; PDP-11 CRC-16 code

; r0=byte
; r1=>data
; r2=count
; r3=crc
; r4=polynomic

mov #&1021,r4	; XMODEM polynomic
.bytelp
movb (r1)+,r0	; Fetch byte from memory

; The following code updates the CRC with the byte in R0
bic #&FF00,r0	; Ensure b8-b15 clear
swab r0
xor r0,r3	; XOR into CRC high byte
mov #8,r0	; Prepare to rotate 8 bits
.rotlp
clc
rol r3		; Rotate CRC
bcc clear	; Carry out clear
xor r4,r3	; CRC=CRC xor &1021
.clear
sub #1,r0
bne rotlp	; Loop for 8 bits

sub #1,r2	; num=num-1
bne bytelp	; Loop until num=0
mov r3,r0	; Return CRC to BASIC in R0/R1
clr r1
rts pc		; Return with R3=updated CRC

;; #include <Debug>