Computer science

Custom subroutine example #$4$

section $.$bss

temp resb $1$ ; used by atoi$\_2$

res resb $2$; used by itoa$\_2$

val resb $1$

section $.$text

global $\_$start

$\_$start:

mov eax, $\text{'}28\text{'}$ ; place input into eax register

call atoi$\_2$; call the subroutine

mov $[$val$],$ eax$$; place return value into "val"

$$

mov eax, $[$val$]$

call print$\_$itoa$2$

$$

call exit

atoi$\_2$:

xor ecx, ecx$$ ; zero out reg.

mov ecx, eax $$; Transfer to a register not being used

xor eax, eax$$ ; Zero out the eax reg.

mov al$,$ cl$$ ; place $1$st character into $8-$bit register

sub al$,\text{'}0\text{'}$ ; remove ASCII offset to reveal integer digit

mov bx$,10$; place $10$ into bx for future multiplication

mul bx$$ ; ax $=$ ax $*$ bx $($recall that al is lowest $8-$bits of ax so they hold the same value in this case$)$

mov $[$temp$],$ ax$$ ; store total $($so far$)$ into temp

xor eax, eax$$; zero out reg.

mov al$,$ ch$$ ; place $2$nd character into $8-$bit register

sub al$,\text{'}0\text{'}$; remove ASCII offset again...

add eax, $[$temp$]$; add the temp. reg. value

ret$$ ; return to caller

print$\_$itoa$2$:

mov bx$,10$

div bx$$

add ax$,\text{'}0\text{'}$

mov $[$res$],$ byte ax

add dx$,\text{'}0\text{'}$

mov $[$res$+1],$ byte dx

xor eax, eax

xor edx, edx

mov ecx, res

mov edx, $2$

call print

ret

print:

mov eax, $4$

mov ebx, $1$

int $0$x$80$

ret

exit:

mov eax, $1$

mov ebx, $0$

int $0$x$80$