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The if-else statement (with a simple condition)

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• The if-else statement in a high level programming language
  • Syntax:

    if ( condition ) statement1; else statement2;

    Meaning:

    If condition is true, then program control is: Execute statement1 Continue with the statement following the if-else statement If condition is false, then program control is: Execute statement2 Continue with the statement following the if-else statement

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  • The program control given of the if-else statement as a flow chart is as follows:

    Flow chart of the if-else statement if ( condition ) ----> | statement1; V else +--------------+ FALSE statement2; | condition |--------+ +--------------+ | | | | TRUE | | | V | statement1 | | | +---------+ | | | | +<---------------+ | | | V | statement2 | | | | +-------->+ | | V

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• Realizing the if-else statement construct in assembler code
  • The assembler construct that realizes (= implements) the program control flow of the if-else statement is as follows:

    Evaluate "condition" (with cmp) FALSE Branch on the FALSE outcome of "condition" to label A: ----+ | | | (TRUE) | | | V | "statement1" assembler code | | Branch (always) to label B: --------+ | | | | | A: "statement2" assembler code <--------------------------+ | | | | B: +<--------------------------+ | V

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• The if-else statement in assembler code - example
  • Example: find the maximum of 2 integers

    The following program fragment will store the larger value of x and y in the variable max:

    int x; int y; int max; if ( x >= y ) max = x; else max = y;

    The flow chart of the above program is:

    The assembler code that implements this high level program is:

    /* -------------------------------------------------- if ( x >= y ) max = x; else max = y; -------------------------------------------------- */ .text main: movw r0, #:lower16:x movt r0, #:upper16:x // r0 = addr(x) ldr r0, [r0] // r0 = x movw r1, #:lower16:y movt r1, #:upper16:y // r1 = addr(y) ldr r1, [r1] // r1 = y cmp r0, r1 // compare x ? y blt else // Branch to "else" if x < y // x >= y ---> max = x movw r2, #:lower16:max movt r2, #:upper16:max // r2 = addr(max) str r0, [r2] // max = x b ifEnd // Skip over the else part !!! else: // x < y ---> max = y movw r2, #:lower16:max movt r2, #:upper16:max // r2 = addr(max) str r1, [r2] // max = y ifEnd: /* -------------------------------------------------- Begin of the permanent program variables -------------------------------------------------- */ .data x: .4byte 7 // Try a value > y y: .4byte 10 max: .4byte 0 .end

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/6-if/if-else1.s

    How to run the program:

    To compile:   as255 if-else1          To run: use EGTAPI