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Andrew Davie

Session 15 - Playfield Continued

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We've had a bit of time to think about the playfield, and hopefully have a go at some of the exercises. Admittedly I threw you in the deep end with the last session - so we'll go back a step and walk through exactly what all this playfield oddity is about. We'll also tackle some of the exercises to show that there's more than one way to skin a fish.

 

Last session we learned that the playfield registers PF0 and PF2 are reversed. Specifically, the order of pixels in the playfield registers (one bit per pixel, remember!) is backward, compared to the order for the first playfield register we encountered - PF1. This backward ordering is rather confusing, but that's just the way it is. Have a close look at the diagram presented in the last session and try and understand exactly the "playfield register/bit" to "pixel position on the scanline" correspondence.

 

There's one new playfield-related capability of the '2600 which I'd like to introduce now - playfield mirroring. I've already introduced this to you when I stated that the right hand side of the playfield was a copy of the left hand side (that is, the left 20 pixels come from the 20 playfield bits held in the TIA registers PF0, PF1 and PF2 - and the right 20 bits are a copy of the same bits). That copy can be displayed "normally" - or "mirrored". When mirrored, the bits are literally a mirrored copy of the left side of the playfield.

 

We're already familiar with two 'types' of TIA register. There's the strobe-type, where a write of any value to the register causes something to happen, independent of the value written (an example is WSYNC, which halts the 6502 until the TIA starts the next scanline). A second type is the normal register to which we write a byte, and the TIA uses that byte for some internal purpose (examples of these are the playfield registers PF0, PF1 and PF2). PF0 was a special-case of this type, where - though we wrote a byte - only four of the bits were actually used by the TIA. The remaining bits were discarded/ignored (have a look at the PF0 register in the diagram in the last session - the X for each bit position in bits D0-D3 indicates those bits are not used).

 

The third type of register (they're not really 'types' - but I want you to understand the difference between the way we're writing data to the TIA) is where we are interested in just the state of a single BIT in a register. Time to introduce a new TIA register, called CTRLPF. It's located at address 10 ($A)

 

CTRLPF

    This address is uded to write into the playfield control

    register (a logic 1 causes action as described below)



    D0 = REF (reflect playfield)



    D1 = SCORE (left half of playfield gets color of player 0,

    right half gets color of player 1)



    D2 = PFP (playfield gets priority over players so they can

    move behind the playfield)



    D4 & D5 = BALL SIZE

              D5   D4   Width

              0    0    1 clock

              0    1    2 clocks

              1    0    4 clocks

              1    1    8 clocks

 

 

Wow! This register has a lot of different stuff associated with it! Most of it is related to playfield display (bits D0, D1, D2) but bits D4 and D5 control the "BALL SIZE" - we'll worry about those bits later :)

 

Bit D0 controls the reflection (mirroring) of our playfield. If this bit is 0, then we have a "normal" non-mirrored playfield, and that's what we've been seeing so far in our demos. If we set this bit to 1, then the '2600 will display a reflected playfield (that is, the right-side of the playfield is a mirror-image of the left-side, instead of a copy). Note that only a single bit is used to control this feature - if we wrote a byte with this bit set (ie: %00000001) to CTRLPF we would also be setting those other bits to 0 - and we should be very sure this is what we want. In fact, it's often NOT what we want, so when we are writing to registers such as this (which contain many bits controlling different parts of the TIA hardware/display), we should be very careful to keep all the bits exactly as we need them. Sometimes this is done with a "shadow" register - a RAM copy of our current register state, and by first setting or clearing the appropriate bit in the shadow register, and THEN writing the shadow register to the TIA register. This is necessary because many/most of the TIA registers are only writeable - that is, you cannot successfully read their contents and expect to get the value last written.

 

Let's have a quick look at those other bits in this register, related to playfield...

 

D1 = SCORE. This is interesting. Setting this bit causes the playfield to have two colours instead of one. The left side of the playfield will be displayed using the colour of sprite 0 (register COLUP0), and the right side of the playfield will be displayed using the colour of sprite 1 (register COLUP1). We won't play with this for now - but keep in mind that it is possible. Remember, this machine was designed for PONG-style games, so this sort of effect makes sense in that context.

