flat assembler
Message board for the users of flat assembler.

flat assembler > Main > xor ax, 1 and xor ax,1

Author
Thread Post new topic Reply to topic
uri



Joined: 09 Apr 2004
Posts: 44
Location: L'viv, Ukraine
there are two different opcodes for one instruction:
Code:
83F001  xor  ax,0001
350100  xor  ax,0001
    

can i say fasm generate one of it, or i must write some macro?
Post 28 Jul 2005, 10:25
View user's profile Send private message Visit poster's website ICQ Number Reply with quote
uri



Joined: 09 Apr 2004
Posts: 44
Location: L'viv, Ukraine
sorry, i already know.

for first i must write xor ax,1
for second - xor ax, word 1
Post 28 Jul 2005, 10:28
View user's profile Send private message Visit poster's website ICQ Number Reply with quote
uri



Joined: 09 Apr 2004
Posts: 44
Location: L'viv, Ukraine
No, this question not closed.

Another situation:
i want to compile instruction "xor bh, al".
But what i have?
Code:
32 F8    xor bh, al
30 C7    xor bh, al
    


Here i can't specify any types, because both instructions have same type:
Code:
32 /r     XOR r8,r/m8
30 /r     XOR r/m8,r8
    


How can i select opcode? Only via macroses? Sad
Post 28 Jul 2005, 14:50
View user's profile Send private message Visit poster's website ICQ Number Reply with quote
Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6981
Location: Kraków, Poland
In this case (as opposed to the previous one) these two encodings are fully functionally equivalent - so assembler always chooses only one of them. It can be even considered a kind of footprint for the assembler.
Post 28 Jul 2005, 14:52
View user's profile Send private message Visit poster's website Reply with quote
uri



Joined: 09 Apr 2004
Posts: 44
Location: L'viv, Ukraine
Yes, fully functionally equivalent, but in case polymorphic code it's very important.

Ok, can you say - what opcode will be selected by fasm in case two identical commands, but two different opcodes? Is present some regularity, or it's depends only from command?
Post 28 Jul 2005, 15:06
View user's profile Send private message Visit poster's website ICQ Number Reply with quote
THEWizardGenius



Joined: 14 Jan 2005
Posts: 382
Location: California, USA
Well I would assume that the opcodes chosen are those that use the smallest code. However, when they are the same size, same speed, and do EXACTLY the same thing, it is up to the compiler. You can ask which is used, or check the source code, or maybe it is documented somewhere else (maybe FASM internals documentation? I don't know).
Post 28 Jul 2005, 19:00
View user's profile Send private message AIM Address Reply with quote
Matrix



Joined: 04 Sep 2004
Posts: 1171
Location: Overflow
uri wrote:
No, this question not closed.

Another situation:
i want to compile instruction "xor bh, al".
But what i have?
Code:
32 F8    xor bh, al
30 C7    xor bh, al
    


Here i can't specify any types, because both instructions have same type:
Code:
32 /r     XOR r8,r/m8
30 /r     XOR r/m8,r8
    


How can i select opcode? Only via macroses? Sad


hi
what do you say on
Code:
32 F8    xor bh,< al
30 C7    xor bh,> al
32 F8    xor bh, al
    

my only idea is
make somethng like r8 direction
Code:
32 /r     XOR r8 , r/m8 so <
30 /r     XOR r/m8 , r8 so >

32 F8    xor bh,< al
30 C7    xor bh,> al

and if not specified direction '<' '>' then
use 32 F8    xor bh,< al
32 /r     XOR r8 , r/m8 so <
because first parameter is r8

this whould mean ',<' and ',>' whould define which one to use in addition to ','

    
Post 28 Jul 2005, 21:55
View user's profile Send private message Visit poster's website Reply with quote
Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6981
Location: Kraków, Poland
Isn't it better to write just machine code for such applications? The assembly language is there to provide you abstraction from encodings and focuses on functionality of instructions (at least on such vision of assembly language the fasm's syntax is based).
Post 28 Jul 2005, 22:13
View user's profile Send private message Visit poster's website Reply with quote
LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4634
Location: Argentina
166C:0100 2D0010 SUB AX,1000
166C:0103 81E80010 SUB AX,1000

