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Add skeleton content for lesson 16.
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‎docs/index.html‎

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<li><a href="#lesson13">Lesson 13 <span>Calculator - subtraction</span></a></li>
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<li><a href="#lesson14">Lesson 14 <span>Calculator - multiplication</span></a></li>
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<li><a href="#lesson15">Lesson 15 <span>Calculator - division</span></a></li>
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<li><a href="#lesson16">Lesson 16 <span>Calculator (atoi)</span></a></li>
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</ul>
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</div>
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</div>
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</div>
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</div>
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<div class="row-fluid">
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<div class="span4">
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<h2>Lesson 16</h2>
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<h5>Calculator (atoi)</h5>
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<p>This program takes a series of passed string arguments, converts them to integers and adds them all together.</p>
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<p><a class="btn" href="#lesson16">View lesson &raquo;</a></p>
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</div>
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</div>
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<hr>
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<div class="row-fluid" id="lesson1">
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</article>
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</div>
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<hr>
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<div class="row-fluid" id="lesson16">
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<article class="span12">
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<header>
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<h2>Lesson 16</h2>
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<h4>Calculator (atoi)</h4>
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</header>
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<div class="snippet">
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<span class="filename">functions.asm</span>
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<pre class="brush: asm;">
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;------------------------------------------
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; int atoi(Integer number)
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; Ascii to integer function (atoi)
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atoi:
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push ebx ; preserve ebx on the stack to be restored after function runs
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push ecx ; preserve ecx on the stack to be restored after function runs
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push edx ; preserve edx on the stack to be restored after function runs
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push esi ; preserve esi on the stack to be restored after function runs
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mov esi, eax ; move pointer in eax into esi (our number to convert)
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mov eax, 0 ; initialise eax with decimal value 0
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mov ecx, 0 ; initialise ecx with decimal value 0
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.multiplyLoop:
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xor ebx, ebx ; resets both lower and uppper bytes of ebx to be 0
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mov bl, [esi+ecx] ; move a single byte into ebx register's lower half
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cmp bl, 48 ; compare ebx register's lower half value against ascii value 48 (char value 0)
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jl .finished ; jump if less than to label finished
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cmp bl, 57 ; compare ebx register's lower half value against ascii value 57 (char value 9)
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jg .finished ; jump if greater than to label finished
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cmp bl, 10 ; compare ebx register's lower half value against ascii value 10 (linefeed character)
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je .finished ; jump if equal to label finished
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cmp bl, 0 ; compare ebx register's lower half value against decimal value 0 (end of string)
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jz .finished ; jump if zero to label finished
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sub bl, 48 ; convert ebx register's lower half to decimal representation of ascii value
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add eax, ebx ; add ebx to our interger value in eax
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mov ebx, 10 ; move decimal value 10 into ebx
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mul ebx ; multiply eax by ebx to get place value
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inc ecx ; increment ecx (our counter register)
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jmp .multiplyLoop ; continue multiply loop
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.finished:
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mov ebx, 10 ; move decimal value 10 into ebx
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div ebx ; divide eax by value in ebx (in this case 10)
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pop esi ; restore esi from the value we pushed onto the stack at the start
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pop edx ; restore edx from the value we pushed onto the stack at the start
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pop ecx ; restore ecx from the value we pushed onto the stack at the start
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pop ebx ; restore ebx from the value we pushed onto the stack at the start
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ret
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</pre>
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</div>
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<div class="snippet">
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<span class="filename">calculator-atoi.asm</span>
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<pre class="brush: asm;">
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; Calculator (ATOI)
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; Compile with: nasm -f elf calculator-atoi.asm
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; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 calculator-atoi.o -o calculator-atoi
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; Run with: ./calculator-atoi 20 1000 317
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%include 'functions.asm'
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SECTION .text
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global _start
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_start:
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pop ecx ; first value on the stack is the number of arguments
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mov edx, 0 ; initialise our data register to store additions
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nextArg:
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cmp ecx, 0h ; check to see if we have any arguments left
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jz noMoreArgs ; if zero flag is set jump to noMoreArgs label (jumping over the end of the loop)
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pop eax ; pop the next argument off the stack
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call atoi ; convert our ascii string to decimal integer
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add edx, eax ; perform our addition logic
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dec ecx ; decrease ecx (number of arguments left) by 1
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jmp nextArg ; jump to nextArg label
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noMoreArgs:
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mov eax, edx ; move our data result into eax for printing
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call iprintLF ; call our integer printing with linefeed function
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call quit ; call our quit function
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</pre>
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<div class="output">
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<div class="inner">
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<span>~$ nasm -f elf calculator-atoi.asm</span>
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<span>~$ ld -m elf_i386 calculator-atoi.o -o calculator-atoi</span>
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<span>~$ ./calculator-atoi</span>
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<span>1337</span>
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</div>
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</div>
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</div>
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</article>
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</div>
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