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The while statement

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• The while statement in a high level programming language
  • The while statement has the following syntax and program flow (as you have learned in CS170):

    while ( condition ) +---------->| statement | | | +--------------+ FALSE | | condition |--------+ | +--------------+ | | | | | | TRUE | | | | | V | | statement | | | | (repeat loop) +-----------+ | (exit loop) | +<---------------+ | V

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• The while statement construct in assembler code

  • Assembler construct that realizes the control program flow of the while statement is as follows:

    LOOP: +--> Evaluate "condition" (use cmp) | | | V | Branch on the FALSE outcome of "condition" to label A: -----+ | | | | | TRUE | | | | | V | | Translate "statement" into assembler code | | | | | V | +--------- b LOOP | | A: +<------------------------------------------------+ | V

    Notice that:

    The while-statement construc always contains: A label (I used LOOP: in the construct, you need to use a unique label name for each loop !) that marks the start of the while-loop A b <STARTLABEL> assembler instruction that jumps to the start of the while loop (so the while-loop code can be repeated !)

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• Example while loop in assembler programming
  • The following is a pretty well-known while-loop used to compute the quotient and the remainder of the integer division A ÷ B:

    int A, B; int Q, R; // Computes: Q = A/B and R = A%B Q = 0; R = A; while ( R >= B ) { Q = Q + 1; R = R - B; }

    The flow chart of the program segment:

    Assembler program:

    main: // Q = 0; movw r0, #:lower16:Q movt r0, #:upper16:Q // r0 = addr(Q) mov r1, #0 str r1, [r0] // Q = 0; // R = A; movw r0, #:lower16:A movt r0, #:upper16:A // r0 = addr(A) ldr r1, [r0] // r1 = A; movw r0, #:lower16:R movt r0, #:upper16:R // r0 = addr(R) str r1, [r0] // R = A; while: // test R >= B movw r0, #:lower16:R movt r0, #:upper16:R // r0 = addr(R) ldr r1, [r0] // r1 = R; movw r0, #:lower16:B movt r0, #:upper16:B // r0 = addr(B) ldr r2, [r0] // r2 = B; cmp r1, r2 // Check: R >= B blt whileEnd // If R < B: exit while loop /* *********************************** While body *********************************** */ // Q = Q + 1 movw r0, #:lower16:Q movt r0, #:upper16:Q // r0 = addr(Q) ldr r1, [r0] // r1 = Q; add r1, r1, #1 // r1 = Q+1 str r1, [r0] // Q = Q+1; // R = R - B movw r0, #:lower16:R movt r0, #:upper16:R // r0 = addr(R) ldr r1, [r0] // r1 = R; movw r0, #:lower16:B movt r0, #:upper16:B // r0 = addr(B) ldr r2, [r0] // r2 = B; sub r1, r1, r2 // r1 = R - B movw r0, #:lower16:R movt r0, #:upper16:R // r0 = addr(R) str r1, [r0] // R = R - B; // End of while body --- branch back to test !! b while whileEnd: Stop: nop /* -------------------------------------------------- Begin of the permanent program variables -------------------------------------------------- */ .data A: .4byte 37 B: .4byte 7 Q: .skip 4 // Quotient R: .skip 4 // Remainder .end

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/7-while/while1.s

    How to run the program:

    To compile:   as255 while1 To run: use EGTAPI