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