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c++assemblyx86zero-extension

Translate assembly instructions to c++

I am having trouble translating the below assembly in to c++

MOVZX EAX, DX

Where EDX is a 32bit register. I need to get the lowest 16 bits(DX).

I've tried the following:

unsigned edx = 0x123ABCDE;
unsigned dx = (edx>>16) & 0xff;

I expect to get an the value of BCDE stored in dx, but it's gone a bit wrong.

Any help would be much appreciated.

Solution

  • Let's analyze your code step by step.
    unsigned edx = 0x123ABCDE;
    Nothing unusual.

    Let's break up the next statement according to order of evaluation.
    (edx >> 16) -- right shift by 16 bits.
    This is right shifting by 2 bytes or 4 hex digits.
    Answer should be 0x123A.

    Now, let's keep the right most 8 bits (ANDing with 0xFF):
    0x123A & 0xFF == 0x3A

    The result should be 0x3A in the variable dx.