PIC Microcontoller Math Method

Multiply 16x8 bit unsigned from Martin Sturm

Code:

• Martin Sturm shares this code:

• ; 16bit by 8bit unsigned multiply
  ;  by Martin Sturm 2010
  ; tested
  ;
  ; aH:aL * b --> r3:r2:r1
  ;
  ; 69 instructions, 69 cycles
  ;
  
  ; helper macro
  mmac MACRO A,bit, u2,u1
  	BTFSC	A,bit
  	 ADDWF	u2,F
  	RRF	u2,F
  	RRF	u1,F
  	ENDM
  
  MULT_16x8_FASTEST MACRO aH,aL, b, r3,r2,r1
  
  	CLRF	r3
  	CLRF	r1
  	CLRC
  	MOVFW	b	; comment out if 8bit multiplicand already in W
  			;  also, b can be removed from macro arguments
  	mmac	aL,0, r3,r1
  	mmac	aL,1, r3,r1
  	mmac	aL,2, r3,r1
  	mmac	aL,3, r3,r1
  	mmac	aL,4, r3,r1
  	mmac	aL,5, r3,r1
  	mmac	aL,6, r3,r1
  	mmac	aL,7, r3,r1
  	
  	CLRF	r2
  	; carry already clear from last RRF of mmac above
  	; 8bit multiplicand still in W
  	mmac	aH,0, r3,r2
  	mmac	aH,1, r3,r2
  	mmac	aH,2, r3,r2
  	mmac	aH,3, r3,r2
  	mmac	aH,4, r3,r2
  	mmac	aH,5, r3,r2
  	mmac	aH,6, r3,r2
  	mmac	aH,7, r3,r2
  
  	ENDM
        
  

Questions:

• dmcewa15 at TakeThisOuTcaledonian.ac.uk asks:

  That looks like pretty nice code!
  Surely it is a signed-unsigned multiply though? Where the a is signed and b is unsigned. It can be thought of as doing a 16*16 where the high byte of b is always 0.
  Is this correct or am I not thinking properly?
  Dave

  Martin Sturm replies: Thanks, I haven't though it through in great detail but my first thoughts are no, it won't work for either signed a or b.+

  +

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