**Multiply 8 bits by 8 bits in software by
http://www.avr-asm-tutorial.net**

; 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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