PIC17C756A MICROCHIP [Microchip Technology], PIC17C756A Datasheet - Page 69

• Manufacturer Part Number: PIC17C756A
• Description: High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D
• Manufacturer: MICROCHIP [Microchip Technology]

Example 9-4 shows the sequence to do a 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:
RES3:RES0
2000 Microchip Technology Inc.
= ARG1H:ARG1L ARG2H:ARG2L
= (ARG1H ARG2H 2
(ARG1H ARG2L 2
(ARG1L ARG2H 2
(ARG1L ARG2L)
(-1 ARG2H<7> ARG1H:ARG1L 2
(-1 ARG1H<7> ARG2H:ARG2L 2
16 x 16 SIGNED
MULTIPLICATION
ALGORITHM
8
8
16
)
)
)
16
16
)
)
+
+
+
+
+
EXAMPLE 9-4:
;
;
;
;
;
SIGN_ARG1
;
CONT_CODE
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
BTFSS
GOTO
MOVFP
SUBWF
MOVFP
SUBWFB
:
ARG1L, WREG
ARG2L
PRODH, RES1 ;
PRODL, RES0 ;
ARG1H, WREG
ARG2H
PRODH, RES3 ;
PRODL, RES2 ;
ARG1L, WREG
ARG2H
PRODL, WREG ;
RES1, F
PRODH, WREG ;
RES2, F
WREG, F
RES3, F
ARG1H, WREG ;
ARG2L
PRODL, WREG ;
RES1, F
PRODH, WREG ;
RES2, F
WREG, F
RES3, F
ARG2H, 7
SIGN_ARG1
ARG1L, WREG ;
RES2
ARG1H, WREG ;
RES3
ARG1H, 7
CONT_CODE
ARG2L, WREG ;
RES2
ARG2H, WREG ;
RES3
16 x 16 SIGNED
MULTIPLY ROUTINE
PIC17C7XX
; ARG1L * ARG2L ->
;
; ARG1H * ARG2H ->
;
; ARG1L * ARG2H ->
;
; Add cross
;
;
;
; ARG1H * ARG2L ->
;
; Add cross
;
;
;
; ARG2H:ARG2L neg?
; no, check ARG1
;
; ARG1H:ARG1L neg?
; no, done
;
PRODH:PRODL
PRODH:PRODL
PRODH:PRODL
PRODH:PRODL
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DS30289B-page 69