Common multi-byte operations in Assembly for the Atmel AVR instruction set

operations.asm

; Include constants for the ATMega324A micro-controller
; This file defines register names and names for I/O ports
.include "m324adef.inc"

; Attempt to perform operations on multi-byte values
; It is assumed the following values are unsigned
; First value = r15:r14 = 0xF8AD
; Second value = r17:r16 = 0xFFDE
START:
ldi r19, 0xF8
ldi r18, 0xAD
mov r15, r19 ; High byte
mov r14, r18 ; Low byte

ldi r17, 0xFF ; High byte
ldi r16, 0xDE ; Low byte

; Multi-byte Two's Complement of r15:r14
clr r18
ldi r19, 0x01
com r14
com r15
add r14, r19 ; Add 1 to lower byte
adc r15, r18 ; Add carry value to upper byte

; Multi-byte Addition
add r14, r16
adc r15, r17

; Set T Flag of status register if overflow occurred during addition
; BST Machine Code: 1111 101d dddd 0bbb
brvs STATUS
jmp CONTINUE
STATUS:
set ; Sets the T Flag of the status register bit
; bset 6 can also be used

; The T Flag can also be set using the BST instruction
ldi r18, 0x01
; bst uses T <- Rd(b), where b is a particular bit in Rd
bst r18, 1

CONTINUE:
; Multi-byte Subtraction
sub r14, r16
sbc r15, r17

; Multi-byte Multiplication (x2)
lsl r14
rol r15

; Multiplication of two 16-bit numbers
; 32-bit result is stored in r13:r12:r11:r10
clr r12
clr r13
clr r18

mul r14, r16
movw r11:r10, r1:r0

mul r16, r15
add r11, r0
adc r12, r1
adc r13, r18

mul r14, r17
add r11, r0
adc r12, r1
adc r13, r18

mul r15, r17
add r12, r0
adc r13, r1

; Multi-byte Division (/2)
lsr r15
ror r14