Solved

mov array[eax * 4], 0  /* when does multiply happen? */

Posted on 1998-11-17
2
675 Views
Last Modified: 2012-06-27
I know this is not a C question, but maybe some assembly
guys look at this section.

In the assembly statement:

mov dword ptr array[eax * 4], 0

When does the eax register get multiplied by four?

Is the resulting index stored anywhere before it
is added to "array"?  I imagine that the result of
"eax * 4" must be stored somewhere, but where?

NOTE: If you want to see this for yourself, compile
this and look at the assembly that results:

int main()
{
      int array[50];
      int x = 5;

      array[x] = 0;

        return 0;
}
0
Comment
Question by:alfredj
  • 2
2 Comments
 
LVL 16

Accepted Solution

by:
heyhey_ earned 50 total points
ID: 1178136
mov dword ptr array[eax * 4], 0 is a standard 486 instruction using some of the more special types of addressing

you can look at some Assembler book for more details
(sorry if i'm wrong with terms - i'm just translating a Russian book)

mov eax, 123456h   - immediate addressing
mov eax, ecx  - registry addressing
mov eax, [123456h] - absolute addressing
mov eax, [ecx] + 123456h - indexed addressing with offset
mov eax, [esi*4] + 1234h - indexed addressing with scaling and offset
..
mov eax, [ebx][edi*2] + 12345h - base index with scaling and offset addressing

so you have ' indexed addressing with scaling and offset'. this is a standard addressing mode in 386+ (or 486+) processors.

hope this helps
  heyhey



0
 
LVL 16

Expert Comment

by:heyhey_
ID: 1178137
you have the following algorithm for calculating the real addres (and maybe it works only in P-mode ?? )

address = offset + [data at base address] + value of index register * multiplier (+base of the segment :).
multiplier can be 1,2,4 or 8 (there are two bits in the instruction opcode reserved for it)

your example
 dword ptr array[eax * 4]
doesn't have a base addres - you have just indexed array
calculated address = offset +  value of index register * multiplier
0

Featured Post

Master Your Team's Linux and Cloud Stack

Come see why top tech companies like Mailchimp and Media Temple use Linux Academy to build their employee training programs.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Often, when implementing a feature, you won't know how certain events should be handled at the point where they occur and you'd rather defer to the user of your function or class. For example, a XML parser will extract a tag from the source code, wh…
Many modern programming languages support the concept of a property -- a class member that combines characteristics of both a data member and a method.  These are sometimes called "smart fields" because you can add logic that is applied automaticall…
The goal of the video will be to teach the user the difference and consequence of passing data by value vs passing data by reference in C++. An example of passing data by value as well as an example of passing data by reference will be be given. Bot…
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.

810 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question