PIC Microcontoller Math Method

Multiply 8 x 8 bits

 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:

• /techref/microchip/math/mul/23x8bbaj.htm microchip math mul 23x8bbaj+

Questions:

• allassoTakeThisOuT at SPAMbacavalley.com asks:

  /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:

• infoTakeThisOuT at SPAMse-es.de " This routine is from AN526 of the year 1997. The documents author was Amar Palacherla from Microchip." +
• allassoTakeThisOuT at SPAMbacavalley.com

  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: +

.