PIC17C4X Datasheet, PDF (51/240 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM/ROM Microcontroller

Example 8-4 shows the sequence to do an 16 x 16
signed multiply. Equation 8-2 shows the algorithm that
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.
EQUATION 8-2:
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)
PIC17C4X
EXAMPLE 8-4: 16 x 16 SIGNED MULTIPLY
ROUTINE
MOVFP ARG1L, WREG
MULWF ARG2L
; ARG1L * ARG2L ->
; PRODH:PRODL
MOVPF PRODH, RES1 ;
MOVPF PRODL, RES0 ;
;
MOVFP ARG1H, WREG
MULWF ARG2H
; ARG1H * ARG2H ->
; PRODH:PRODL
MOVPF PRODH, RES3 ;
MOVPF PRODL, RES2 ;
;
MOVFP ARG1L, WREG
MULWF ARG2H
; ARG1L * ARG2H ->
; PRODH:PRODL
MOVFP PRODL, WREG ;
ADDWF RES1, F
; Add cross
MOVFP PRODH, WREG ; products
ADDWFC RES2, F
;
CLRF
WREG, F
;
ADDWFC RES3, F
;
;
MOVFP ARG1H, WREG ;
MULWF ARG2L
; ARG1H * ARG2L ->
; PRODH:PRODL
MOVFP
ADDWF
MOVFP
ADDWFC
CLRF
ADDWFC
;
BTFSS
GOTO
MOVFP
SUBWF
MOVFP
SUBWFB
;
SIGN_ARG1
BTFSS
GOTO
MOVFP
SUBWF
MOVFP
SUBWFB
;
CONT_CODE
:
PRODL, WREG ;
RES1, F
; Add cross
PRODH, WREG ; products
RES2, F
;
WREG, F
;
RES3, F
;
ARG2H, 7 ; ARG2H:ARG2L neg?
SIGN_ARG1 ; no, check ARG1
ARG1L, WREG ;
RES2
;
ARG1H, WREG ;
RES3
ARG1H, 7 ; ARG1H:ARG1L neg?
CONT_CODE ; no, done
ARG2L, WREG ;
RES2
;
ARG2H, WREG ;
RES3
© 1996 Microchip Technology Inc.
DS30412C-page 51