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The if-statement construct in assembler code

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• The if-statement
  • High level programming languages provides the if-statement construct that has the following form:

    if ( condition ) statement

    Comment:

    The body of the if-statement contains only one (1) statement If you need multiple statements in the body, you can use curly braces { ... } to enclose the statements The compiler will treat (= consider) this construct: { statement1; statement2; ... } as one statement !!!

  • Program control control of the if-statement given in a flow chart is as follows:

    Flow chart of the if-statement if ( condition ) ----> | statement | +--------------+ FALSE | condition |--------+ +--------------+ | | | | TRUE | | | V | statement(s) | | | | | V | +<---------------+ | V

    I.e.:

    If the condition is true: The statement is execute and The program continues with the next statement If the condition is false: The program continues with the next statement

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• Assembler code construct to realize the if-statement construct
  • The assembler code construct that realizes the program control flow of the if-statement is as follows:

    Assembler code to evaluate "condition" (use cmp instruction) | V FALSE Branch on the FALSE outcome of "condition" to label A: ----+ | | | don't branch on the TRUE outcome of condition | | | V | Assembler code to execute "statement" | | | | | V | A: +<------------------------------------------------+ | V

    I will illustrate the assembler coding technique with 2 examples in this webpage

    In class - due to time limitations - I will explain the example 1.

    Come to class if you want to see how I develop the code for this example

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• The if-statement - Example 1
  • Example 1: find the absolute value of the value in the variable x

    The high level programming language statement is:

    int x; if ( x < 0 ) x = -x;

    The flow chart of the program is:

    The ARM assembler code for the high level program is:

    main: movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r1, [r0] // r1 = x cmp r1, #0 // Compare x ? 0 bge IfEnd // Branch to "isPos" if x >= 0 neg r1, r1 // r1 = 0 - r1 (negate) movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) str r1, [r0] // Update x with -x // NOTE: if you OMIT this instruction, // the variable x will NOT be // updated !!! IfEnd: /* -------------------------------------------------- Begin of the permanent program variables -------------------------------------------------- */ .data x: .4byte -4 // x = -4 -- try changing the sign and run the prog .end

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/6-if/if1.s

    How to run the program:

    To compile:   as255 if1          To run: use EGTAPI

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• If-statement in assembler code - exmaple 2
  • Given 2 values in x and y

    Make sure that y has the largest value

    I.e.: swap the values in x and y if x > y

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  • The high level program is as follows:

    int x; int y; int help; if ( x > y ) { help = x; x = y; y = help; }

    The flow chart of the program is:

    Note:

    In a high level language, we must use a help variable to perform the swap operation Because registers can store variable values, we don't need an extra help variable to to perform the swap operation in assembler code !!!

    ARM assembler code for the above if-statement:

    main: movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r1, [r0] // r1 = x (I save r0 because I need it later !) movw r2, #:lower16:y movt r2, #:upper16:y // r2 = addr(y) ldr r3, [r2] // r3 = y (I save r2 because I need it later !) cmp r1, r3 // Compare x ? y ble ifEnd // Branch if x <= y to ifEnd // x > y: swap x and str r1, [r2] // Store the x-copy in y (in memory): y = x in r1 str r3, [r0] // Store the y-copy in x (in memory): x = y in r3 ifEnd: /* -------------------------------------------------- Begin of the permanent program variables -------------------------------------------------- */ .data x: .4byte 9 // x = 9 -- try a value smaller that 4 y: .4byte 4 // y = 4 .end

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/6-if/if2.s

    How to run the program:

    To compile:   as255 if2              To run: us EGTAPI