CRC-16 Super fast and tiny! 17 instructions by Scott Dattalo

Code:

• ; W = CRC Message byte
  ; CRC16 - current 16-bit CRC check sum
  ;
  ; let CRC16 = abcd
  ;     Message Byte = xy
  ;
  ; where abcd, xy are nibble variables
  ;
  ; Upon exit,
  ;
  ;  a = c ^ (i1>>3) ^ i2
  ;	jmp	= d ^ ((i1<<1)&$f) ^ (i2>>3)
  ;  c = i1 ^ ((i2<<1)&0xf)
  ;  d = i2
  ;
  ; First compute the nibble array indices:
  ;  i1 = a ^ x
  ;  i2 = i1 ^ b ^ y
  ;
  ;
  ; Notation  (I) : (J)  is a byte with I= high nibble, J= low nibble
  ;
  
  CRC_Update
  	xor	W, CRC16_High	; (a^x):(b^y)
  	mov	Index, W	;
  	and	W, #$f0	; W = (x^x):0
  	swap	Index	; Index = (b^y):(a^x)
  	xor	Index, W	; Index = (a^b^x^y):(a^x) = i2:i1
  
  
    ; High byte
  	mov	W, <>3) : (((i1<<1)&0xf) | (i2>>3))
  	xor	W, CRC16_Low	; W = (i1>>3)^c : ((((i1<<1)&0xf) |
                                  ;     (i2>>3)) ^ d)
  	mov	CRC16_High, W	; low nibble of high byte is done
  
  	mov	W, Index
  	and	W, #$f0	; W = i2 : 0
  	xor	CRC16_High, W	; High nibble is of high byte is done
  
    ; now low byte
  	mov	CRC16_Low, W	; Low = i2 : 0
  	add	CRC16_Low, W	; Low = (i2<<1) : 0
  	mov	W, <>Index	; W = i1 : i2
  	xor	CRC16_Low, W	; Low = i1 ^ (i2<<1) : i2
  
  	retw	0
  
  

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