File format

Bytes from files are sent in 256 byte packets, each packet is followed by a 2 byte CRC checksum word.

CRC computation

The calculation is described as a "standard CRC-CCITT that uses polynomial $1021" and produces a 16 bit output, the Z80 code (slow, unoptimized) to generate it is as follows:

;--Z80 code to make CRC --------------------------------------------------------

; makes checksum in HL, src addr = DE, length = C bytes

crc_checksum

	ld hl,$ffff		
crcloop	ld a,(de)			
	xor h			
	ld h,a			
	ld b,8
crcbyte	add hl,hl
	jr nc,crcnext
	ld a,h
	xor 10h
	ld h,a
	ld a,l
	xor 21h
	ld l,a
crcnext	djnz crcbyte
	inc de
	dec c
	jr nz,crcloop
	ret

See also: Serial link protocol