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Introduction to "if" and "while" statements

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• The control structures in a high level programming languae
  • Control structures in high level programming language can change the program (execution) flow

    For a discussion on program flow, refer to this webpage: click here

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  • The control structures in high level programming language are:

    The if-statement The if-else-statement The while-statement The for-statement The do-while-statement

    The control constructs "if", "if-else", ("switch") "while-loop", "for-loop" and "do-while-loop" is handled in assembler code by (only) 2 kinds of assembler instructions:

    Compare instruction: compares 2 values and set the status flags (N, Z, V, C) according to the compared result Branch instruction(s): makes the CPU "jump" to a certain memory location when a ceratin condition is satisfied. And if that condition is not satisfied, the CPU continues execution with the next instruction (the one after the branch instruction).

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    So you may wonder how can just 2 assember instructions ((1) compare and (2) conditional/unconditional branch) be enough to implement "if", "if-else", ("switch") "while-loop", "for-loop" and "do-while-loop" etc.

    The answer is: these high level programming language constructs ("if", "if-else", ("switch") "while-loop", "for-loop" and "do-while-loop") only changes the flow of program control in a computer program.

    Remember that the flow of program control is the order in which the program instructions are executed (See: click here)

    The mechanism to change the flow of program control is very simple

    By using the same mechanism, we can change the program execution order in any way that we want.

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  • So in the next few webpages, I will:

    First explain the mechanism used in the CPU to change the program flow How to use this mechanism to construct the program flow that reflects the execution of if/if-else statements How to use this mechanism to construct the program flow that reflects the execution of while/for statements

    I have to stress that the highlight of the discussions of the "if", "if-else", ("switch") "while-loop", "for-loop" and "do-while-loop" statement is:

    How to construct the program flow that reflects the statement

    We will use flow charts to help us visualize the program flow corresponding to each type of high level language statements

    After constructing the flow chart of the statement, we will write assembler instructions to implement the program flow.