PIC18F258 Datasheet, PDF (78/384 Pages) Microchip Technology – High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN

PIC18FXX8
Example 7-3 shows the sequence to do a 16 x 16
unsigned multiply. 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
RES3:RES0 = ARG1H:ARG1L • ARG2H:ARG2L
= (ARG1H • ARG2H • 2¹⁶)+
(ARG1H • ARG2L • 2⁸)+
(ARG1L • ARG2H • 2⁸)+
(ARG1L • ARG2L)
EXAMPLE 7-3:
16 x 16 UNSIGNED
MULTIPLY ROUTINE
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
; Add cross
MOVF PRODH, W
; products
ADDWFC RES2
;
CLRF WREG
;
ADDWFC RES3
;
;
MOVF ARG1H, W
;
MULWF ARG2L
; ARG1H * ARG2L ->
; PRODH:PRODL
MOVF PRODL, W
;
ADDWF RES1
; Add cross
MOVF PRODH, W
; products
ADDWFC RES2
;
CLRF WREG
;
ADDWFC RES3
;
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 argu-
ments, each argument pairs Most Significant bit (MSb)
is tested and the appropriate subtractions are done.
DS41159B-page 76
EQUATION 7-2: 16 x 16 SIGNED
MULTIPLICATION
ALGORITHM
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)
EXAMPLE 7-4:
16 x 16 SIGNED
MULTIPLY ROUTINE
MOVF
MULWF
MOVFF
MOVFF
;
MOVF
MULWF
MOVFF
MOVFF
;
MOVF
MULWF
MOVF
ADDWF
MOVF
ADDWFC
CLRF
ADDWFC
;
MOVF
MULWF
MOVF
ADDWF
MOVF
ADDWFC
CLRF
ADDWFC
;
BTFSS
BRA
MOVF
SUBWF
MOVF
SUBWFB
;
SIGN_ARG1
BTFSS
BRA
MOVF
SUBWF
MOVF
SUBWFB
;
CONT_CODE
:
ARG1L, W
ARG2L
PRODH, RES1
PRODL, RES0
ARG1H, W
ARG2H
PRODH, RES3
PRODL, RES2
ARG1L, W
ARG2H
PRODL, W
RES1
PRODH, W
RES2
WREG
RES3
ARG1H, W
ARG2L
PRODL, W
RES1
PRODH, W
RES2
WREG
RES3
ARG2H, 7
SIGN_ARG1
ARG1L, W
RES2
ARG1H, W
RES3
ARG1H, 7
CONT_CODE
ARG2L, W
RES2
ARG2H, W
RES3
; ARG1L * ARG2L ->
; PRODH:PRODL
;
;
; ARG1H * ARG2H ->
; PRODH:PRODL
;
;
; ARG1L * ARG2H ->
; PRODH:PRODL
;
; Add cross
; products
;
;
;
;
; ARG1H * ARG2L ->
; PRODH:PRODL
;
; Add cross
; products
;
;
;
; ARG2H:ARG2L neg?
; no, check ARG1
;
;
;
; ARG1H:ARG1L neg?
; no, done
;
;
;
 2002 Microchip Technology Inc.