What does instruction `shl 8` do?

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acel

Joined: 22 Jul 2012
Posts: 8

acel

I saw this instruction in fasm source, what's the target of this instruction? Usually this op (shl) contains 2 operand, why the target is ignored by fasm?

02 Oct 2012, 13:41

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18472
Location: In your JS exploiting you and your system

revolution

Where do you see that?

02 Oct 2012, 14:11

acel

Joined: 22 Jul 2012
Posts: 8

acel

Sorry for the confusion, the full instruction is actually `mov eax, 1 shl 8`, so shr here is calculated by the assembler instead of being included in the instruction encoding?

02 Oct 2012, 14:35

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly

Yes, it is calculated by the assembler (i.e. it assembles `mov eax, 256`).

02 Oct 2012, 14:56

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 18472
Location: In your JS exploiting you and your system

revolution

In this case SHL is not used as an instruction, it is a numeric operator. The instruction in the above line is actually MOV.

02 Oct 2012, 15:07

AsmGuru62

Joined: 28 Jan 2004
Posts: 1440
Location: Toronto, Canada

AsmGuru62

And here is a small macro, which creates a color in Win32 using this feature:

Code:

macro SetRGB dest,R,G,B
{
       mov     dest, (B shl 16) or (G shl 8) or R
}

...

;
; ECX = RGB (32, 87, 228)
;
SetRGB    ecx, 32, 87, 228

02 Oct 2012, 16:41

uart777

Joined: 17 Jan 2012
Posts: 369

uart777

How about a runtime version? Assembler can resolve constant subexpressions.

Code:

macro RGB r, g, b {
mov eax, r   ; 00.00.00.RR
shl eax, 16  ; 00.RR.00.00
mov ecx, g   ; 00.00.00.GG
shl ecx, 8   ; 00.00.GG.00
or eax, ecx  ; 00.RR.GG.00
mov edx, b   ; 00.00.00.BB
or eax, edx  ; 00.RR.GG.BB
}

06 Oct 2012, 21:50

uart777

Joined: 17 Jan 2012
Posts: 369

uart777

Just to elaborate. Assemblers resolve constant expressions to one immediate value. When you write - add eax, (1+2+3)*2 - assembler sees it as "add eax, 12" and FASM uses the 5 byte instruction: add eax, i32 ; 05 i32

07 Oct 2012, 01:52

AsmGuru62

Joined: 28 Jan 2004
Posts: 1440
Location: Toronto, Canada

AsmGuru62

Come to think of it - runtime version was never needed for me.
Once the colors are set by default I use the ChooseColor function and it provides a color
as 32 bit value, so there was no need to make it from RGB pieces.

07 Oct 2012, 01:58

uart777

Joined: 17 Jan 2012
Posts: 369

uart777

AsmGuru: Depends on what you're doing. In art/image/game/graphics software, there are many situations where we need to alter pixels/colors directly and calculate RGBs at runtime using registers and/or memory operands (which your macro won't accept).

07 Oct 2012, 13:56

AsmGuru62

Joined: 28 Jan 2004
Posts: 1440
Location: Toronto, Canada

AsmGuru62

I understand.

07 Oct 2012, 16:11

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