PIC17C75X Datasheet, PDF (63/320 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers

Example 9-4 shows the sequence to do an 16 x 16
signed multiply. Equation 9-2 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.
EQUATION 9-2:
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)
PIC17C75X
EXAMPLE 9-4: 16 x 16 SIGNED MULTIPLY
ROUTINE
MOVFP
MULWF
MOVPF
MOVPF
;
MOVFP
MULWF
MOVPF
MOVPF
;
MOVFP
MULWF
MOVFP
ADDWF
MOVFP
ADDWFC
CLRF
ADDWFC
;
MOVFP
MULWF
MOVFP
ADDWF
MOVFP
ADDWFC
CLRF
ADDWFC
;
BTFSS
GOTO
MOVFP
SUBWF
MOVFP
SUBWFB
;
SIGN_ARG1
BTFSS
GOTO
MOVFP
SUBWF
MOVFP
SUBWFB
;
CONT_CODE
:
ARG1L, WREG
ARG2L
; ARG1L * ARG2L ->
; PRODH:PRODL
PRODH, RES1 ;
PRODL, RES0 ;
ARG1H, WREG
ARG2H
; ARG1H * ARG2H ->
; PRODH:PRODL
PRODH, RES3 ;
PRODL, RES2 ;
ARG1L, WREG
ARG2H
; ARG1L * ARG2H ->
; PRODH:PRODL
PRODL, WREG ;
RES1, F
; Add cross
PRODH, WREG ; products
RES2, F
;
WREG, F
;
RES3, F
;
ARG1H, WREG ;
ARG2L
; ARG1H * ARG2L ->
; PRODH:PRODL
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
© 1997 Microchip Technology Inc.
Preliminary
DS30264A-page 63