 

D2 = PFP. Playfield priority. You may have the playfield appear in front of, or behind, sprites. If you set this bit, then the playfield will be displayed in front, and all sprites will appear to go behind the playfield pixels. If this bit is not set, then all sprites appear to go in front of the playfield pixels.

 

That's a very quick rundown of this register. We know now that it controls the playfield mirroring (=reflection), the playfield colour control for left/right halves, the playfield priority (if sprites go in front of or behind the playfield), and finally it does something with the "BALL SIZE" which we're not worring about yet.

 

I've indicated that it's useful to have a "shadow" copy of the register in RAM, so that we can easily keep track of the state of this sort of register. In practise, this is rarely done - as we generally just set the reflection on or off, the score colouring on or off, the priority on or off, and the ball size as appropriate... and then forget it. But if, for example, you were doing a game where you were changing the priority on the fly (so your sprites went behind SOME bits background, but not other bits) then you'd need to know what those other values should be.

 

In any case, the point of this is to introduce you slowly to more TIA capabilities, and at the same time build your proficiency with 6502 programming. Here's how we set and clear bits with 6502.

 



CTRLPF_shadow = $82  ; a RAM location for our shadow register



   lda #%00000000

   sta CTRLPF_shadow          ; init our shadow register as required



  ; lots of code here





   lda CTRLPF_shadow

   sta CTRLPF                 ; copy shadow register to TIA register

 

 

The above code snippet shows the general form of shadow register usage. The shadow register is initialised - and at some point later in the code, we copy it to the TIA register. Now for the fun bit - setting and clearing individual bits in the shadow register...

 



  ; how to set a single bit in a byte



   lda CTRLPF_shadow          ; load the shadow register from RAM

   ora #%00000001             ; SET bit 0 (D0 = 1)

   sta CTRLPF_shadow          ; save new register value back to RAM





  ; how to clear a single bit in a byte



   lda CTRLPF_shadow

   and #%11111110             ; keep all bits BUT the one we want to clear

   sta CTRLPF_shadow



 

 

OK, that's not too difficult to understand. The two new 6502 instructions we have just used are "ORA", which does a logical-OR (that is, combines the accumulator with the immediate value bit-by-bit using a OR operation) - and the "AND", which does a logical-AND (again, combines the accumulator with the immediate value bit-by-bit using an AND operation). Now this is getting into pretty basic binary math - and you should read up on this stuff if you don't already know - but here's some truth tables for you...

 

OR operation



BIT |   0     1

-----+------------

 0  |   0     1

    |

 1  |   1     1





AND operation



BIT |   0     1

-----+------------

 0  |   0     0

    |

 1  |   0     1

 

 

Basically the above two tables give you the result FOR A SINGLE BIT POSITION, where you either OR or AND together two bits. For example, if I "OR" together 1 and 0, the resultant value (bit) is 1. Likewise, if I "AND" together a 1 and 0, I get a 0. This logical operation is performed on each bit of the accumulator, with the corresponding bit of the immediate value as part of the instruction. So "ora #%00000001" will actually leave the accumulator with the lowest bit SET. No matter what. Likewise, "and #%11111110" will leave the accumulator with the lowest bit CLEAR. No matter what. And in the other bits, their value will remain unchanged. You should try some values and check this out, because understanding this binary logical operation on bits is pretty fundamental to '2600 programming.

 

 

In the initialisation section of your current kernel, add the following lines...

 

   lda #%00000001

   sta CTRLPF

 

 

That's our playfield reflection in operation - if you're running any sort of playfield code, you will see that the right-side is now a mirror-image of the left-side. Now have a think about the exercise I offered in session 14...

 

5. How would you make a "wall" which was 8 scanlines high, full screen width, followed by left and right walls just 1  pixel wide each, at extreme left/right edges of the screen, 176 scanlines high, followed by another horizontal "wall",  full screen width and 8 scanlines high?  

 

Note: this would form a "box" border around the entire playfield.

