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;+=======================================================================+
;A simple assembly program that computes average of 3 two-digit numbers.
;Copyright (C) 2014 Ferriel Lisandro B. Melarpis
;
; This program is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or (at
; your option) any later version.
;
; This program is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
; General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program.  If not, see <http://www.gnu.org/licenses/>.
;+========================================================================+
section .data
    menu db 'Main Menu',0xa,\
    '[1] Addition',0xa,\
    '[2] Subtraction',0xa,\
    '[3] Multiplication',0xa,\
    '[4] Division',0xa,\
    '[5] Exit',0xa,\
    'Enter choice: '
    menulen equ $-menu
    ask db 'Enter number: '
    asklen equ $-ask
    nsign db '-'
    nslen equ $-nsign
    inv db 'Invalid Input!',0xa
    ilen equ $-inv
    n db 0xa
    nlen equ $-n
section .bss
    choice resb 1
    sign1 resb 1
    num1a resb 1
    num1b resb 1
    num1 resw 1
    sign2 resb 1
    num2a resb 1
    num2b resb 1
    num2 resw 1
    temp resw 1
    res1 resb 1
    res2 resb 1
    res3 resb 1
section .text
    global _start
_start:
_menu:
    ;print menu
    mov eax, 4
    mov ebx, 1
    mov ecx, menu
    mov edx, menulen
    int 0x80
    ;scan choice
    mov eax, 3
    xor ebx, ebx
    mov ecx, choice
    mov edx, 2
    int 0x80
    ;convert choice to int
    sub byte [choice], 0x30
    ;if choice >=5
    cmp byte [choice], 5
    ja _inv                                             ;if choice > 5 invalid
    je _exit                                            ;if choice = 5 exit
    ;ask for first number
    mov eax, 4
    mov ebx, 1
    mov ecx, ask
    mov edx, asklen
    int 0x80
    ;scan sign of first number
    xor ah, ah
    mov eax, 3
    xor ebx, ebx
    mov ecx, sign1
    mov edx, 1
    int 0x80
    ;scan tens digit
    xor ah, ah
    mov eax, 3
    xor ebx, ebx
    mov ecx, num1a
    mov edx, 1
    int 0x80
    ;scan ones digit
    xor ah, ah
    mov eax, 3
    xor ebx, ebx
    mov ecx, num1b
    mov edx, 2
    int 0x80
    ;ask for second number
    mov eax, 4
    mov ebx, 1
    mov ecx, ask
    mov edx, asklen
    int 0x80
    ;scan sign of second number
    xor ah, ah
    mov eax, 3
    xor ebx, ebx
    mov ecx, sign2
    mov edx, 1
    int 0x80
    ;scan tens digit
    xor ah, ah
    mov eax, 3
    xor ebx, ebx
    mov ecx, num2a
    mov edx, 1
    int 0x80
    ;scan ones digit
    xor ah, ah
    mov eax, 3
    xor ebx, ebx
    mov ecx, num2b
    mov edx, 2
    int 0x80
    ;convert first number's digits to int
    sub byte [num1a], 0x30
    sub byte [num1b], 0x30
    ;combine tens and ones digit
    mov al, [num1a]
    mov bl, 10
    mul bl
    add al, [num1b]
    mov [num1], al
    ;convert second number's digits to int
    sub byte [num2a], 0x30
    sub byte [num2b], 0x30
    ;combine tens and ones digit
    mov al, [num2a]
    mul bl
    add al, [num2b]
    mov [num2], al

    cmp byte [sign1], '-'
    jne _step1
    neg word [num1]
    _step1:
    cmp byte [sign2], '-'
    jne _step2
    neg word [num2]
    _step2:
    cmp byte [choice], 1
    je _add
    cmp byte [choice], 2
    je _sub
    cmp byte [choice], 3
    je _mul
    cmp byte [choice], 4
    je _div

_add:
    mov ax, [num1]
    add ax, [num2]
    jns _res
    jmp _neg

 _sub:
    jmp _menu

_mul:
    jmp _menu

_div:
    jmp _menu

_inv:
    mov eax, 4
    mov ebx, 1
    mov ecx, inv
    mov edx, ilen
    int 0x80

    jmp _menu

_neg:
    mov [temp], ax

    mov eax, 4
    mov ebx, 1
    mov ecx, nsign
    mov edx, nslen
    int 0x80

    neg word [temp]
    mov ax, [temp]

_res:
    mov bl, 10
    xor ah, ah
    div bl
    mov byte[res3], ah
    add byte[res3], 0x30
    mov [temp], al
    mov ax, [temp]
    xor ah, ah
    div bl
    mov byte[res2], ah
    add byte[res2], 0x30
    mov byte[res1], al
    add byte[res1], 0x30

    mov eax, 4
    mov ebx, 1
    mov ecx, res1
    mov edx, 1
    int 0x80

    mov eax, 4
    mov ebx, 1
    mov ecx, res2
    mov edx, 1
    int 0x80

    mov eax, 4
    mov ebx, 1
    mov ecx, res3
    mov edx, 1
    int 0x80

    mov eax, 4
    mov ebx, 1
    mov ecx, n
    mov edx, nlen
    int 0x80

    jmp _menu

_exit:
    mov eax, 1
    xor ebx, ebx
    int 0x80