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flat assembler > Main > Store address of label in another variable

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nisargshah95



Joined: 23 Jun 2016
Posts: 7
Location: India
Store address of label in another variable
I am new to FASM (ver 1.71.16) and trying to write a program which has the following simplified structure -


Code:
start:
    ;; some condition using cmp
    je @f
    mov [block], block1
    jmp .forward
  @@
    mov [block], block2
  .forward:
    ;; go to block.sub1

block1:
  .sub1:
  .sub2:

block2:
  .sub1:
  .sub2:

;; data
block rd 1



I wish to store the address of block1 or block2 (depending on a condition into block variable and then jump to .sub1 sub label. How do I do this?
Post 23 Jun 2016, 17:40
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15303
Location: Bigweld Industries

Code:
mov [block],block1.sub1
jmp [block]

Post 23 Jun 2016, 23:56
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nisargshah95



Joined: 23 Jun 2016
Posts: 7
Location: India

revolution wrote:

Code:
mov [block],block1.sub1
jmp [block]



I know I can do that, but I was looking for a way such that I would not have to move local label's address into register every time I want to access it.
Post 24 Jun 2016, 04:14
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15303
Location: Bigweld Industries
I don't understand what you mean. There are no registers used in any code posted in this thread so far.
Post 24 Jun 2016, 05:59
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nisargshah95



Joined: 23 Jun 2016
Posts: 7
Location: India
Sorry I did not make it clear. Throughout the program, I will only be executing the code in either block1 or block2 (both have the same local label names) based on a condition (ie, one of them will not be used during the entire execution). So throughout the program instead of checking every time whether block1 is to be used or block2 I simply want to call the local labels using block.labelname
Post 24 Jun 2016, 06:19
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nisargshah95



Joined: 23 Jun 2016
Posts: 7
Location: India
Based on a discussion with a friend, I was given the following suggestion. Keep the address of the local labels at the start of block1/block2, And access the code at that local label this way

Code:

start:
    ;; some condition using cmp
    je @f
    mov [block], block1
    jmp .forward
  @@
    mov [block], block2
  .forward:
    jmp [block+sub1]

block1:
  dd .sub1
  dd .sub2

  .sub1:
    ;; code
  .sub2:
    ;; code


Post 24 Jun 2016, 06:22
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shutdownall



Joined: 02 Apr 2010
Posts: 518
Location: Munich
Why don't you use a direct call rather than moving into a variable ?


Code:

jmp block1.sub1
or even faster
jpe block2.sub1


Post 24 Jun 2016, 11:23
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Furs



Joined: 04 Mar 2016
Posts: 891
You can't jump to the respective block's sub by just storing one value. Jump instructions store offsets to the respective location (either relative, if direct jump, or absolute, if indirect).

Since I assume block1 and block2 are different pieces of code, then their subs are definitely different code, thus different sizes. You can't do, for example, block+x, where x is block1.sub1-block1, because then it would fail on block2.sub1, assuming it has a different offset (relative to block2) which is probably the case...?

However, you can store an index to a table, or simply offset to the table as you did in your second attempt, the table holding offsets for the sub1 and sub2 for each block etc.

Your discussion with a friend as you mentioned it does indeed use that method, which works fine. Any problems with it or did you just post because you found the solution? Smile
Post 01 Jul 2016, 23:29
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nisargshah95



Joined: 23 Jun 2016
Posts: 7
Location: India
@Furs I posted it because I wanted an opinion on how effective a solution it is, and whether a better approach exists. Nevertheless, I used that approach in the end.
Post 09 Jul 2016, 04:11
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Furs



Joined: 04 Mar 2016
Posts: 891
Don't worry about it, it is the best solution if you have many such blocks or many conditions, that's how you can make "objects" in C or asm (which don't have abstract OOP things), because basically those offsets are "function pointers" (if you know C).

Of course in asm you have more control and they don't have to be functions (you use jmp instead), as long as they all have same entry conditions it should work fine. Smile
Post 13 Jul 2016, 16:47
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