 

 

It should be apparent, now, that in this sort of situation we really only need to worry about the left side of the playfield! If we let the '2600 reflect the right side, we will get a symmetrical copy of the left, and we'll have our box if only we do the left-side borders. This is a huge advantage to the programmer, because we suddently don't have to write new PF0, PF1, PF2 values each scanline. Remember (and I'll drum this into you until the very last session!) we only have 76 cycles per scanline - the less we have to do on any line, the better. At the very least, rewriting PF0, PF1 and PF2 twice per scanline would cost 30 cycles IF you were being clever. That's almost half the available time JUST to draw background - and there's still colours, sprites, balls and missiles to worry about! However, if you just use a reflected playfield, then we are only looking at single writes to PF0, PF1, PF2, cutting our playfield update to only 15 cycles per line (eg: lda #value / sta PF0 / lda #value2 / sta PF1 / lda #value3 / sta PF2).

 

Just an aside, here - some people have been posting code IN UPPERCASE. It is quite acceptable to use upper or lowercase for the mnemonics of your 6502 code. I prefer lowercase, as I find it easier to read and LESS LIKE SHOUTING! But its totally up to you - you will typically (but not always) find my code is lowercase, and you may feel free to adopt a style that suits you. I make my constants UPPERCASE, my variables typically a mixture, and my mnemonics lower-case. Your mileage may vary.

 

So, let's get down to it - here's a solution for exercise 5, of session 14...

 

; '2600 for Newbies
; Session 15 - Playfield Continued
; This kernel draws a simple box around the screen border
; Introduces playfield reflection 





               processor 6502

               include "vcs.h"

               include "macro.h"


;------------------------------------------------------------------------------



               SEG

               ORG $F000



Reset



  ; Clear RAM and all TIA registers



               ldx #0 

               lda #0 

Clear           sta 0,x 

               inx 

               bne Clear



      ;------------------------------------------------

      ; Once-only initialisation...



               lda #$45

               sta COLUPF             ; set the playfield colour



               lda #%00000001

               sta CTRLPF             ; reflect playfield



      ;------------------------------------------------



StartOfFrame



  ; Start of new frame

  ; Start of vertical blank processing



               lda #0

               sta VBLANK



               lda #2

               sta VSYNC



               sta WSYNC

               sta WSYNC

               sta WSYNC               ; 3 scanlines of VSYNC signal



               lda #0

               sta VSYNC

               

      ;------------------------------------------------

      ; 37 scanlines of vertical blank...

           

               ldx #0

VerticalBlank   sta WSYNC

               inx

               cpx #37

               bne VerticalBlank



      ;------------------------------------------------

      ; Do 192 scanlines of colour-changing (our picture)



               ldx #0                 ; this counts our scanline number



               lda #%11111111

               sta PF0

               sta PF1

               sta PF2



                  ; We won't bother rewriting PF0-PF2 every scanline of the top 8 lines - they never change!



Top8Lines       sta WSYNC

               inx

               cpx #8                 ; are we at line 8?

               bne Top8Lines          ; No, so do another



                  ; Now we want 176 lines of "wall"

                  ; Note: 176 (middle) + 8 (top) + 8 (bottom) = 192 lines



               lda #%00010000         ; PF0 is mirrored <--- direction, low 4 bits ignored

               sta PF0

               lda #0

               sta PF1

               sta PF2



                  ; again, we don't bother writing PF0-PF2 every scanline - they never change!





MiddleLines     sta WSYNC

               inx

               cpx #184

               bne MiddleLines



                  ; Finally, our bottom 8 scanlines - the same as the top 8

                  ; AGAIN, we aren't going to bother writing PF0-PF2 mid scanline!



               lda #%11111111

               sta PF0

               sta PF1

               sta PF2



Bottom8Lines    sta WSYNC

               inx

               cpx #192

               bne Bottom8Lines



      ;------------------------------------------------





               lda #%01000010

               sta VBLANK          ; end of screen - enter blanking



  ; 30 scanlines of overscan...



               ldx #0

Overscan        sta WSYNC

               inx

               cpx #30

               bne Overscan



               jmp StartOfFrame




;------------------------------------------------------------------------------



           ORG $FFFA



InterruptVectors



           .word Reset          ; NMI

           .word Reset          ; RESET

           .word Reset          ; IRQ



     END

 

 

This kernel is interesting in that it achieves the box effect by writing the playfield registers BEFORE the scanline loops to do the appropriate section. It uses the knowledge that the TIA has an internal state and will keep displaying whatever it already has in the plafyield registers. So, in fact, the actual cost (in cycles) of drawing the "box" playfield on each scanline is 0 cycles - ie; it's free. We just had that short initial load before each section (taking a few cycles out of the very first scanline of each section). This is how you need to think about '2600 programming - how to remove cycles from your scanlines - and do the absolute minimal necessary.

