A multi-platform 256-byte intro

“Amor para Dos” is a multi-platform 256-byte intro. The binary, without any kind of modification, can run both on:

• a 80386 (or better) + DOS
• and on a Commodore 64.

We (L.I.A) released it at Flashparty 2021. I did the coding.

Multi-platform internals

A bit of context:

• DOS: A .com file has no header. The first byte of the .com is code: this first byte will get executed first.
• C64: A .prg file has a header of two bytes. These two bytes indicate the load address of program. E.g: If the first two bytes are 0x01 and 0x08, it means that program will be loaded at address: 0x0801.

Taking that into account, there are different ways to support both DOS and C64 at the same time:

• Using the standard 0x0801 address (the one used by this intro).
• Or autorun: using an address like 0x02NN, where NN could be any of the single-byte instructions. E.g: A good candidate for autorun could be 0x02cc.

Let’s see in detail how using 0x0801 start address work:

$01 $08         ; Start Address ($0801)
$0b $08         ; Next basic instruction: Address $080b
$75 $08         ; BASIC line number. E.g: 2165, but could be any number
$9e             ; BASIC "SYS" token
$32 $32 $32 $34 ; "2224"
$00             ; End of line
$00 $00         ; Address: $080b. End of BASIC program

• In summary, the first two bytes are fixed: $01 $08.
• Bytes 2 and 3 can be somewhat controlled.
• Bytes 4 and 5 can be fully controlled.
• …and the rest is not important since we can fully control bytes 4 and 5.

If we disassemble our C64 program like if it were a DOS .com, it would look like:

.org 0x100

    ; Assumes: (see: http://www.fysnet.net/yourhelp.htm)
    ;  BX=0x0000
    ;  CX=0x00ff
    ;  SI=0x0100
    add     [bx + si], cx           ; Bytes $01 08$
    or      cx, [bx + si]           ; Bytes $0b $08

    ; we control the next 2 bytes: $0875 (2165)
    ; Meaning that the BASIC line will be 2165
    ; "Z" won't be set, so the jump is guaranteed.
    jnz     start                   ; Bytes $75 $08

    db      0x9e                    ; BASIC "SYS" opcode
    db      0x32, 0x32, 0x32, 0x34  ; "2224" or 0x08b0: C64 start address    
    db      0x00                    ; End of line
    db      0x00, 0x00              ; End of BASIC program

start:
    ;Our start code

The first 2 instruction could potentially break our DOS program. But if you look at the initial values of BX, SI and CX, it is safe to assume that:

• BX = 0x0000
• SI = 0x0100
• CX = 0x00FF

What will happen is that the first instruction will overwrite itself, and the second will overwrite CX. The OR will set Z=0. And the following instruction will jump to our start address.

The generated binary will run both on a DOS machine and on a C64. This technique does not use any emulator trick. The binary runs in real hardware.

What’s nice about this technique is that it doesn’t add any overhead: no additional bytes are needed to support both platforms.

Challenge: Create a binary that can run in 3 different platforms.

Size distribution

The 256 bytes of the intro are distributed like the following:

• Green: C64 bootstrap (SYS 2224), 14 bytes (~5%)
• Blue: DOS code, 112 bytes (~43%)
• Pink: C64 scroller text, 51 bytes (~20%)
• Red: C64 code, 79 bytes (~30%)

Total:

• DOS: 112 bytes (~43%)
• C64: 14 + 51 + 79: 144 bytes (~57%)

Download + Source code + Misc links

Binary is available here:

• amor_para_dos_lia2.zip

Commented source code is available here:

• https://gitlab.com/ricardoquesada/c64-pc-intro-amor-para-dos

Misc links:

• Pouet: https://www.pouet.net/prod.php?which=89786
• CSDB: https://csdb.dk/release/?id=207946