“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.
A bit of context:
- DOS: A
.comfile has no header. The first byte of the
.comis code: this first byte will get executed first.
- C64: A
.prgfile has a header of two bytes. These two bytes indicate the load address of program. E.g: If the first two bytes are
0x08, it means that program will be loaded at address:
Taking that into account, there are different ways to support both DOS and C64 at the same time:
- Using the standard
0x0801address (the one used by this intro).
- Or autorun: using an address like
NNcould be any of the single-byte instructions. E.g: A good candidate for autorun could be
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:
- 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
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
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.
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%)
- DOS: 112 bytes (~43%)
- C64: 14 + 51 + 79: 144 bytes (~57%)
Download + Source code + Misc links
Binary is available here:
Commented source code is available here: