PIC18LF24K Datasheet, PDF (148/594 Pages) –

PIC18(L)F26/45/46K40
Example 12-3 shows the sequence to do a 16 x 16
unsigned multiplication. Equation 12-1 shows the
algorithm that is used. The 32-bit result is stored in four
registers (RES<3:0>).
EQUATION 12-1: 16 x 16 UNSIGNED
MULTIPLICATION
ALGORITHM
RES3:RES0 = ARG1H:ARG1L  ARG2H:ARG2L
= (ARG1H  ARG2H  2¹⁶) +
(ARG1H  ARG2L  2⁸) +
(ARG1L  ARG2H  2⁸) +
(ARG1L  ARG2L)
EXAMPLE 12-3: 16 x 16 UNSIGNED
MULTIPLY ROUTINE
MOVF
MULWF
ARG1L, W
ARG2L
MOVFF
MOVFF
;
MOVF
MULWF
PRODH, RES1
PRODL, RES0
ARG1H, W
ARG2H
MOVFF
MOVFF
;
MOVF
MULWF
PRODH, RES3
PRODL, RES2
ARG1L, W
ARG2H
MOVF
PRODL, W
ADDWF RES1, F
MOVF
PRODH, W
ADDWFC RES2, F
CLRF
WREG
ADDWFC RES3, F
;
MOVF
ARG1H, W
MULWF ARG2L
MOVF
ADDWF
MOVF
ADDWFC
CLRF
ADDWFC
PRODL, W
RES1, F
PRODH, W
RES2, F
WREG
RES3, F
; ARG1L * ARG2L->
; PRODH:PRODL
;
;
; ARG1H * ARG2H->
; PRODH:PRODL
;
;
; ARG1L * ARG2H->
; PRODH:PRODL
;
; Add cross
; products
;
;
;
;
; ARG1H * ARG2L->
; PRODH:PRODL
;
; Add cross
; products
;
;
;
Example 12-4 shows the sequence to do a 16 x 16
signed multiply. Equation 12-2 shows the algorithm
used. The 32-bit result is stored in four registers
(RES<3:0>). To account for the sign bits of the argu-
ments, the MSb for each argument pair is tested and
the appropriate subtractions are done.
EQUATION 12-2: 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 12-4: 16 x 16 SIGNED
MULTIPLY ROUTINE
MOVF
MULWF
ARG1L, W
ARG2L
MOVFF PRODH, RES1
MOVFF PRODL, RES0
;
MOVF
ARG1H, W
MULWF ARG2H
MOVFF PRODH, RES3
MOVFF PRODL, RES2
;
MOVF
ARG1L, W
MULWF ARG2H
MOVF
PRODL, W
ADDWF RES1, F
MOVF
PRODH, W
ADDWFC RES2, F
CLRF
WREG
ADDWFC RES3, F
;
MOVF
ARG1H, W
MULWF ARG2L
MOVF
ADDWF
MOVF
ADDWFC
CLRF
ADDWFC
;
BTFSS
BRA
MOVF
SUBWF
MOVF
SUBWFB
;
SIGN_ARG1
BTFSS
BRA
MOVF
SUBWF
MOVF
SUBWFB
;
CONT_CODE
:
PRODL, W
RES1, F
PRODH, W
RES2, F
WREG
RES3, F
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
;
;
;
 2016 Microchip Technology Inc.
Preliminary
DS40001816C-page 148