The first one is sub accum, immediate and the second is sub reg, immediate

Is there a way to force "sub" to be assembled with reg operand instead of accum? I can't remember where I read using 2D instead of 81 E8 not pair in some situations on Pentium 1
Post 28 Jul 2005, 22:39
View user's profile Send private message Reply with quote
LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4634
Location: Argentina
Well I remember now but it wasn't "sub" Razz

http://www.codingnow.com/2000/download/pentopt.htm#26_13
Quote:

26.13 MOV [MEM], ACCUM (PPlain and PMMX)

The instructions MOV [mem],AL MOV [mem],AX MOV [mem],EAX are treated by the pairing circuitry as if they were writing to the accumulator. Thus the following instructions do not pair:

MOV [mydata], EAX
MOV EBX, EAX

This problem occurs only with the short form of the MOV instruction which can not have a base or index register, and which can only have the accumulator as source. You can avoid the problem by using another register, by reordering your instructions, by using a pointer, or by hard-coding the general form of the MOV instruction.

In 32 bit mode you can write the general form of MOV [mem],EAX:

DB 89H, 05H
DD OFFSET DS:mem

In 16 bit mode you can write the general form of MOV [mem],AX:

DB 89H, 06H
DW OFFSET DS:mem

To use AL instead of (E)AX, you replace 89H with 88H

This flaw has not been fixed in the PMMX.


[edit]
166C:0100 A33412 MOV [1234],AX
166C:0103 89063412 MOV [1234],AX
[/edit]


Last edited by LocoDelAssembly on 29 Jul 2005, 01:47; edited 1 time in total
Post 28 Jul 2005, 23:16
View user's profile Send private message Reply with quote
comrade



Joined: 16 Jun 2003
Posts: 1118
Location: Russian Federation
there are also two ways to encode:

Code:
33C0    xor eax,eax
31C0    xor eax,eax
    


The first one is common across MS products and others I have seen (Borland). Actually I have only seen the second one in FASM.
Post 29 Jul 2005, 01:39
View user's profile Send private message Visit poster's website AIM Address Yahoo Messenger MSN Messenger ICQ Number Reply with quote
comrade



Joined: 16 Jun 2003
Posts: 1118
Location: Russian Federation
and yes, these different encodings are used by assemblers as a footprint

i hard somewhere that unregistered copies of a86 or a386 made such special encodings, so the author could track down those who distributed programs that were compiled with unregistered a86... damn capitalists
Post 29 Jul 2005, 02:24
View user's profile Send private message Visit poster's website AIM Address Yahoo Messenger MSN Messenger ICQ Number Reply with quote
MCD



Joined: 21 Aug 2004
Posts: 604
Location: Germany
This is just a completly useless discussion for the overwhelming majority of programs. It's only important for polymorphic code...but anyway, which sensible software uses that?
Post 29 Jul 2005, 09:01
View user's profile Send private message Reply with quote
Matrix



Joined: 04 Sep 2004
Posts: 1171
Location: Overflow
well yes, it is only a difference in the machine code,
but anyway it could be added to the features request, in a smaller priority at least for completeness.

another option is to define typecast r8 r16 r32 r64 m8 m16 m32 m64 typecast, so syntax whouldn't has to be changed with other character combinations example
Code:
32 F8    xor bh,< al
30 C7    xor bh,> al
32 F8    xor bh, al
    


instead this could be written (advanced typecast)
Code:
32 /r     XOR r8 , r/m8 so <
30 /r     XOR r/m8 , r8 so >

32 F8    xor r8 bh, al
30 C7    xor bh, r8 al
32 F8    xor bh, al
    
Post 29 Jul 2005, 09:33
View user's profile Send private message Visit poster's website Reply with quote
Madis731



