what is the name for instruction like this.

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vbVeryBeginner

Joined: 15 Aug 2004
Posts: 884
Location: \\world\asia\malaysia

vbVeryBeginner 21 Mar 2006, 06:07

recently, i saw Nikolay Petrov code on http://board.flatassembler.net/topic.php?t=4968

Code:

cmp     ecx,[edx+4*(eax+1)]
mov     ecx,[edx+4*(eax+2)] 
mov     [edx+4*(eax+1)],ecx

i didn't know we could code like this in assembly language. so i start wondering, where (any web) got information regarding something like above.
maybe, mov ecx,edx*eax ;would works?

so, i tried something like this

Code:

mov  ecx,[edx+4*((ebx/10)*2)]

it gets assembled, but from olly, i saw
MOV ECX,DWORD PTR DS:[EDX]

i search on intel pdf but didn't found examples like above, so, am wish to know like if anyone got any reference that could tell maybe like.

wat reg we could use, how many reg we could use, are + - * / works? or how many level () we could use, does % works? and so on for instruction like above.

thanks.

21 Mar 2006, 06:07

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 21 Mar 2006, 07:49

Code:

mov  ecx,[edx+4*((ebx/10)*2)]

It doesn't get assembled: Invalid use of symbol

You can only scale memory addresses and they are very limited:

Code:

mov eax,[402000h]               ; Displacement
mov eax,[esi]                   ; Base or Index
mov eax,[esi+402000h]           ; Displacement+(Base or Index)
mov eax,[esi*8+402000h]         ; Base*Scale+Displacement
mov eax,[esi*8+esi]             ; Base*Scale+Index
mov eax,[esi*8+esi+402000h]     ; Base*Scale+Displacement+Index
                                ; While scale can be any of x1, x2, x4, x8
; LEA works the same way but it doesn't access memory
; It only calculates the address and puts it to a register

'/' works only on some (or all) variables or constants YOU define, but you can't divide a register.

21 Mar 2006, 07:49

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8358
Location: Kraków, Poland

Tomasz Grysztar 21 Mar 2006, 07:49

The instruction sample you gave does not assemble with the latest fasm's versions, there was a bug that caused older versions to ignore register divided by some value, while it should signalize an error.

As for the allowed addressing modes, I'd recommend checking out Intel manuals. In general, with 32-bit addressing you can specify address in a form [base+index*scale+displacement], where base and index are registers, scale can be 1,2,4 or 8 and displacement (offset) can be a 32-bit number. fasm is able to do the calculations on expressions with scaled registers, so for example:

Code:

mov ecx,[((edx+ecx*3)+(edx+ecx*5))/2]

gets reduced to

Code:

mov ecx,[edx+ecx*4]

and thus assembles correctly. Also:

Code:

mov eax,[ecx*5]

assembles, as it can be transformed into:

Code:

mov eax,[ecx+ecx*4]

21 Mar 2006, 07:49

vbVeryBeginner

Joined: 15 Aug 2004
Posts: 884
Location: \\world\asia\malaysia

vbVeryBeginner 21 Mar 2006, 16:07

oh yeah, i am using fasm 1.65.8
getting 1.65.17 now

btw, thanks a lot!!
i look at the intel manuals under the MOV section, maybe due to my ignorant, i didn't found any examples showing something like above Crying or Very sad

maybe they are in another section, i check them again later.

21 Mar 2006, 16:07

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8358
Location: Kraków, Poland

Tomasz Grysztar 21 Mar 2006, 16:17

It is not related to MOV or any particular instruction, it's about the addressing modes and it affects almost any instruction. You should find detailed information on this nearer the beginnning of Intel Manuals.

21 Mar 2006, 16:17

Reverend

Joined: 24 Aug 2004
Posts: 408
Location: Poland

Reverend 21 Mar 2006, 20:35

Also I may add that there is one exception. ESP cannot be index register. So it cannot be multiplied and combination like [esp+esp] will not work.

21 Mar 2006, 20:35

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