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CS 456 - Systems Programming
Spring 2024
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Displaying ./code/catasm/functions.asm
;------------------------------------------
; int atoi(Integer number)
; Ascii to integer function (atoi)
atoi:
push ebx ; preserve ebx on the stack to be restored after function runs
push ecx ; preserve ecx on the stack to be restored after function runs
push edx ; preserve edx on the stack to be restored after function runs
push esi ; preserve esi on the stack to be restored after function runs
mov esi, eax ; move pointer in eax into esi (our number to convert)
mov eax, 0 ; initialise eax with decimal value 0
mov ecx, 0 ; initialise ecx with decimal value 0
.multiplyLoop:
xor ebx, ebx ; resets both lower and uppper bytes of ebx to be 0
mov bl, [esi+ecx] ; move a single byte into ebx register's lower half
cmp bl, 48 ; compare ebx register's lower half value against ascii value 48 (char value 0)
jl .finished ; jump if less than to label finished
cmp bl, 57 ; compare ebx register's lower half value against ascii value 57 (char value 9)
jg .finished ; jump if greater than to label finished
sub bl, 48 ; convert ebx register's lower half to decimal representation of ascii value
add eax, ebx ; add ebx to our integer value in eax
mov ebx, 10 ; move decimal value 10 into ebx
mul ebx ; multiply eax by ebx to get place value
inc ecx ; increment ecx (our counter register)
jmp .multiplyLoop ; continue multiply loop
.finished:
cmp ecx, 0 ; compare ecx register's value against decimal 0 (our counter register)
je .restore ; jump if equal to 0 (no integer arguments were passed to atoi)
mov ebx, 10 ; move decimal value 10 into ebx
div ebx ; divide eax by value in ebx (in this case 10)
.restore:
pop esi ; restore esi from the value we pushed onto the stack at the start
pop edx ; restore edx from the value we pushed onto the stack at the start
pop ecx ; restore ecx from the value we pushed onto the stack at the start
pop ebx ; restore ebx from the value we pushed onto the stack at the start
ret
;------------------------------------------
; void iprint(Integer number)
; Integer printing function (itoa)
iprint:
push eax ; preserve eax on the stack to be restored after function runs
push ecx ; preserve ecx on the stack to be restored after function runs
push edx ; preserve edx on the stack to be restored after function runs
push esi ; preserve esi on the stack to be restored after function runs
mov ecx, 0 ; counter of how many bytes we need to print in the end
divideLoop:
inc ecx ; count each byte to print - number of characters
mov edx, 0 ; empty edx
mov esi, 10 ; mov 10 into esi
idiv esi ; divide eax by esi
add edx, 48 ; convert edx to it's ascii representation - edx holds the remainder after a divide instruction
push edx ; push edx (string representation of an intger) onto the stack
cmp eax, 0 ; can the integer be divided anymore?
jnz divideLoop ; jump if not zero to the label divideLoop
printLoop:
dec ecx ; count down each byte that we put on the stack
mov eax, esp ; mov the stack pointer into eax for printing
call sprint ; call our string print function
pop eax ; remove last character from the stack to move esp forward
cmp ecx, 0 ; have we printed all bytes we pushed onto the stack?
jnz printLoop ; jump is not zero to the label printLoop
pop esi ; restore esi from the value we pushed onto the stack at the start
pop edx ; restore edx from the value we pushed onto the stack at the start
pop ecx ; restore ecx from the value we pushed onto the stack at the start
pop eax ; restore eax from the value we pushed onto the stack at the start
ret
;------------------------------------------
; void iprintLF(Integer number)
; Integer printing function with linefeed (itoa)
iprintLF:
call iprint ; call our integer printing function
push eax ; push eax onto the stack to preserve it while we use the eax register in this function
mov eax, 0Ah ; move 0Ah into eax - 0Ah is the ascii character for a linefeed
push eax ; push the linefeed onto the stack so we can get the address
mov eax, esp ; move the address of the current stack pointer into eax for sprint
call sprint ; call our sprint function
pop eax ; remove our linefeed character from the stack
pop eax ; restore the original value of eax before our function was called
ret
;------------------------------------------
; int slen(String message)
; String length calculation function
slen:
push ebx
mov ebx, eax
nextchar:
cmp byte [eax], 0
jz finished
inc eax
jmp nextchar
finished:
sub eax, ebx
pop ebx
ret
;------------------------------------------
; void sprint(String message)
; String printing function
sprint:
push edx
push ecx
push ebx
push eax
call slen
mov edx, eax
pop eax
mov ecx, eax
mov ebx, 1
mov eax, 4
int 80h
pop ebx
pop ecx
pop edx
ret
;------------------------------------------
; void sprintLF(String message)
; String printing with line feed function
sprintLF:
call sprint
push eax ; push eax onto the stack to preserve it while we use the eax register in this function
mov eax, 0Ah ; move 0Ah into eax - 0Ah is the ascii character for a linefeed
push eax ; push the linefeed onto the stack so we can get the address
mov eax, esp ; move the address of the current stack pointer into eax for sprint
call sprint ; call our sprint function
pop eax ; remove our linefeed character from the stack
pop eax ; restore the original value of eax before our function was called
ret ; return to our program
;------------------------------------------
; void exit()
; Exit program and restore resources
quit:
mov ebx, 0
mov eax, 1
int 80h
ret
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