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  1. PO
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    <blockquote> <p>how can a computer understand it is executing an "if" or a "while" statement?</p> </blockquote> <p>The short answer is that the computer does not understand if or while.</p> <p>The computer executes one simple instruction after another. Each instruction is located at a particular address. There are special "branch" (or "jump") instructions to jump forwards and backwards in the instruction list. Some of these jump instructions are conditional on the value of certain bits in the CPU. For example there is usually a bit that indicates whether the previous calculation was zero. There are also instructions to set these bits, and special jump instructions that only jump <em>conditionally</em> based on the value of these bits.</p> <p>Consider this code:</p> <pre><code>if( a == 0 ){ b = 1; }else{ b = 2; } </code></pre> <p>This might get compiled to machine code something like the following made-up assembly code (and I'm simplifying this to explain how an if statement might be compiled):</p> <pre><code>;;(address instruction params ;; comments) 10 TEST a ;; set the "ZERO" bit based on whether a is zero 20 JNZ 50 ;; if ZERO bit is *not* set, jump to 50, otherwise fall through to 30 30 STORE b, 1 ;; store 1 in b 40 JUMP 60 ;; jump over else branch 50 STORE b, 2 ;; store 2 in b 60 </code></pre> <p>So the machine knows how to test values and set bits in the <a href="http://en.wikipedia.org/wiki/Status_register" rel="nofollow">status register</a> (that's where the zero bit is stored). And it knows how to conditionally jump forwards and backwards in the instruction stream (you can probably guess how a while loop is implemented: you make a test and jump <em>backwards</em> in the instruction list to make a loop). I guess you could say that the machine has a kind of "if" built in, but it is very simple: it can only jump forwards or backwards.</p> <p>You might find it informative to study the <a href="http://en.wikipedia.org/wiki/Instruction_set" rel="nofollow">instruction set</a> of a simple CPU such as the <a href="http://en.wikipedia.org/wiki/Atmel_AVR_instruction_set" rel="nofollow">Atmel AVR</a> to get an idea about what a CPU can natively do.</p> <blockquote> <p>And how did the early programmer's know what to type, using only binary?</p> </blockquote> <p>I guess they had a big list of all the instructions that the processor knew how to interpret. Or maybe an <a href="http://www.atmel.com/images/doc0856.pdf" rel="nofollow">instruction set manual</a>.</p> <p>By the way, early programmers didn't type, they entered code on <a href="http://en.wikipedia.org/wiki/Punched_card" rel="nofollow">punch cards</a>, or by flipping banks of switches. </p>
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    1. POHow can a computer create "if" statements using only ones and zeros
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    1. USRoss Bencina
    1. USRoss Bencina
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    1. POHow can a computer create "if" statements using only ones and zeros
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    1. COThanks a lot for those links, they are really helpful! Was the language you wrote in assembly?
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      1. USAnton D
    2. CO@AntonD no problems. The language I wrote in was a kind of made up assembly. There will usually be some kind of TEST, JNZ, STORE, and JUMP instructions. They might be named differently though. And I used the variables a and b, which you can sometimes do but this is cheating: not all assemblers (or CPUs) make dealing with variables so easy. I did that to simplify the explanation.
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      1. USRoss Bencina
    3. COComputers which ran electronically-stored programs were pretty advanced. Computers before that were often programmed using wires. A that could run a 32-step program using 8 registers and had 8 operations it could perform might have four attachment points for each allowable program step, three points for each register (fetch op1, fetch op2, and store), and a socket for each allowable operation. If step 3 was supposed to add register 3 to register 5 and store the result to register 6, wires would connect the "step 3" attachment points to "register 3 fetch1", "register 5 fetch2", ...
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      1. USsupercat
 

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