From Newsgroup: alt.lang.asm

nasm

inp equ 64 ; input maximum size
section .text
global _start
getnum:
xor rax, rax
mov dl, [rdi]
mov sil, dl
sub sil, '-'
jnz .pos
.loop: inc rdi
.pos:
mov dl, [rdi]
sub dl, '0'
jc .end0
imul rax, 10
add rax, rdx
jmp .loop
.end0:
test sil, sil
jnz .end
neg rax
.end: ret
_start:
enter inp, 0
xor rax, rax
xor rdi, rdi
mov rsi, rbp
mov rdx, inp
syscall
mov rdi, rbp
call getnum
push rax
mov cl, [rdi]
inc rdi
call getnum
mov rbx, rax
pop rax
cmp cl, '+'
je .add
cmp cl, '-'
je .sub
xor rdx, rdx
cmp cl, '*'
je .mul
idiv rbx
cmovl rax, rdx ; intel, so eflags stay :)
jmp print
.mul: imul rbx
jmp print
.add: add rax, rbx
jmp print
.sub: sub rax, rbx
print:
mov byte [rbp + inp-1], 10
mov rcx, 1
lea rsi, [rbp + inp-2]
test rax, rax
sets bl
mov r9, rbx
jns .loop
neg rax
.loop:
; search: GranlundrCoMontgomeryrCOs algorithm
mov r8, rax
mov rdx, 0xCCCC_CCCC_CCCC_CCCD ; 1/5
mul rdx
mov rbx, rdx
shr rbx, 3

mov rax, r8
mov rdx, rbx
shl rdx, 3
lea rdx, [rdx + rbx * 2]
; shr rdx, 3
sub rax, rdx
mov rdx, rbx
xchg rax, rdx

add dl, '0'
mov [rsi], dl
dec rsi
inc rcx
test rax, rax
jnz .loop

mov rbx, r9
test bl, bl
jz .out
mov byte [rsi], '-'
inc rcx
dec rsi
.out:
inc rsi
mov rax, 1
mov rdi, 1
mov rdx, rcx
syscall
mov rax, 60
syscall
--
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