Blake 32(pouet.net) |
This is a good overview of some of the best demoscene stuff of 2020: https://youtu.be/CVAvuhNBt-Y
mov al,13h ; 2
int 10h ; 2
frameloop:
les ax,[bx] ; 2
mov ah,0xcc ; 2
mul di ; 2
mov al,16 ; 2
fractalloop:
ja snobby ; 2
inc ax ; 1
snobby:
adc dl,[fs:0x46C] ; 5 (f&^k this shit!)
sbb dh,dl ; 2
ror dl,cl ; 2
adc dl,dh ; 2
jno fractalloop ; 2
stosb ; 1
jmp frameloop ; 2
nop ; bonus, because 32 bytes was too much free space for me.For the rest it would take a while to understand the math and I'm not even sure where 0xa0000 would come from
32 bytes!
; Inspired by "Auguries of Innocence", a poem by William Blake
;
; To see a World in a Grain of Sand
; And a Heaven in a Wild Flower
; Hold Infinity in the palm of your hand
; And Eternity in an hour
And 32 is of course the size in bytes of the executable.
The key is the LES AX,[BX] opcode. BX is initialized to 0 by DOS, and the "Set video mode" BIOS call preserves BX's value. So the LES opcode sets ES:AX by reading a dword from DS:0.
What's there? A COM file is a single-segment program, so DS equals CS, and the code segment starts with the Program Segment Prefix. (The actual code is loaded at offset 100h.) So it loads the first two words of the PSP. What are those?
The first word is an "INT 20h" instruction, 20CDh, for compatibility with CP/M.
The second word is the segment number of the end of the memory allocated for the program. But DOS always allocates all memory to COM programs, so this will be 0x9FFF (assuming you have a full 640K conventional memory installed).
So ES:AX is set to 9FFF:20CD. And with x86 segmented memory 9FFF:0010 equals A000:0000.
I’ve toyed through out the years that if I ever got a tattoo, it would be:
mov ax, 13h;
int 10h