you are new to Forth, so don't worry if you don't understand it all. I just wanted to give you an appreciation of the power and flexibility of Forth. It's not essential to understand this stuff right now. And now a quick demo using our new words. We haven't covered a lot of the code below yet. For now, just sit back and enjoy. : FWD-BOX ( -- ) 30 0 DO 12 0 DO CALL VCHAR( I I I 33 + J + 24 I 2* - ) CALL VCHAR( I 31 I - I 33 + J + 24 I 2* - ) CALL HCHAR( I I I 33 + J + 32 I 2* - ) CALL HCHAR( 23 I - I I 33 + J + 32 I 2* - ) LOOP LOOP ; : REV-BOX ( -- ) 0 29 DO 12 0 DO CALL VCHAR( I I I 33 + J + 24 I 2* - ) CALL VCHAR( I 31 I - I 33 + J + 24 I 2* - ) CALL HCHAR( I I I 33 + J + 32 I 2* - ) CALL HCHAR( 23 I - I I 33 + J + 32 I 2* - ) LOOP -1 +LOOP ; : BOXES ( -- ) \ top-level - run me 1 GMODE 5 0 DO FWD-BOX REV-BOX LOOP 0 GMODE ." Thanks for watching!" CR ; Note the additional spaces in the paremeters so that it's easier to identify each paremeter. References: HCHAR - http://turboforth.net/lang_ref/view_word.asp?ID=220 VCHAR - http://turboforth.net/lang_ref/view_word.asp?ID=232 IMMEDIATE - http://turboforth.net/lang_ref/view_word.asp?ID=163 ASCII - http://turboforth.net/lang_ref/view_word.asp?ID=210 STATE - http://turboforth.net/lang_ref/view_word.asp?ID=189 COMPILE - http://turboforth.net/lang_ref/view_word.asp?ID=156 IF - http://turboforth.net/lang_ref/view_word.asp?ID=81 THEN - http://turboforth.net/lang_ref/view_word.asp?ID=88 ELSE - http://turboforth.net/lang_ref/view_word.asp?id=76 CONSTANT - http://turboforth.net/lang_ref/view_word.asp?ID=157 CR - http://turboforth.net/lang_ref/view_word.asp?ID=129 ." - http://turboforth.net/lang_ref/view_word.asp?ID=206 DO - http://turboforth.net/lang_ref/view_word.asp?ID=75 LOOP - http://turboforth.net/lang_ref/view_word.asp?id=84 +LOOP - http://turboforth.net/lang_ref/view_word.asp?id=69 I - http://turboforth.net/lang_ref/view_word.asp?ID=80 J - http://turboforth.net/lang_ref/view_word.asp?ID=82
  8. I'm going to take a break from fixing disks images for now and instead talk about the language Forth. Forth grew out of the programming work by Charles Moore in the late 1960s. By the late 1970s interest in it was such that a group of enthusiasts banded together to program it for the microcomputers that were then becoming available. They formed the Forth Interest Group (or FIG) and placed several implementations of the language in the public domain. The implementation for the 6502 was done primarily by William Ragsdale, and was complete in 1980, and it is from this implementation it seems all the various Atari versions derived. That 6502 fig-Forth listing can be found at https://archive.org/details/fig_FORTH_6502_Assembly_Source_Listing, and I've cleaned up the OCR to create a more readable listing at https://ksquiggle.neocities.org/ff6502.htm. It was a significantly different language than the more commonly available BASIC, and due to its low cost and better performance it attracted many adherents. Since the code was available on such liberal terms (essentially public domain), many versions of it proliferated from user groups and some companies, and for a short time it was given modest coverage in the Atari-specific magazines such as Analog and Antic. Forth is built around a small (~6k) kernel of core words which manipulate a pair of stacks and a handful of registers. The compiler converts the code to series of addresses which are interpreted by a tiny virtual machine, and the emphasis is placed on building up the code in small routines (called words). Almost all parts of the language can be rewritten if they are not to the programmer's liking, and a clever inline assembler that let's one rewrite timing critical routines. Despite these strengths, the language failed to become especially popular. Large programs implemented using the fig-Forth model could consume all the memory available if care wasn't taken, and the language itself was typically more difficult to learn than the BASIC's of the day. Programmer's who wanted to escape the limitations of BASIC were encouraged to investigate assembler more frequently, or perhaps a BASIC compiler. This wasn't just a problem with Forth, all the languages for the Atari seemed to struggle to attract users. The earliest version of Forth to appear on the Atari was almost certainly the Forth used to implement the Dealer Demo, most likely in 1980. Developed in-house at Atari, it was released to demonstrate the Atari computers capabilities. It later became the more sophisticated "Coin-Op" Forth. Antic Forth and the fig-Forth 1.4S distributed originally by SoftSide magazine all share this later kernel, but at the time the Dealer Demo was released, the Forth kernel in it was not significantly different than the original fig-Forth kernel, and with a copy of the demo disk you could construct a Forth kernel from it with a little bit of sector editing. To show this, I'm going to decompile the Dealer Demo and extract its kernel for comparison with the original fig-Forth listing. That comparison is the goal, but I'm going to spend quite a bit of time developing the tooling to help do this, as we can leverage those tools in the future to examine other programs. Rather than simply drop all the code at once, I'm going to develop it up again like I did with the atr patching tools, hopefully giving some motivation for the choices I made in the code, and giving readers enough context to adapt the code for other purposes.
  9. This is a some of a message stream I just had with Willsy on installing TurboForth. Hope it helps if some one has a problem. From what I have seen it's a worthy product to program in. Installing the 'turboforth' cart in classic99: To run it, start classic99, and from the menu, select Cartridge --> User --> Open Then navigate to the MODS folder in your classic99 installation, then select TurboForthC.bin Then you're ready to rock. The blocks disk will boot from DSK1 when you select TF from the TI-99/4A selection screen. Just for your reference, you can bypass the auto-boot by holding down enter after selecting TurboForth from the cart selection screen. My note: you can also install the turboforthC.bin and turboforthd.bin in the MODS folder then modify the classic99.ini so that the new version of turboforth will then show up in the user carts area on the menu. installing the BLOCKS disk. Firstly (I didn't make it clear in my post above) you don't need to process the blocks file in any way with TI99DIR or any some such program, it's ready to go - just drag it out of the zip file and drop it into the DSK1 folder. Unfortunately, what I didn't mention (sorry) is that the file is in V9T9 format. I put it in this format as V9T9 format is recognized by a lot of other emulators such as V9T9, PC99 and TI994W. So, you need to click on the Disk menu option in Classic99, and from the DSK1 menu, select the menu option .\DSK1\ Then, make sure your emulated DSK1 'drive' is configured as per the following configuration: my note: It's a FIAD file. I made the dumb (on my part) assumption that it was a .dsk image file and kept trying to read it as such. Also, be sure to check 'recognizes v9t9 headers'. His website is turboforth.net If he doesn't already, he said he have all the latest files up shortly. This is well worth a look. If you have done any programming in the latest generation languages like Java this should be an easy pick up. The stack might be the only thing a little odd, but if you have done assembler, you should be familiar with the concept.
  10. I got to wondering how much code it would need to implement the BrainFuck language in Forth. Turns out not much: create array 8000 chars allot variable pointer array pointer ! : reset ( -- ) \ zero bf memory and reset pointer array 8000 0 fill array pointer ! ; : peek ( -- ) pointer @ [email protected] ; : poke ( -- ) pointer @ c! ; : > ( -- ) \ increment the pointer pointer @ 1+ pointer ! ; : < ( -- ) \ decrement the pointer pointer @ 1- pointer ! ; : + ( -- ) \ increment the byte at the pointer peek 1+ poke ; : - ( -- ) \ decrement the byte at the pointer peek 1- poke ; : . ( -- ) \ output the byte at the pointer peek emit ; : , ( -- ) \ input a byte and store it in the byte at the pointer pad 3 expect pad 3 number drop poke ; : [ \ ( -- ) \ jump forward past the matching ] if the byte at the pointer is zero [compile] begin compile peek [compile] while ; immediate : ] ( -- ) \ jump backward to the matching [ unless the byte at the pointer is zero [compile] repeat ; immediate That's a complete BrainFuck *compiler*. Most implementations I have seen are interpreters. Here, you feed in BF code using normal colon definitions. Because it's a normal colon definition, what you get out is compiled Forth code. Here's Hello World! in BrainFuck: : hw ( hello world in BrainFuck ) reset + + + + + + + + [ > + + + + [ > + + > + + + > + + + > + < < < < - ] > + > + > - > > + [ < ] < - ] > > . > - - - . + + + + + + + . . + + + . > > . < - . < . + + + . - - - - - - . - - - - - - - - . > > + . > + + . ; To run it, just type hw and press enter. To prove it's compiled Forth, use SEE (in TurboForth, it's block 27 on the accompanying disk): SEE HW And you get: Forth, you gotta love it. Of course, some say Forth is a bit of a BrainFuck, and it is, until the penny drops. Even then, managing the stack is still a puzzle. But it's so powerful .
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