How to set eax register in, inline assembly C++

Question

0.00/5 (No votes)

See more:

eax and ebx are 32 bits registers. a[] and b[] arrays have 4 chars and 32 bits data. i can separately set each index of array in each register for example mov eax,a[1].
but I wanna set all 4 chars of array 'a' to eax register. the important thing is the sequence. I want to set those two registers without any changes in the arrays content sequence. for example first array is a[0]='a',a[1]='b',a[2]='c',a[3]='d' and the eax register must be as "abcd". how can I do that?

What I have tried:

<pre lang="c++">
int _tmain()
{
char b[4],a[4];
b[0]='A',b[1]='T',b[2]='C',b[3]='G';
a[0]='A',a[1]='T',a[2]='C',a[3]='G';
__asm
{
    movzx eax,a[1]  //here i want to load all 4 char of a
    movzx ebx,b[1]  //here i want to load all 4 char of b 
}
getchar();
return 0;
}

Posted 2-Aug-18 11:13am

Member 13932705

Updated 2-Aug-18 12:05pm

Add a Solution

Comments

Richard MacCutchan 3-Aug-18 4:12am

Why? As soon as that code is passed then the registers will be used for other purposes, as decided by the compiler.

2 solutions

Solution 2

Quote:
eax and ebx are 32 bits registers. a[] and b[] arrays have 4 chars and 32 bits data. i can separately set each index of array in each register for example mov eax,a[1].

first of all, you should try to replace

C++

movzx eax,a[1]  //here i want to load all 4 char of a
movzx ebx,b[1]  //here i want to load all 4 char of b

by

C++

movzx eax,a[0]  //here i want to load all 4 char of a
movzx ebx,b[0]  //here i want to load all 4 char of b

because arrays are 0 based in C++.

Quote:
How to set eax register in, inline assembly C++

otherwise, what is your problem with this code ?

Posted 2-Aug-18 12:05pm

Patrice T

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Jochen Arndt · Accepted Answer · 2018-08-02T11:51:00

The compiler knows that the arrays are of type char and generates a

MOV EAX,BYTE PTR [addr]

If you want to load a 32 bit value from a byte array, tell the compiler that he has to load 32 bit words (untested but should do it):

C++

__asm
{
    mov eax,DWORD PTR [a]
    mov ebx,DWORD PTR [b]
}

Or cast the char array pointers to DWORD pointers:

uint32_t* a32 = reinterpret_cast<uint32_t*>(a); uint32_t* b32 = reinterpret_cast<uint32_t*>(b); __asm { mov eax,a32[0] mov ebx,b32[0] }
Note that in both cases using MOVZX makes no sense when source and destination have the same size.

[EDIT]
Have tested it meanwhile. Using DWORD PTR works as expected while using a casted pointer does not work (loads register with address of a32 resp. b32).
[/EDIT]