 

 

That will do for today's session. We've had an introduction to controlling individual TIA register bits, and seen how to achieve a reflected playfield at next to no cost. We've had a brief introduction to the CTRLPF register, and seen how it has a myriad (well, more than 3) uses. Although some of the previous sessions have asked you to think about tricky subjects like horizontal scrolling, and asymmetrical playfields - now is not the time to actually discuss these tricky areas. Those who have been posting their sample solutions are on the right track. We'll get to those areas in future sessions - so until next time (when we'll develop our playfield skills a bit more)... ciao!

 

 

 

Exercises:

 

1. Introduce a RAM shadow of the CTRLPF register, and modify it differently in each section of the kernal. For example turn reflection on and off partway through the midsection of the box, and see what happens.

 

2. Have a play with the SCORE bit in the CTRLPF register, and in conjunction with that the COLUP0 and COLUP1 colour registers. Note how this SCORE bit changes where the colour for the playfield comes from.

post-214-1055160673_thumb.png

kernel15.zip

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What a newbie I am! Setting/clearing bits with ORA and 'AND' (no pun intended :) ) didn't become clear until playing with them for a while. I thought I could use them interchangeably to switch on/off selected bits as I pleased. :dunce: Since the tought did cross my mind:

 

Is there a 6502 command to switch selected bits to their opposite value?

 

 

 

 

---

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Exercises:

 

1. Introduce a RAM shadow of the CTRLPF register, and modify it differently in each section of the kernal.  For example turn reflection on and off partway through the midsection of the box, and see what happens.

 

Ok here is my code:

 

; 2600 for newbies
; Session 15 - More Playfield
;
; Exercise 1 -  Shadow RAM technique with CTRLPF example.
;  The following exercise uses a part of RAM as a 
;  shadow copy of the CTRLPF register. The objective
;  is to practive the use of this technique by
;  modifying the CTRLPF register using its shadow copy.
;  The reflection bit will be reversed at the middle
;  of the scanline loop and turned back to normal
;  at the end of it.



 processor 6502

 include "vcs.h"

 include "macro.h"



 seg

 org $F000



CTRLPF_shadow = $82; Variable will hold a copy of the value of CTRLPF .



Reset



 ldx #0

 lda #0



Clear



 sta 0,x

 inx

 bne Clear



 lda #$45

 sta COLUPF



 lda #%00000001	; Set shadow copy to "mirror" mode.

 sta CTRLPF_shadow



StartOfFrame



 lda #0

 sta VBLANK



 lda #2

 sta VSYNC



 sta WSYNC

 sta WSYNC

 sta WSYNC



 lda #0

 sta VSYNC



 ldx #0 ; x will count the scanlines



 lda CTRLPF_shadow; Load shadow ram copy into real CTRLPF

 sta CTRLPF  



VerticalBlank



 sta WSYNC

 inx

 cpx #37

 bne VerticalBlank



 ldx #0



 lda #%11111111

 sta PF0

 sta PF1

 sta PF2



Top8Lines



 sta WSYNC

 inx

 cpx #8

 bne Top8Lines



 lda #%00010000

 sta PF0

 lda #0

 sta PF1

 sta PF2



 lda #0    ; Modify CTRLPF shadow ram to #%00000000

 sta CTRLPF_shadow; This will turn off mirror mode.



