LIST P = 16C54, n = 66
;
;*******************************************************************
; 8x8 Software Multiplier
; ( Fast Version : Straight Line Code )
;*******************************************************************
;
; The 16 bit result is stored in 2 bytes
;
; Before calling the subroutine " mpy ", the multiplier should
; be loaded in location " mulplr ", and the multiplicand in
; " mulcnd " . The 16 bit result is stored in locations
; H_byte & L_byte.
;
; Performance :
; Program Memory : 35 locations
; # of cycles : 37
; Scratch RAM : 0 locations
;
;
; Program: MULT8x8F.ASM
; Revision Date:
; 1-13-97 Compatibility with MPASMWIN 1.40
;
; This routine is optimized for speed efficiency ( straight line code )
; For code efficiency, refer to "mult8x8S.asm" ( looped code )
;*******************************************************************
;
mulcnd equ 09 ; 8 bit multiplicand
mulplr equ 10 ; 8 bit multiplier
H_byte equ 12 ; High byte of the 16 bit result
L_byte equ 13 ; Low byte of the 16 bit result
;
;
include "p16c5x.inc"
Same equ 1
;
;**** Define a macro for adding & right shifting **
;
mult MACRO bit ; Begin macro
btfsc mulplr,bit
addwf H_byte,Same
rrf H_byte,Same
rrf L_byte,Same
ENDM ; End of macro
;
; ***************************** Begin Multiplier Routine
mpy_F clrf H_byte
clrf L_byte
movf mulcnd,W ; move the multiplicand to W reg.
bcf STATUS,C ; Clear the carry bit in the status Reg.
mult 0
mult 1
mult 2
mult 3
mult 4
mult 5
mult 6
mult 7
;
retlw 0
;
;********************************************************************
; Test Program
;*********************************************************************
main movlw 0FF
movwf mulplr ; multiplier (in mulplr) = 0FF
movlw 0FF
movwf mulcnd ; multiplicand(in mulcnd ) = 0FF
;
call mpy_F ; The result 0FF*0FF = FE01 is in locations
; ; H_byte & L_byte
;
self goto self
;
org 01FF
goto main
;
END
See:
Questions:
/techref/microchip/math/mul/8x8u.htm This is a wonderful little routine. So wonderful I can't figure out how it works. Is there something I am missing, or something about the PIC that I am not familiar with? Here is my question:
How is the overflow from the rrf of H_byte put into the top of L_byte? It seems like it just magically appears. Yes, it works, I've seen it. But how?
Thanks, Allasso
Comments:
Answer to my own question:
I did not understand how rotate instructions work. I see now that whatever is sitting in the carry will be brought into whatever file is being rotated, regardless of where the carry came from. So, if there is a carry generated from a rotate just prior to the next, the next rotate will bring that carry in.
Now I understand what "rotate through carry" means.
Allasso
Stefan Eder Says: