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  1. Here's the 6502 assembler I mentioned recently on the Atari 8-bit forum. The reasons to write this were: 1.) None of the assemblers I tried could generate correct code for code assembled to run in zero page and have forward references to other code in zero page, changing their operand in real-time. 2.) I wanted to write an assembler in sh (years ago, I came across osimplay, which I thought was pretty neat). shasm65 is written in sh, the Unix Bourne Shell, with a few extensions used which are not available in all sh incarnations. So far, I have adapted it to work with bash, zsh (~28% faster than bash) and mksh (ksh93, ~52% faster than bash). ash, dash, ksh (ksh88) and pdksh all fail to work, either because they lack array support or do not allow function names to start with a dot. Both issues could be "fixed", but that would make it slower and/or pollute the internal namespace, so I decided against it. Its syntax is different from all other 6502 assemblers, because an input file is treated as just another shell script, which is sourced by the assembler. Mnemonics are function calls and its arguments are the operands. Labels are defined by using the special function L and assembler directives are functions starting with a dot, like .org, .byte, .word, et cetera. Labels are referenced as shell variable names (ex. jmp $label). Numbers/memory locations can be specified in decimal, hexadecimal (ex. 0xfffe) or octal (ex. 0377). To fix the main reason for writing this assembler (see point 1. above), shasm65 uses different mnemonics for some addressing modes. For example, loading A from a zero-page location is lda.z. This way, the assembler knows immediately exactly how much storage an instruction requires. addressing modes: implied no suffix, ex. cli accu .a, ex. rol.a zp .z, ex. lda.z 0xfe zp,x .z, ex. adc.z 128,x zp,y .z, ex. stx.z 64,y (ind,x) .i [,x], ex. lda.i [23,x] (ind),y .i [],y, ex. cmp.i [017],y immediate ., ex. lda. 17 absolute no suffix, ex. dec 0x6ff absolute,x no suffix, ex. inc 0x0678,x absolute,y no suffix, ex. ldx $fubar,y (abs) .i, jmp.i [0xfffe] relative no suffix, ex. beq $loop directives: .org start [dest] start address of next block (optionally loaded at different location) .byte x y z ... include literal bytes (no comma but spaces between the arguments) .word x y z ... include literal 16-bit little-endian words .ascii "ascii string" include literal string .screen "string" include literal string of Antic screen codes .space size reserve size space .align b align to b-bytes boundary .binary filename include _raw_ binary file filename . include source file (shell script, library functions, etc.) L name define label Because both the assembler and the source files it assembles are just shell scripts, you have all of the shell functionality (including calling external applications) as your "macro" language. You can create your own functions, use for loops, tests, if/then/else/fi conditional assembly, arithmetic, all you can think of. # lines starting with a hash are comments # the code below demonstrates a few of the features START_ADDRESS=0x3000 clear_pages() { # start number_of_pages ldx. 0 txa L loop for foo in `seq $1 0x0100 $(($1+($2-1)*256))` ; do sta $foo,x done inx bne $loop } .org $START_ADDRESS L clear_some_mem # inline unrolled loop to clear 0x4000-0xbfff clear_pages 0x4000 $((0xc0-0x40)) rts L host_info .ascii $(uname -a) There are two built-in functions: lsb() least-significant byte, ex. lda. `lsb $dlist` msb() most-significant byte, ex. lda. `msb $handler` Variable-, function- and label names should not start with an underscore or a dot. Both are reserved for the assembler itself. Also, all shell reserved words are prohibited. shasm65 has the following command line options: -oFILE write output (Atari 8=bit $FF$FF binary format) to FILE -v verbose output -h -help --help -? output credits, license and command line options with a short description and their defaults So, why a shell script? Well, because I can, it is fun, the code is short (~300 lines), it runs on many, many platforms, it provides a very powerful scripting/macro language and it's fun So, no drawbacks? Yes, there are. Shell scripts are interpreted and therefore shasm65 is a lot slower than the usual assemblers written in C and compiled to native machine code. I have probably missed describing some features or quirks, but basically, this is it. Have fun Any questions, post below. shasm65-0.91.tar.gz
  2. Long time, no blog. Let's start at the beginning. In 2009 I wrote my first version of a SID Emulator, running directly on the Atari 8-bit hardware. The emulation loop ran in the main loop and synchronisation was done with STA WSYNC. I had created a raw register dump of a .sid file with a modified version of TinySid and every frame I left the main loop a few times for a few cycles to stuff the SID emulation engine with new values. I didn't take long to realize I didn't have quite enough time to do much else. I wanted to ditch the idea of playing a register dump. Even RLE compressed it took way too much memory to store a decently sized song. I wanted to run the original C64 player routine. That led me to the source code of siddasm and a rewrite with more features, which I called siddasm2 (really original). I moved the emulation code to page zero to save some cycles, but that did not really help enough. At the same time, I stumbled upon a bug in Atasm and instead of fixing that, I ended up writing my own assembler. I had always wanted to do that. I won't go into details here. See my previous blog entries on shasm65 and siddasm2. After that, I kind of lost interrest, but a few weeks ago I continued this project. I completely rewrote my SID emulator in shasm65. Now the emulation engine was running of a timer interrupt, leaving the main loop free to periodically call a C64 player routine. This routine had to be patched though to write to shadow SID registers instead of the C64 hardware registers. This is where siddasm2 came in again. Early in the process of disassembling Laxity's song "Freeze" I noticed that the resulting assembly code couldn't be assembled again by atasm AT ALL. Which shouldn't be the case. I fixed a few bugs in siddasm2 and shortly after that I was able to recreate the .sid file from source code with the help of atasm -r and unix dd (for the header). So far, so good. Now I was able to patch the source code of 'Freeze' and run it alongside my sid emulation code. I added code that converted the shadow sid registers to values my engine could work with, added ADSR envelope handling, everything I used to do in my modified TinySid. A few debugging sessions later, I had it all working! After that, I disassembled and patched the International Karate tune. This triggered a new bug in my emulation, which is now fixed. I sent out an example to Freddy (F#Ready on AA), which he turned into a youtube video and posted a link on AA to see what all of you would think about it. And now I figured it's time to release the source code and a few binaries to play with Attached you'll find version 2.1 of siddasm2. A slightly modified shasm65 which is now verbose in hex instead of decimal (but the original shasm65 is still fine to assemble the code, too. The assembler itself did not change). .xex binaries for Laxity's Freeze, Rob Hubbard's International Karate and Jeroen Tel's JT42. The source code for everything. Regards and have fun! binaries.zip shasm65.zip siddasm2-2.1.zip sid-songs.zip atarisid2.zip
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