MiddleLines



Alter1stTime



 cpx #96  	; Wait until the 96th scanline to...

 bne Continue



 lda CTRLPF_shadow; ..load new shadow ram copy into real CTRLPF

 sta CTRLPF  



Continue



 sta WSYNC

 inx

 cpx #184

 bne MiddleLines



 lda #%11111111

 sta PF0

 sta PF1

 sta PF2



Bottom8Lines



 sta WSYNC

 inx

 cpx #192

 bne Bottom8Lines



 lda #01  	; Modify CTRLPF shadow ram back to mirror mode.

 sta CTRLPF_shadow; The CTRLPF register will be actualized when the loop begins a new

 	; iteration (near the 'VerticalBlank' label).



 lda #%01000010

 sta VBLANK



 ldx #0



Overscan



 sta WSYNC

 inx

 cpx #30

 bne Overscan



 jmp StartOfFrame





 ORG $FFFA



InterruptVectors



 .word Reset 

 .word Reset 

 .word Reset 



END

post-3560-1055203788_thumb.png

lesson15_ex1.zip

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Is there a 6502 command to switch selected bits to their opposite value?

---

 

 

Yes, the "eor" instruction does that. "eor" stands for exclusive-or. Here's the corresponding truth-table for that function...

 



 EOR  |   0   1

 -----+----------

   0  |   0   1

   1  |   1   0



 

Wherever you want to 'flip' the status of a bit, just use eor #value, where the value has the bits you want flipped... set. For example, eor #%10000001 will flip bits 0 and 7 in the accumulator, and leave the rest the same.

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Exercises:

 

2. Have a play with the SCORE bit in the CTRLPF register, and in conjunction with that the COLUP0 and COLUP1 colour registers.  Note how this SCORE bit changes where the colour for the playfield comes from.

 

This was fun to do. I added the following lines to my init routine:

 

lda #$80	; NTSC blue for player 0

sta COLUP0



lda #$1E	; NTSC yellow for player 1

sta COLUP1



lda #%00000001	; Set both shadow copy and CTRLPF to "mirror" mode. 

sta CTRLPF_shadow

sta CTRLPF

 

I initialized the CTRLPF shadow copy and CTRLF to the same value. At first the code would only use player sprite colors, but I started playing around with the code. In the end I figured how to get three colors. The top part of the screen uses the playfield color, and the bottom uses the player sprite colors. The code uses the CTRLPF shadow ram to change the SCORE bit value of this register around the middle of the scanline loop.

 

StartOfFrame	; Beginning of scanline loop

.

.

.

lda #0

sta VSYNC



lda #%00000011	; set shadow copy to mirror mode and

sta CTRLPF_shadow; to use player prite colors. 



ldx #0  ; x counts the scanlines.



VerticalBlank

.

.

.

.

Top8Lines



sta WSYNC

inx

cpx #8

bne Top8Lines

.

.

.

MiddleLines



Alter1stTime



cpx #96	; Wait until the 96th scanline (middle of screen) to...

bne Continue



lda CTRLPF_shadow; Load shadow copy into real CTRLPF

sta CTRLPF



and #%11111101	; Clear SCORE bit in shadow copy of CTRLPF

sta CTRLPF_shadow; to use playfield color again. This won't be set

 ; until the end of the scanline loop.



Continue



sta WSYNC

inx

cpx #184

bne MiddleLines

.

.

.

Bottom8Lines



sta WSYNC

inx

cpx #192

bne Bottom8Lines



lda CTRLPF_shadow; Set CTRLPF to use playfield color again.

sta CTRLPF



lda #%01000010

sta VBLANK

.

.

.

 

 

(I tried to summarize the major changes. The whole exercise solution is in the zip. If asked I will gladly shorten the post)

post-3560-1055205305_thumb.png

lesson15_ex2.zip

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MiddleLines



Alter1stTime



cpx #96	; Wait until the 96th scanline (middle of screen) to...

bne Continue



lda CTRLPF_shadow; Load shadow copy into real CTRLPF

sta CTRLPF



and #%11111101	; Clear SCORE bit in shadow copy of CTRLPF

sta CTRLPF_shadow; to use playfield color again. This won't be set

 ; until the end of the scanline loop.



Continue



sta WSYNC

inx

cpx #184

bne MiddleLines

 

 

The above code fragment is a perfectly legitimate way to do this. In summary, it checks for scanline x=96 and on that line it makes its change. The drawback with this, though, is that the check is performed every scanline. It actually costs 5 cycles (of your 76 available) per line just to do this! That is, the cpx #96 / bne Continue, is taking a good 6.5% or so of your total available processing time - for just two instructions!