Joined: 25 Sep 2003
Posts: 2146
Location: Estonia
r8 is reserved in 64-bit systems, or was it r08?
Post 02 Aug 2005, 13:40
View user's profile Send private message Visit poster's website Yahoo Messenger MSN Messenger Reply with quote
vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7109
Location: Slovakia
when speaking about it - there is a bigger problem. you cannot force size of constant operand when there are multiple possibilities, like:
cmp eax,0
0 here gets encoded as byte (instruction cmp r32,c8 ) where c8 gets sign extended. But if you aren't sure if constant fits in byte, you cannot force it, like
cmp eax,byte 0
where assembler says it cannot assemble even though it assembles it to byte.
This forces us to hardcode instruction in sefl-overwriting code, which is against "The assembly language is there to provide you abstraction from encodings and focuses on functionality of instructions".
Post 02 Aug 2005, 15:59
View user's profile Send private message Visit poster's website AIM Address MSN Messenger ICQ Number Reply with quote
Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6981
Location: Kraków, Poland
Because it "focuses on functionality" it cannot allow "cmp eax,byte 0" because clearly the operand sizes doesn't match here, and this is likely to mean a mistake by programmer (like if you do "cmp eax,[var]" with byte variable, where also fasm detects an error). If you aren't sure if constant fits in byte and you need to be sure that it does, just use the "if" directive to check it out (or something like "cmp eax,value and 07Fh" if you care only about instruction code, not overflows).

The other case are direct jumps and calls, as size operator in this case doesn't apply to the size of target address (as this actually depends on the processor's mode rather than on the instruction itself), but to the size of relative address displacement. Thus the size operators for all possible values of displacements are allowed there.

And as for the "cmp eax,byte 0" not being allowed, consider also this kind of code:
Code:
var db ?
cmp [var],al    

if we change the "db" to "dd" in "var" definition, we will get an error - for obvious reasons (see Design Principles. And thus it was clear for me that "cmp [var],byte 0" should cause the same kind of error when we change "db" to "dd", since it's just analoguous. And then to allow choosing between shorter and longer encodings I had to make a scheme explained in the section 1.2.6 of the manual. You know, the arguments like the ones you used were all considered by me when I was designing fasm's syntax and it actually took me more time to think over the syntax than to implement it. I did not write fasm in a "let's get it working and we will worry about details later" way.


PS. Sorry if it sounds a bit conceited, I may need a rest...
Post 02 Aug 2005, 16:17
View user's profile Send private message Visit poster's website Reply with quote
vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7109
Location: Slovakia
Quote:
Because it "focuses on functionality" it cannot allow "cmp eax,byte 0" because clearly the operand sizes doesn't match here, and this is likely to mean a mistake by programmer


i still don't agree, operand sizes doesn't ALWAYS have to match. Just like with movzx, if there is isntruction which allows different sizes of operands, then assembler should be able to interpret it.

Quote:
And thus it was clear for me that "cmp [var],byte 0" should cause the same kind of error when we change "db" to "dd", since it's just analoguous.

It just isn't. Processor doesn't have instruction cmp-r16-r8, so error here is allright, but it does have instruction cmp-r16-c8, so i don't see reason to throw error for perfectly encodeable instruction.
it isn't general rule that all instructon arguments have to have same size.
Your reasoning seems to me as more "internal problem" of FASM, i think if coder writes "cmp eax, byte 8" he can await instruction to be generated and doesn't have to remember if FASM does create shortened form or it doesn't. It's like writing "public" before things which would be public anyway in C, if you want to be sure cmp-r16-c8 opcode is generated, you can make it sure this way. And it's much more clear for reader of code who don't know FASM so well.
Also i don't see any problem this could cause, i can only see benefits.
Post 02 Aug 2005, 16:57
View user's profile Send private message Visit poster's website AIM Address MSN Messenger ICQ Number Reply with quote
rugxulo



Joined: 09 Aug 2005
Posts: 2311
Location: Usono (aka, USA)
He hasn't gone after anyone yet, and he says he'll only do it if their program is commercial (i.e., you make money, you should register) although you aren't supposed to distribute any kind of program assembled by A86 without registering.


comrade wrote:
and yes, these different encodings are used by assemblers as a footprint

i hard somewhere that unregistered copies of a86 or a386 made such special encodings, so the author could track down those who distributed programs that were compiled with unregistered a86... damn capitalists
Post 09 Sep 2005, 05:31
View user's profile Send private message Visit poster's website Reply with quote
Display posts from previous:
Post new topic Reply to topic

Jump to:  


< Last Thread | Next Thread >
Forum Rules:
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Copyright © 1999-2018, Tomasz Grysztar.

Powered by rwasa.