lea esi,[esi+ecx*8]

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eskizo

Joined: 22 Nov 2005
Posts: 59

eskizo 28 Jun 2009, 22:53

Hello all,

I would like to know if this expression is actually some kind of "macro" or it is a real machine instruction... I mean:

Code:

lea esi, [esi + ecx * 8]

;is compiled as follow?

push esi
push ecx
mul ecx, 8
add esi, ecx
lea esi, [esi]
pop ecx
pop esi

28 Jun 2009, 22:53

arigity

Joined: 22 Dec 2008
Posts: 45

arigity 28 Jun 2009, 23:16

yes that is a legitimate use of the lea instruction and is not a macro, you should see the intel developers manuals for more information on questions like these.

28 Jun 2009, 23:16

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 29 Jun 2009, 00:22

There is no multiplication involved with lea. It is shift left. That is, multiplication by power of 2.

2, 4, 8, 16, 32, 64, 128... and so on, are powers of 2. You can't use lea to multiply by some other value.

EDIT: what's the point of:

Code:

lea esi, [esi]

29 Jun 2009, 00:22

eskizo

Joined: 22 Nov 2005
Posts: 59

eskizo 29 Jun 2009, 01:28

thanks arigity & Borsuc.

Borsuc, forget my mistake.

29 Jun 2009, 01:28

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20460
Location: In your JS exploiting you and your system

revolution 29 Jun 2009, 14:45

Also the push/pop of esi will nullify your result completely. The only change from the code you posted is that the flags are altered. In a real lea instruction no flage are altered. Maybe this?

Code:

pushfd
push ecx
mul ecx, 8
add esi, ecx
pop ecx
popfd

29 Jun 2009, 14:45

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 29 Jun 2009, 14:52

You mean "imul ecx,8", don't you? Wink

29 Jun 2009, 14:52

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 29 Jun 2009, 14:56

Actually "shl ecx, 3". Multiplication in addressing is left shift by scale field in opcode's SIB byte.

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29 Jun 2009, 14:56

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20460
Location: In your JS exploiting you and your system

revolution 29 Jun 2009, 15:01

Code:

pushfd
rept 8 {add esi, ecx}
popfd

29 Jun 2009, 15:01

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 29 Jun 2009, 15:07

manfred wrote:

Actually "shl ecx, 3". Multiplication in addressing is left shift by scale field in opcode's SIB byte.

And why not "sal ecx,3"? Wink

revolution: good one. Smile

But I'd prefer:

Code:

times 8: add esi,ecx

Last edited by Tomasz Grysztar on 29 Jun 2009, 15:08; edited 1 time in total

29 Jun 2009, 15:07

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 29 Jun 2009, 15:07

revolution wrote:

Code:

pushfd
rept 8 {add esi, ecx}
popfd

Not (AFAIK)

Code:

pushfd
times 8 dw 0CE01h
popfd

@up: Because SAL is same opcode as SHL.

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Sorry for my English...

29 Jun 2009, 15:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20460
Location: In your JS exploiting you and your system

revolution 30 Jun 2009, 00:56

Code:

pushfd
rept 2{rept 2\{rept 2\\{add esi,ecx\\}\}}
popfd

Can't do that with times Wink

30 Jun 2009, 00:56

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 30 Jun 2009, 01:16

No?

Code:

times 2 times 2 times 2 add esi, ecx

Assembled as a 24 bytes binary by fasm 1.69.00

30 Jun 2009, 01:16

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20460
Location: In your JS exploiting you and your system

revolution 30 Jun 2009, 01:30

Can it be mixed?

Code:

times 2 rept 4{add esi,ecx}

30 Jun 2009, 01:30

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 30 Jun 2009, 02:23

eskizo wrote:

thanks arigity & Borsuc.

Borsuc, forget my mistake.

no worries I just pointed out that you can't multiply by any value -- so if you run into a compile error, you should know. Saves time looking up or asking Wink

@revolution: times is assembly-time directive, rept is macroinstruction... rept will make multiple "add esi,ecx" during assembly stage as if you wrote them multiple times.

30 Jun 2009, 02:23

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 30 Jun 2009, 06:50

manfred wrote:

Because SAL is same opcode as SHL.

It may be the same opcode, but you've got the assembly language so that you can verbalize you ideas better than just with opcodes.

30 Jun 2009, 06:50

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 30 Jun 2009, 13:43

revolution wrote:

Can it be mixed?

Maybe

Code:

rept 4 { times 2 add esi, ecx }

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30 Jun 2009, 13:43

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20460
Location: In your JS exploiting you and your system

revolution 30 Jun 2009, 14:04

manfred passes the test, well done.

30 Jun 2009, 14:04

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 30 Jun 2009, 15:31

The reason it works is because at assembly time this is what fasm will see:

Code:

times 2 add esi, ecx
times 2 add esi, ecx
times 2 add esi, ecx
times 2 add esi, ecx

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30 Jun 2009, 15:31

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