如何查找8086程序集中的位数？

Question

我是一个新的汇编程序员，我无法找到一个数字有多少位数。我的目的是找到阶乘。 我在程序集8086的模拟器中编程。

Answer 1

执行此操作的最有效方法是使用bsr指令（请参阅此幻灯片 ，20到25）。

这应该是这样的代码：

    .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

但是，我想你需要基数10日志而不是基数2 ...所以，这里将是代码：

    .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

我很想知道是否有人能找到比这更好的东西！ 实际上，使用SSE指令可能会有所帮助。