; Mult8.asm multiplies two 8-bit-numbers
; to yield a 16-bit-result
;
.NOLIST
.INCLUDE "8515def.inc"
.LIST
;
; Flow of multiplication
;
; 1.The binary to be multiplicated with, is shifted bitwise
; into the carry bit. If it is a one, the binary number
; is added to the result, if it is not a one that was
; shifted out, the number is not added
; 2.The binary number is multiplied by 2 by rotating it
; one position left, shifting a 0 into the void position.
; 3.If the binary to be multiplied with, is not zero, the
; multiplication loop is repeated. If it is zero, the
; multiplication is done.
;
; Used registers
;
.DEF rm1=R0 ; Binary number to be multiplicated (8 Bit)
.DEF rmh=R1 ; Interim storage
.DEF rm2=R2 ; Binary number to be multiplicated with (8 Bit)
.DEF rel=R3 ; Result, LSB (16 Bit)
.DEF reh=R4 ; Result, MSB
.DEF rmp=R16 ; Multi purpose register for loading
;
.CSEG
.ORG 0000
rjmp START
START:
ldi rmp,0xAA ; example binary 1010.1010
mov rm1,rmp ; to the first binary register
ldi rmp,0x55 ; example binary 0101.0101
mov rm2,rmp ; to the second binary register
; Here we start with the multiplication of the two binaries
; in rm1 und rm2, the result will go to reh:rel (16 Bit)
MULT8:
; Clear start values
clr rmh ; clear interim storage
clr rel ; clear result registers
clr reh
; Here we start with the multiplication loop
MULT8a:
; Step 1: Rotate lowest bit of binary number 2 to the carry
; flag (divide by 2, rotate a zero into bit 7)
clc ; clear carry bit
ror rm2 ; bit 0 to carry, bit 1 to 7 one position to
; the right, carry bit to bit 7
; Step 2: Branch depending if a 0 or 1 has been rotated to
; the carry bit
brcc MULT8b ; jump over adding, if carry has a 0
; Step 3: Add 16 bits in rmh:rml to the result, with overflow
; from LSB to MSB
add rel,rm1 ; add LSB of rm1 to the result
adc reh,rmh ; add carry and MSB of rm1
MULT8b:
; Step 4: Multiply rmh:rm1 by 2 (16 bits, shift left)
clc ; clear carry bit
rol rm1 ; rotate LSB left (multiply by 2)
rol rmh ; rotate carry into MSB and MSB one left
; Step 5: Check if there are still one's in binary 2, if
; yes, go on multiplicating
tst rm2 ; all bits zero?
brne MULT8a ; if not, go on in the loop
; End of the multiplication, result in reh:rel
; Endless loop
LOOP:
rjmp loop

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