PIC18F2X1X Datasheet, PDF (82/380 Pages) Microchip Technology – 28/40/44-PIN FLASH MICROCONTROLLERS WITH 10-BIT A/D AND NANO WATT TECHNOLOGY

PIC18F2X1X/4X1X
Example 7-3 shows the sequence to do a 16 x 16
unsigned multiplication. 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
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
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 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
arguments, the MSb for each argument pair is tested
and the appropriate subtractions are done.
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 ARG1L, W
MULWF 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
;
;
;
DS39636A-page 80
Preliminary
 2004 Microchip Technology Inc.