计算一个字节中的零和一的数目 [英] counting number of zeros and ones in a byte

问题描述

我以前发布过一个程序来查找一个字节中的总1s.现在，我尝试查找一个字节中的0.以下是我的代码:

I previously posted a program to find the total number of 1s in a byte. Now I am trying to find the number of 0s in a byte. Following is my code:

MOV AL,1
MOV CX,08H
MOV BX,0000H
MOV DX,0000H

Zero:


SHR AL,01H

JZ ero

JNZ ZrO

ero: INC BX

ZrO: INC DX


LOOP Zero

hlt

程序没有给出正确的答案.你猜我在哪里错了吗?

The program isn't giving the right answer. Any guesses where am I wrong?

推荐答案

从AL移出的位进入进位标志，而不是零标志.更改您的条件跳转:

The bit which is shifted-out from AL goes to the carry-flag, not to the zero-flag. Change your conditional jumps:

    MOV AL,1     ; An investigated byte.    
    MOV CX,08H   ; Number of bits in the byte. 
    MOV BX,0000H ; Result: number of 1s.
    MOV DX,0000H ; Result: number of 0s.
Zero:SHR AL,01H  ; Shift the byte, least significant bit to CF.
    JNC ZrO
ero:INC BX      ; Count 1s. 
    JMP Skip
ZrO:INC DX      ; Count 0s.
Skip:LOOP Zero   ; Repeat CX times.
    hlt

顺便说一句，在新的英特尔处理器(NEHALEM)上有专门针对此任务的说明: https://www.felixcloutier.com/x86/popcnt

BTW there is a specialized instruction for this task on new Intel processors (NEHALEM): https://www.felixcloutier.com/x86/popcnt

    MOV AL,1     ; An investigated byte.  
    XOR AH,AH    
    POPCNT BX,AX ; Count the number of 1s in AX and put result to BX.
    MOV DX,8
    SUB DX,BX    ; The number of 0s in AL is 8-BX.

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