The following program is in nasm format, and assembles to a DOS .com executable of 240 bytes.

The program is based on Horst Schäffer's 784-byte CRC32 calculator, which is part of the PBATS32 collection.

crc32.asm: A CRC32 calculator [bits 16] org 0x0100 start: mov di,CMDBUF mov cx,0x7800 xor ax,ax rep stosw mov di,LUTBUF xor cx,cx .lutolp: xor dx,dx mov ax,cx mov ch,8 .lutlp: shr dx,1 rcr ax,1 jnc .xorskip xor dx,0xEDB8 xor ax,0x8320 .xorskip: dec ch jnz .lutlp stosw mov ax,dx stosw inc cx and ch,ch jz .lutolp mov si,0x81 mov di,CMDBUF mov dx,di .getarg: lodsb cmp al,' ' je .getarg jb .aend cmp al,',' je .aend cmp al,'/' je .aend stosb jmp short .getarg .aend: cmp dx,di je printhelp push di mov ax,0x3D40 int 0x21 jc printhelp mov bp,ax mov di,LUTBUF xor ax,ax dec ax cwd .crcolp: push ax push dx mov si,READBUF mov dx,si mov cx,READBLEN mov bx,bp mov ah,0x3F int 0x21 mov cx,ax pop dx pop ax jc .crcend jcxz .crcdone .crclp: mov bx,ax lodsb xor bx,ax shl bx,2 mov al,ah mov ah,dl mov dl,dh xor dh,dh xor ax,[bx+di] xor dx,[bx+di+2] loop .crclp cmp si,READBUF+READBLEN jnc .crcolp .crcdone: not ax not dx .crcend: push ax mov bx,bp mov ah,0x3e int 0x21 pop bx pop di mov al,' ' stosb mov cx,4 .prn: mov al,dh mov dh,dl mov dl,bh mov bh,bl call byteascii loop .prn mov ax,0x0a0d stosw mov al,'$' stosb mov dx,CMDBUF jmp short printmsg printhelp: mov dx,msghelp printmsg: mov ah,9 int 0x21 ret byteascii: xor ah,ah div byte [divider] call .inner xchg ah,al .inner: cmp al,10 sbb al,0x69 das stosb ret divider: db 16 msghelp: db "Usage: CRC32 <file>",13,10,'$' CMDBUF equ 0x03C3 LUTBUF equ 0x0468 READBUF equ 0x0968 READBLEN equ 0xC000

Article dated: 9th Feb 2009