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flat assembler > Main > which flags "test" updates?

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vivik



Joined: 29 Oct 2016
Posts: 78
which flags "test" updates?
This page https://flatassembler.net/docs.php?article=manual says:

>test performs the same action as the and instruction, but it does not alter the destination operand, only updates flags. Rules for the operands are the same as for the and instruction.

>and, or and xor instructions perform the standard logical operations. They update the SF, ZF and PF flags. Rules for the operands are the same as for the add instruction.

, but this page http://stackoverflow.com/questions/147173/x86-assembly-testl-eax-against-eax says:

>CF and OF cleared (AND/TEST always does that, and subtracting zero never produces a carry)

>ZF, SF and PF according to the value in EAX. (a = a&a = a-0)

So I guess, that's something to add in the fasm documentation?

I'm asking this because in the second link you can find this:

>It tests whether eax is 0, or above, or below.

But I'm not sure if it can test for "above" or "below", does it set enough flags for that? Also, is that a signed or unsigned comparasion?
Post 20 Mar 2017, 17:43
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zhak



Joined: 12 Apr 2005
Posts: 453
Location: Belarus
From Intel 64 manual:


Quote:

TEST—Logical Compare
. . .
Flags Affected
The OF and CF flags are set to 0. The SF, ZF, and PF flags are set according to the result. The state of the AF flag is undefined.



test above and below depend on CF (particularly, CF is set if below). Thus you cannot use TEST for this as it always clears Carry flag
Post 20 Mar 2017, 19:27
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 14584
Location: Planet Dirt
vivik: I suggest you download either the Intel Manuals, or the AMD manuals, or both. There are thousands of little details that the fasm manual doesn't cover. Without the source manuals you will be lost.
Post 21 Mar 2017, 02:04
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vivik



Joined: 29 Oct 2016
Posts: 78
@zhak
Thanks for clearing things out!

@revolution
>There are thousands of little details that the fasm manual doesn't cover.
Can I get some examples?
Post 21 Mar 2017, 05:40
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system error



Joined: 01 Sep 2013
Posts: 438
@vivik

FASM manual is a description about FASM - it describes mostly how it implements x86 instruction sets and how it may or may not differ from the ISA. So, FASM manual just goes a quick walkthrough of the entire instruction sets without bothering about the details. The technical details of such implementations are described in the Intel/AMD manuals instead.

For example, it could be that FASM implements a symbolic MOVTOREG to represent intel's <mov reg,source> instruction. Assembler's manual is used to explain such things.
Post 21 Mar 2017, 05:55
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vivik



Joined: 29 Oct 2016
Posts: 78
hm... fasm manual seems to be enough for practical needs, it's verbose just enough for me to understand.

Can you give me other details that may be important during assembly programming but are not mentioned in fasm manual?
Post 21 Mar 2017, 06:25
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system error



Joined: 01 Sep 2013
Posts: 438

vivik wrote:
hm... fasm manual seems to be enough for practical needs, it's verbose just enough for me to understand.

Can you give me other details that may be important during assembly programming but are not mentioned in fasm manual?



Example: not all instructions mentioned in FASM manuals come with explanations of the affected flags.

Example: there's no instruction mnemonics (which are important for debuggers, disassemblers)

We are lucky enough that FASM emulates all instructions as close as possible to the Intel/AMD definitions. In MASM for example, MOVQ is implemented differently than the Intel Manual. So assuming MOVQ of FASM and MOVQ of MASM behaves the same is wrong. That's the purpose of assembler's manual - to explain WHAT it implements and HOW they may differ.
Post 21 Mar 2017, 06:38
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vivik



Joined: 29 Oct 2016
Posts: 78
Always thought that MOV, AND, TEST actually are the mnemonics, was I wrong?
Post 21 Mar 2017, 07:12
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system error



Joined: 01 Sep 2013
Posts: 438
What I meant to say was encoding. I personally prefer the term SYMBOLS when referring to textual representations of instructions because it applies to all programming languages when it comes to syntax.
Post 21 Mar 2017, 07:54
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vivik



Joined: 29 Oct 2016
Posts: 78
Ok, I see. Thanks.
Post 21 Mar 2017, 08:12
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