CODEWITHSUNDEEP
mathassemblyx86x86-16digits
TJR
TJR

Reputation: 484

how to find number of digits in assembly 8086?

I am a new assembly programer and I could not succeed in finding how many digits a number has.My purpose is to find factorials. I program in an emulator of assembly 8086.

Upvotes: 4

Views: 2212

Answers (1)

perror
perror

Reputation: 7426

The most efficient way to perform this operation is to use the bsr instruction (see this slides, 20 to 25).

This should make a code like this:

    .text
    .globl  main
    .type   main, @function
main:
    movl    $1024, %eax ;; pushing the integer (1024) to analyze
    bsrl    %eax, %eax  ;; bit scan reverse (give the smallest non zero index)
    inc     %eax        ;; taking the 0th index into account

But, I guess you need the base 10 log and not the base 2... So, here would be the code:

    .text
    .globl  main
    .type   main, @function
main:
    movl    $1024, %eax ;; pushing the integer (1024) to analyze
    bsrl    %eax, %eax  ;; bit scan reverse (give the smallest non zero index)
    inc     %eax        ;; taking the 0th index into account

    pushl   %eax        ;; saving the previous result on the stack

    fildl   (%esp)      ;; loading the previous result to the FPU stack (st(0))
    fldlg2              ;; loading log10(2) on the FPU stack
    fmulp   %st, %st(1) ;; multiplying %st(0) and %st(1) and storing result in %st(0)

    ;; We need to set the FPU control word to 'round-up' (and not 'round-down')
    fstcw  -2(%esp)      ;; saving the old FPU control word
    movw   -2(%esp), %ax ;; storing the FPU control word in %ax
    andw   $0xf3ff, %ax  ;; removing everything else
    orw    $0x0800, %ax  ;; setting the proper bit to '1'
    movw   %ax, -4(%esp) ;; getting the value back to memory
    fldcw  -4(%esp)      ;; setting the FPU control word to the proper value

    frndint              ;; rounding-up

    fldcw  -2(%esp)      ;; restoring the old FPU control word

    fistpl (%esp)        ;; loading the final result to the stack
    popl   %eax          ;; setting the return value to be our result

    leave
    ret

I am curious to know if somebody can find better than that ! Indeed, using SSE instructions might help.

Upvotes: 2

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