 

An alternate solution is to have 96 lines without the comparison, THEN a single scanline where you change the value, then another 95 lines without a comparison. This costs more ROM, of course. It's always a balancing act - ROM vs RAM vs time. Things get really tight when you want to do playfield AND sprites AND colour modifications on the scanline... at the same time :)

 

Cheers

A

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The "happy" kernal

 

Ok, now write it using a lookup table instead -- i.e:

HappyBitmap:

.byte %00010000

.byte %00000000


; etc etc etc

 

Hint: you can do it a couple of ways -- you can have one table, with values for PF1 and PF2 interlaced (i.e, PF1, PF2, PF1, PF2, etc, etc). Or you can have two tables -- one for PF1 and PF2.

 

"Happy" coding :D

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Ok, how's this:

 

;Reflected playfield goodness v2

(snip/snip)

 

 

Very nice indeed. I've attached a snapshot so people can see what it does. Since you've stolen my next lesson idea, I'll have to come up with somethin else :)

post-214-1055600616_thumb.png

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Playing around with bit shifting

 

Another lovely bit of PF work. Attached is a piccy of the effect - though it should be viewed 'live' to get the full effect.

post-214-1055600812_thumb.png

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I'm guessing that screenshot is doing something. :ponder:

 

Well heres my first attempt, it's only a snap shot

but if you tell us whats meant to be going on.... ;) :D

;set variables

BACK = $80

BACK1 = $81

BUFFER = $82

 sta COLUBK  

 lda #%10101010	; set PF0 to edge

 sta PF0

 sta PF2

 sta BACK

 lda #%01010101

 sta PF1

 sta BACK1



<snip>



     ; 192 scanlines of picture...

               ldy 0

               ldx #24

Picture         sta WSYNC

 iny

 lda BACK

 sta BUFFER

 sta WSYNC

 iny

 sta WSYNC

 iny

 lda BACK1

 sta BACK

 sta WSYNC

 iny

 lda BUFFER

 sta BACK1

 sta WSYNC

 iny

 sta WSYNC

 iny

 sta WSYNC

 iny

 sta WSYNC

 iny

 lda BACK

 sta PF0

 sta PF2

 lda BACK1

 sta PF1

 sty COLUPF

 dex

 bne Picture



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I worked up a playfield graphic template in Excel (for NTSC) systems. It can easily be altered to fit PAL, also. I pasted the Playfield Weirdness graphic and other helpful information from session 15 along the right side. The document is formatted to print on 11x17 paper.

 

 

-=TNT2Gamer=-

graph_paper_playfield_11x17.zip

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Hello All,

 

I wasn't here for the beginning of the course, but I'm catching up. I started 2600 programming a year and a half ago, but didn't get very far. I'm going all the way this time :)

 

Thanks to Andrew Davie for doing an incredible job!

 

This kernel draws a diamond with different colored sides, and the colors rotate clockwise.

 

-Tom Dunn

 

s153.jpg

s153.zip

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All right, Im a little behind, but Im loving all the info that is here. Wow there is a lot to chew on. Thanks for all the time invested. Anyways, here is my question, I want to stop the dark brown stripes about half way down the screen. I don't really know where to go from here though? Any suggestions? Thanks.

 

	processor 6502

include "vcs.h"

include "macro.h"

;------------------------------------------------------------------------

SEG

ORG $F000

Reset

; Clear RAM and all TIA registers

ldx #0

lda #0

Clear sta 0,x

inx

bne Clear

;------------------------------------------------

; Once-only initialisation...

lda #$22

sta COLUPF; set the playfield colour

lda #%00000001

sta CTRLPF; reflect playfield

;------------------------------------------------

StartOfFrame

; Start of new frame

; Start of vertical blank processing

lda #0

sta VBLANK

lda #2

sta VSYNC

sta WSYNC

sta WSYNC

sta WSYNC; 3 scanlines of VSYNC signal

lda #0

sta VSYNC

;------------------------------------------------

; 37 scanlines of vertical blank...

