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Index > Compiler Internals > [strange, but not BUG] LODSB and LODS BYTE [GS:SI]

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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
Code:
lodsb [gs:si]      ; BUG
lods byte [gs:si]  ; Fine
    


FASM core bug or intentional behaviour ?


Last edited by DOS386 on 11 Jan 2009, 14:36; edited 1 time in total
Post 11 Jan 2009, 09:27
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asmfan



Joined: 11 Aug 2006
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asmfan
i think it's purposely cuz lodsb is a separate 1 byte (w/o REX) command with defined size operand, but lods is a hint to use that command with appropriate segment (if overridden) and size for operand.
Post 11 Jan 2009, 09:45
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edfed



Joined: 20 Feb 2006
Posts: 4237
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edfed
error: extra characters on line

lodsb[gs:si]

i think it is normal
as lodsb is a short version of lods, it don't need operand under 1.67.26
Post 11 Jan 2009, 09:47
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vid
Verbosity in development


Joined: 05 Sep 2003
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vid
Proper syntax would be "lods byte [gs:si]", but I am not sure if that would work
Post 11 Jan 2009, 10:12
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
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Tomasz Grysztar
You can look it up in Intel manuals - LODSB, LODSW, XLATB, etc. are all the "shortcuts", which do not take any arguments and use the default operands. To specify other than default, you need to use the proper instruction. Like:
Code:
use32

lodsb ; = lods byte [ds:esi]
lods byte [gs:si] ; some different variant

xlatb ; = xlat byte [ds:ebx]
xlat byte [fs:bx] ; some different variant    
Post 11 Jan 2009, 12:44
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
Thanks ... Smile

> You can look it up in Intel manuals

Not much in. Sad

vid wrote:

> Proper syntax would be "lods byte [gs:si]", but I am not sure if that would work

Why ? It seems to work.

But since "lodsb" and "lods byte" is the very same instruction it was and still is inobvious to me why only the latter accepts segment prefix.
Post 11 Jan 2009, 14:39
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Tomasz Grysztar
Assembly Artist


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Tomasz Grysztar
DOS386 wrote:
> You can look it up in Intel manuals

Not much in. Sad

This is what my copy of "IA-32 Intel® Architecture Software Developer’s Manual Volume 2A" says. Wink


Description:
Filesize: 47.08 KB
Viewed: 5086 Time(s)

lods_intel.jpg


Post 11 Jan 2009, 21:06
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
Tomasz Grysztar wrote:
This is what my copy of "IA-32 Intel® Architecture Software Developer’s Manual Volume 2A" says.


That's what the famous manual you originally learned ASM from says:

Quote:
LODS loads the AL, AX, or EAX register with the memory byte, word,
or doubleword at the location pointed to by the source-index register.
After the transfer is made, the source-index register is automatically
advanced. If the direction flag is 0 (CLD was executed), the source index
increments; if the direction flag is 1 (STD was executed), it decrements.
The increment or decrement is 1 if a byte is loaded, 2 if a word is loaded,
or 4 if a doubleword is loaded.

If the address-size attribute for this instruction is 16 bits, SI is used
for the source-index register; otherwise the address-size attribute is 32
bits, and the ESI register is used. The address of the source data is
determined solely by the contents of ESI/SI. Load the correct index value
into SI before executing the LODS instruction. LODSB, LODSW, LODSD are
synonyms for the byte, word, and doubleword LODS instructions.

LODS can be preceded by the REP prefix; however, LODS is used more typically
within a LOOP construct, because further processing of the data moved into
EAX, AX, or AL is usually necessary.

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug
Post 12 Jan 2009, 08:42
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MazeGen



Joined: 06 Oct 2003
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MazeGen
LODSB/MOVSB/CMPSB/... takes no operands and therefore you can't override default DS: segment.

The manual explains it clearly: there is short form and long form of all string instructions' syntax. Always use the long form if you want to use segment override.
Post 12 Jan 2009, 10:16
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
MazeGen wrote:
LODSB/MOVSB/CMPSB/... takes no operands and therefore you can't override default DS: segment. The manual explains it clearly: there is short form and long form of all string instructions' syntax.


NOT the one I quoted from Neutral

Quote:
Always use the long form if you want to use segment override.


OK ... minor thing.
Post 12 Jan 2009, 10:47
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MazeGen



Joined: 06 Oct 2003
Posts: 974
Location: Czechoslovakia
MazeGen
Always use the latest manuals when in doubt.

...
At the assembly-code level, two forms of this instruction are allowed: the “explicit operands”
form and the “no-operands” form. The explicit-operands form (specified
with the LODS mnemonic) allows the source operand to be specified explicitly. Here,
the source operand should be a symbol that indicates the size and location of the
source value.
...
The no-operands form provides “short forms” of the byte, word, and doubleword
versions of the LODS instructions. Here also DS:(E)SI is assumed to be the source
operand and the AL, AX, or EAX register is assumed to be the destination operand.
The size of the source and destination operands is selected with the mnemonic:
LODSB (byte loaded into register AL), LODSW (word loaded into AX), or LODSD
(doubleword loaded into EAX).
Post 12 Jan 2009, 11:03
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution
MazeGen wrote:
Always use the latest manuals when in doubt.
Yes, indeed, very true.

Just like the software and hardware, manuals/documentation can also have "bugs". So the latest version is always the best place to start. Perhaps the old adage "if in doubt RTFM" can be slightly modified to "if in doubt RTLFM" where "L" stands for "latest"? Wink
Post 12 Jan 2009, 11:15
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