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16bit X 16bit Unsigned
Author: Jim Bixby
This code was originally located @ http://www.piclist.com
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;**********************************************************************************************
; PIC16 fast unsigned multiply routines
; Jim Bixby
; bix@san.rr.com
;**********************************************************************************************
list r=dec,x=on,t=off,p=16F877
include p16F877.INC
; STATUS bit definitions
#define _C STATUS,0
#define _Z STATUS,2
cblock 0x20 ;Register definitions
TEMP0, TEMP1, TEMP2, TEMP3 ;convention: '0' is most sig byte
AARG0, AARG1, AARG2, AARG3
BARG0, BARG1
endc
org 0x0000
nop
goto MAIN
; a short test routine
MAIN
movlw 0x45 ;init a couple of arguments
movwf AARG0
movlw 0x30
movwf AARG1
movlw 0xC2
movwf BARG0
movlw 0x51
movwf BARG1
call MUL16x16U ;call the multiply
goto MAIN
;*****************************************************************************************
; Unsigned multiply routine
; Multiplies of any size can easily be made from the macros - for this code, only MUL16X16
; is made
;
; The approach here is linear code for speed at the expense of RAM and registers
; Adapted from : http://www.piclist.org/techref/microchip/math/mul/8x8u.htm
;*****************************************************************************************
; Add16AB macro - add AH:AL to BH:BL, result goes to BH:BL
Add16AB MACRO AH,AL,BH,BL
movfw AL
addwf BL,f
movfw AH
btfsc _C
incfsz AH,w
addwf BH,f
ENDM
;*****************************************************************************************
; Macro for adding & right shifting - used once per bit by mulmac
mult MACRO bit,A,H,L ;A=multiplier,H:L=result, other arg in W
btfsc A,bit
addwf H,f
rrf H,f
rrf L,f
ENDM ; End of macro
;*****************************************************************************************
; 8x8-->16 multiply macro A * B --> H:L
; Invokes mult macro above
mulmac MACRO A,B,H,L ; H:L = A*B
clrf H
clrf L
movf B,W ; move the multiplicand to W reg.
bcf _C ; Clear the carry bit in the status Reg.
mult 0,A,H,L
mult 1,A,H,L
mult 2,A,H,L
mult 3,A,H,L
mult 4,A,H,L
mult 5,A,H,L
mult 6,A,H,L
mult 7,A,H,L
ENDM
;
NOEXPAND
;*****************************************************************************************
; MUL16x16U
; AARG0:1 * BARG0:1 --> AARG0:3
; Invokes the three macros above
; 164 prog words, 168 inst cycles, 10 registers (6 for call/return arguments + 4)
; Uses TEMP0:3 also. BARG0:1 unchanged on return
;*****************************************************************************************
MUL16x16U
; 32 bit result is first calculated into T0:T1:A2:A3
; When done, T0:1 is moved to A0:1 so the final result is in A0:3
; Register names can be changed at will, and any register can be located
; anywhere - order is not important.
mulmac ARG0, BARG0, TEMP0, TEMP1 ;T0:1 <--A0*B0
mulmac AARG1, BARG1, AARG2, AARG3 ;A2:3 <--A1*B1
mulmac AARG1, BARG0, TEMP2, TEMP3 ;T2:3 <--A1*B0
Add16AB TEMP2, TEMP3, TEMP1, AARG2 ; Add T2:3 to T1:A2, carry into T0
btfsc _C ; nb: this relies on the _C bit being
incf TEMP0,f ; correct after the Add16AB macro
mulmac AARG0, BARG1, TEMP2, TEMP3 ;T2:3 <--A0*B1
Add16AB TEMP2, TEMP3, TEMP1, AARG2 ; Add T2:3 to T1:A2, carry into T0
btfsc _C
incf TEMP0,f
movfw TEMP1 ;Move T0:1 to A0:1
movwf AARG1 ;to finish
movfw TEMP0
movwf AARG0
retlw 0
EXPAND
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