ldx #0

VerticalBlank sta WSYNC

inx

cpx #37

bne VerticalBlank

;------------------------------------------------

; Do 192 scanlines of colour-changing (our picture)

ldx #$8A

stx COLUBK

ldx #0; this counts our scanline number

Sky

inx

sta WSYNC

cpx #80

bne Sky



lda %00000001

sta CTRLPF



lda #%10101010

sta PF0

sta PF1

sta WSYNC

sta WSYNC



ldy #$2F

sty COLUBK

ldx #0

Ground

inx

sta WSYNC	

cpx #110

bne Ground



ldx #0

stx COLUBK





lda #0

sta PF0

sta PF1

sta PF2



lda #%01000010

sta VBLANK; end of screen - enter blanking

; 30 scanlines of overscan...

ldx #0

Overscan sta WSYNC

inx

cpx #30

bne Overscan

jmp StartOfFrame

;--------------------------------------------------------------------------

ORG $FFFA

InterruptVectors

.word Reset; NMI

.word Reset; RESET

.word Reset; IRQ

END

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>I want to stop the dark brown stripes about half way down the screen. I >don't really know where to go from here though? Any suggestions? >Thanks

 

What does this section of code do?

 

ldx #0

Ground

inx

sta WSYNC

cpx #110

bne Ground

 

 

Chris...

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Right...I think that he was pointing out where that striped code will appear on the screen. If you want those darker stripes to only appear halfway through the "ground", use 2 loops instead of 1 (changing the bitpattern saved to PF0-2 between them).

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I've got a really weird problem with this session. I understand everything and I've managed to do everything in the previous sessions so far. But when I try to run the code above, Stella gives me an error. I tried everything I could think of to fix it but nothing worked. Finally I added the old "rainbow effect" code from the original kernel, and amazingly it worked. So now I have the rainbow background and the purple box around it. I can't really think of why that helps it work, but I guess the Mac version of Stella has to see "stx COLUBK" on every scanline or else it doesn't work. Anybody know why it's doing this??

 

Edit: Also, I tried downloading all of the examples people posted above, but none of them work. Adding the rainbow background, however, makes them work flawlessly. It think it's pretty obvious that MacStella is messed up.

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It think it's pretty obvious that MacStella is messed up.

Nah, I don't think so. MacStella is outdated, but not that buggy.

 

Please post your source code and I am sure someone will find the error.

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Thanks for offering your help, but I already figured it out. It was Stella after all, I just had the old version (0.7). It took me forever to find a working version of 1.2, most of the links were broken. But the only one I found is in French, so if anyone knows where I can DL an English version of MacStella 1.2, please let me know! Thanks! :D

 

Edit...

How can something so simple make me so happy? :D :D :D

 

pf.jpg

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Never too late to start programming my favorite console. ;)

It's a mirrored playfield demo with a color scrolling.

 

I coded this demo thinking to my wife :D

 

Heart_Color.jpg

 

Here is the code :

 




;/////////////////////////////////////////////////////////////////
;// Heart Playfied Demo V1.00 - Christian Bogey, April 16, 2004 //
;//      Mirrored Playfield with Playfield Color Scrolling      //  
;/////////////////////////////////////////////////////////////////



               processor 6502 

               include "vcs.h" 

               include "macro.h" 




;///////////////////////// Vars ///////////////////////////////////////



Color_Start = $80     ; Playfield Color (Color of the 1st Line)

Color = $81           ; Temp Var Address


;/////////////////// Playfield Datas //////////////////////////////////
; PF0
;PFData0_0 = #%11110000   ; Up and Down * 8 Lines  // Idem as PFData2_0

PFData0_1 = #%00010000    ;  Middle * 176


; PF1
;PFData1_0 = #%11111111   ; Up and Down * 8 Lines // Idem as PFData2_0
;PFData1_1 = #%00000000   ; Middle * 176          // Idem as PFData2_1

       
; PF2

PFData2_0 = #%11111111    ; Up and Down * 8 Lines

PFData2_1 = #%00000000    ; Empty Lines * 8 (5 Up and 5 Down)




