How work relativeto ?

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Roman

Joined: 21 Apr 2012
Posts: 2019

Roman 16 Jun 2025, 12:08

fasmw 1.73
i found this code

Code:

if a relativeto b & a > b
  db a - b
end if

Not understand clear what do relativeto b & a > b

16 Jun 2025, 12:08

Roman

Joined: 21 Apr 2012
Posts: 2019

Roman 16 Jun 2025, 12:10

I do this

Code:

a = 30
  b = 4
if a relativeto b & a > b
  mov al, a - b.  ;I get mov al,30-4
end if

But we can do simple if a > b
Why need a relativeto ?

16 Jun 2025, 12:10

macomics

Joined: 26 Jan 2021
Posts: 1197
Location: Russia

macomics 16 Jun 2025, 13:54

Code:

macro movzz a,b {
  display 'movzz a, b'
  if b relativeto ebx
    display 9, 'ebx'
  end if
  if b relativeto edi
    display 9, 'edi'
  end if
  display 13,10
}
label a1 dword at ebx + 5
label a2 dword at edi + 7
virtual at ebx + 6
  a3 dd ?
end virtual
virtual at edi + 8
  a4 dd ?
end virtual
label a5 dword at ebx + edi * 4 + 5
label a6 dword at edi + ebx * 4 + 7
virtual at ebx + esi * 4 + 6
  a7 dd ?
end virtual
virtual at edi + ebp * 4 + 8
  a8 dd ?
end virtual
movzz eax, a1
movzz ecx, a2
movzz edx, a3
movzz eax, a4
movzz eax, a5
movzz ecx, a6
movzz edx, a7
movzz eax, a8

16 Jun 2025, 13:54

Roman

Joined: 21 Apr 2012
Posts: 2019

Roman 18 Jun 2025, 08:39

Thanks.
relativeto not work with xmm registers.

18 Jun 2025, 08:39

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 18 Jun 2025, 09:02

xmm registers can't be used for addressing. So relaiveto wouldn't make sense with them.

Code:

mov eax,[xmm0] ; ???
virtual at xmmo ; ???

18 Jun 2025, 09:02

Roman

Joined: 21 Apr 2012
Posts: 2019

Roman 18 Jun 2025, 21:43

xmm registers can store address!
one xmm register can store four addresses.

Code:

mov eax,mylabel
movd xmm0,eax

if b relativeto xmm
    movd ebx,xmm0
  end if

18 Jun 2025, 21:43

macomics

Joined: 26 Jan 2021
Posts: 1197
Location: Russia

macomics 18 Jun 2025, 22:30

Roman wrote:

xmm registers can store address!

The storage option has nothing to do with it. relativeto checks the set from the addressing block (expressions in [] or rex+modr/m+sib fields)

18 Jun 2025, 22:30

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8466
Location: Kraków, Poland

Tomasz Grysztar 19 Jun 2025, 10:50

The XMM registers can, in fact, occur in a [] address, like in this example straight from the manual:

Code:

    vgatherqps xmm0,[xmm2],xmm3        ; gather two floats

For this reason, XMM registers are among the terms that RELATIVETO is able to distinguish:

Code:

label a at xmm0+3
assert a relativeto xmm0

However, an issue that you likely encountered is that fasm 1 has different lexemes for a register operand and for the same register name used in an address expression. While the second argument to RELATIVETO is always assumed to be an address expression and is forcibly parsed as such, the first one is not (at the time when fasm's parser sees RELATIVETO, the register symbol has already been parsed). But there is a way to ensure that the argument is parsed as an address expression, just prefix it with "+":

Code:

;assert xmm0 relativeto xmm0            ; fails
assert +xmm0 relativeto xmm0

You may need to add a plus symbol before the name of argument in your macro, then.

In case of fasm2, such distinction no longer exists. All registers are defined as linear polynomial terms (with ELEMENT), so even when you use a plain register argument to a simple instruction, it is parsed as an expression (what makes "add eax,ebx+0" accepted syntax, while in fasm 1 it was not).

19 Jun 2025, 10:50

Roman

Joined: 21 Apr 2012
Posts: 2019

Roman 19 Jun 2025, 11:51

Thanks.
I knew it Smile

work fine

Code:

label myrq at +xmm3
        if myrq relativeto xmm3
        display 're'         ;I get 're'
        end if

19 Jun 2025, 11:51

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20759
Location: In your JS exploiting you and your system

revolution 19 Jun 2025, 11:56

Sorry for the misinformation from me, and thank to Tomasz for setting the record straight.

19 Jun 2025, 11:56

Roman

Joined: 21 Apr 2012
Posts: 2019

Roman 19 Jun 2025, 17:17

relativeto what symbols using ?
I mean symbols -,+,&,:,#

19 Jun 2025, 17:17

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