使用偏移访问数据段

Question

I'm reading assembly language for x86 processor 's book

I'm trying to solve one of the chapter problems

Question is :

Insert the following variables in your program:

.data
Uarray WORD 1000h,2000h,3000h,4000h
Sarray SWORD -1,-2,-3,-4

Write instructions that use direct-offset addressing to move the four values in Uarray to the EAX, EBX, ECX, and EDX registers.

I write this code :

TITLE MASM Template                     (main.asm)

INCLUDE Irvine32.inc
.data
    arr1 WORD 1000h,2000h,3000h,4000h
    arr2 SWORD -1,-2,-3,-4

.code   
main PROC
    mov esi,OFFSET arr1
    mov eax,[esi]
    call DumpRegs
exit
main ENDP
END main

But the value of eax is 20001000 !

I cant understand why it isn't 00001000 ? why the first part become 2000?

And how can I fix it ?

Answer 1

发生这种情况是因为您将arr1声明为word ，这是一个16位值，但是您使用mov eax,...从地址加载32位双字。

Answer 2

arr1 is an Array of WORDs ie 16-bit-values. mov eax,[esi] receives a 32-bit-value in this case two 16-bit-values. So you'll find in EAX the first and the second element of arr1 . If you want to fill the whole EAX only with the first value use movzx eax, word ptr [esi] instead. This instruction fills the lower 16 bits of EAX with the element and nullifies the upper 16 bits.