;/////////////////  Start of Code ///////////////////////////////////// 



               SEG 

               ORG $F000 



Reset 



      ; Clear RAM, TIA registers and Set Stack Pointer to #$FF

               SEI

               CLD	

               LDX #$FF

               TXS

               LDA #0

Clear_Mem

               STA 0,X

               DEX

               BNE Clear_Mem  	



               STA Color_Start; Init Playfield Color

               LDA #1

               STA CTRLPF; Mirrored Playfiels

               LDA #$00

               STA COLUBK; Set Background to Black


;///////////////////  Picture Starts Here /////////////////////////////



Start_Frame 



      ; Start VSYNC



               LDA #2 

               STA VSYNC 



               STA WSYNC 

               STA WSYNC 

               STA WSYNC    ; 3 Scanlines of VSYNC 



               LDA #0 

               STA VSYNC; End VSYNC         





      ; 37 Scanlines of Vertical Blank... 

          

               LDX 37

Vertical_Blank  

               STA WSYNC 

               DEX 

               BNE Vertical_Blank 

               

               LDA #0 

               STA VBLANK ; Enable TIA Output

                               
;////////////// Start To Draw Playfield ///////////////////////////////      	

         

               LDX Color_Start

               STX Color	

               STX COLUPF  

      ; Draw Top Bar  

               LDY #8	; 8 Lines to Draw

               LDA #PFData2_0

               STA PF0

               STA PF1

               STA PF2

Draw_Bar1       

               STA WSYNC   

               DEX

               STX COLUPF  

               DEY

               BNE Draw_Bar1 

               

            

               

      ; Draw 5 Empty Lines

               LDY #5*8

               LDA #PFData0_1

               STA PF0

               LDA PFData2_1

               STA PF1

               STA PF2

Empty_Lines1

               STA WSYNC 

               DEX

               STX COLUPF  

               DEY

               BNE Empty_Lines1                



 

      ; Draw Heart

               STX Color

               LDX #$0C       ; Init Index

Draw_Heart  

               LDA PF2HeartData-1,X       ; (Keep PF0 & PF1)

               STA PF2

               LDY #8            ; 8 Lines

               TXA                 ; Save Index

               LDX Color

Draw_Heart_Line

               STA WSYNC

               DEX

               STX COLUPF  

               DEY

               BNE Draw_Heart_Line

               STX Color        ; Save Color Index

               TAX                 ; Restore Index

               DEX

               BNE Draw_Heart



      ; Draw 5 Empty Lines

               LDX Color; Restore Color Index

               LDY #5*8

               LDA PFData2_1

               STA PF2           ; Clear PF2 (Keep PF0 & PF1)

Empty_Lines2

               STA WSYNC   

               DEX

               STX COLUPF  

               DEY

               BNE Empty_Lines2



      ; Draw Bottom Bar  

               LDY #8	; 8 Lines to Draw

               LDA #PFData2_0

               STA PF0

               STA PF1

               STA PF2

Draw_Bar2            

               STA WSYNC   

               DEX

               STX COLUPF             

               DEY

               BNE Draw_Bar2

               
;////////////// End To Draw Playfield /////////////////////////////////      



      ; Makes Colors to Scroll Up                 

               LDX Color_Start

               DEX                      ; Use INX instead of DEX to Scroll Up

               STX Color_Start


;////////////// End Of Display ////////////////////////////////////////      	

   

               LDA #%01000010     ; Disable VIA Output

               STA VBLANK           



      ; 30 scanlines of overscan... 



               LDX #30 

Overscan        

               STA WSYNC 

               DEX 

               BNE Overscan 



               JMP Start_Frame       ; Build Next Frame


;////////////// Heart Data ////////////////////////////////////////////



PF2HeartData

               .byte #%10000000; Heart Pic Data End * 8 Lines

               .byte #%11000000;

               .byte #%11100000;

               .byte #%11110000;

               .byte #%11111000;

               .byte #%11111100;

               .byte #%11111110; 12 * 8 = 96

               .byte #%11111110;	

               .byte #%11111110;

               .byte #%11111100;

               .byte #%11111000;

               .byte #%01110000   ; Heart Pic Data Start * 8 Lines

 
;////////////// Set Vectors ///////////////////////////////////////////



           ORG $FFFA 


; Interrupt Vectors 



           .word Reset          ; NMI 

           .word Reset          ; RESET 

           .word Reset          ; IRQ 



         END 





heart_color.zip

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