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PIC18F2450/4450
DS39760A-page 84
Advance Information
2006 Microchip Technology Inc.
Example 7-3 shows the sequence to do a 16 x 16
unsigned
multiplication.
Equation 7-1
shows
the
algorithm that is used. The 32-bit result is stored in four
registers (RES3:RES0).
EQUATION 7-1:
16 x 16 UNSIGNED
MULTIPLICATION
ALGORITHM
EXAMPLE 7-3:
16 x 16 UNSIGNED
MULTIPLY ROUTINE
Example 7-4 shows the sequence to do a 16 x 16
signed multiply. Equation 7-2 shows the algorithm
used. The 32-bit result is stored in four registers
(RES3:RES0). To account for the sign bits of the
arguments, the MSb for each argument pair is tested
and the appropriate subtractions are done.
EQUATION 7-2:
16 x 16 SIGNED
MULTIPLICATION
ALGORITHM
EXAMPLE 7-4:
16 x 16 SIGNED
MULTIPLY ROUTINE
RES3:RES0
=
ARG1H:ARG1L
ARG2H:ARG2L
=
(ARG1H
ARG2H 216) +
(ARG1H
ARG2L 28) +
(ARG1L
ARG2H 28) +
(ARG1L
ARG2L)
MOVF
ARG1L, W
MULWF
ARG2L
; ARG1L * ARG2L->
; PRODH:PRODL
MOVFF
PRODH, RES1
;
MOVFF
PRODL, RES0
;
MOVF
ARG1H, W
MULWF
ARG2H
; ARG1H * ARG2H->
; PRODH:PRODL
MOVFF
PRODH, RES3
;
MOVFF
PRODL, RES2
;
MOVF
ARG1L, W
MULWF
ARG2H
; ARG1L * ARG2H->
; PRODH:PRODL
MOVF
PRODL, W
;
ADDWF
RES1, F
; Add cross
MOVF
PRODH, W
; products
ADDWFC
RES2, F
;
CLRF
WREG
;
ADDWFC
RES3, F
;
MOVF
ARG1H, W
;
MULWF
ARG2L
; ARG1H * ARG2L->
; PRODH:PRODL
MOVF
PRODL, W
;
ADDWF
RES1, F
; Add cross
MOVF
PRODH, W
; products
ADDWFC
RES2, F
;
CLRF
WREG
;
ADDWFC
RES3, F
;
RES3:RES0= ARG1H:ARG1L
ARG2H:ARG2L
= (ARG1H
ARG2H 216) +
(ARG1H
ARG2L 28) +
(ARG1L
ARG2H 28) +
(ARG1L
ARG2L) +
(-1
ARG2H<7> ARG1H:ARG1L 216) +
(-1
ARG1H<7> ARG2H:ARG2L 216)
MOVF
ARG1L, W
MULWF
ARG2L
; ARG1L * ARG2L ->
; PRODH:PRODL
MOVFF
PRODH, RES1
;
MOVFF
PRODL, RES0
;
MOVF
ARG1H, W
MULWF
ARG2H
; ARG1H * ARG2H ->
; PRODH:PRODL
MOVFF
PRODH, RES3
;
MOVFF
PRODL, RES2
;
MOVF
ARG1L,W
MULWF
ARG2H
; ARG1L * ARG2H ->
; PRODH:PRODL
MOVF
PRODL, W
;
ADDWF
RES1, F
; Add cross
MOVF
PRODH, W
; products
ADDWFC
RES2, F
;
CLRF
WREG
;
ADDWFC
RES3, F
;
MOVF
ARG1H, W
;
MULWF
ARG2L
; ARG1H * ARG2L ->
; PRODH:PRODL
MOVF
PRODL, W
;
ADDWF
RES1, F
; Add cross
MOVF
PRODH, W
; products
ADDWFC
RES2, F
;
CLRF
WREG
;
ADDWFC
RES3, F
;
BTFSS
ARG2H, 7
; ARG2H:ARG2L neg?
BRA
SIGN_ARG1
; no, check ARG1
MOVF
ARG1L, W
;
SUBWF
RES2
;
MOVF
ARG1H, W
;
SUBWFB
RES3
;
SIGN_ARG1
BTFSS
ARG1H, 7
; ARG1H:ARG1L neg?
BRA
CONT_CODE
; no, done
MOVF
ARG2L, W
;
SUBWF
RES2
;
MOVF
ARG2H, W
;
SUBWFB
RES3
;
CONT_CODE
:
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