Copy from real space to virtual

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Jin X

Joined: 06 Mar 2004
Posts: 133
Location: Russia

Jin X 26 Mar 2020, 14:23

Hello.
How can I copy the code from real space to virtual one?

I can do it like this:

Code:

  len = $-start
  virtual at $100 as 'com'
    jmp copy
    dbg::
  end virtual
  repeat len
    load x byte from start+%-1
    virtual dbg
      db x
    end virtual
  end repeat
  virtual dbg
    copy:
  end virtual

But it seems like I can do it more optimal.

26 Mar 2020, 14:23

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8434
Location: Kraków, Poland

Tomasz Grysztar 26 Mar 2020, 14:31

Your snippet is compatible with both fasm 1 and fasmg, but I assume that you have fasm in mind. There you can load no more than 8 bytes at a time, so this the best you could do:

Code:

  repeat len / 8
    load x qword from start+(%-1)*8
    virtual dbg
      dq x
    end virtual
  end repeat
  repeat len mod 8
    load x byte from start+(len/8)*8+%-1
    virtual dbg
      db x
    end virtual
  end repeat

With fasmg the are no limits on the length of loaded string, so you could do a simple:

Code:

  load x: len from start
  virtual dbg
    emit len: x
  end virtual

26 Mar 2020, 14:31

Jin X

Joined: 06 Mar 2004
Posts: 133
Location: Russia

Jin X 26 Mar 2020, 14:59

So, I can't get access to real code inside virtual block?

26 Mar 2020, 14:59

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8434
Location: Kraków, Poland

Tomasz Grysztar 26 Mar 2020, 15:04

Jin X wrote:

So, I can't get access to real code inside virtual block?

Oh, I did not realize that this is what you were asking for. You can do it as long as you use "format binary" and you need to use output offsets:

Code:

  start = $%
; ...
  len = $% - start
  virtual at $100 as 'com'
    jmp copy
    dbg::
    repeat len
      load x byte from :start+%-1
      db x
    end repeat
    copy:
  end virtual

This is a feature that has been recently back-ported from fasmg.

26 Mar 2020, 15:04

Jin X

Joined: 06 Mar 2004
Posts: 133
Location: Russia

Jin X 26 Mar 2020, 15:40

Oh but can I write:

Code:

  start:
; ...
  startbyte = start-$$
  len = $ - start
  virtual at $100 as 'com'
    jmp copy
    dbg::
    repeat len
      load x byte from :startbyte+%-1
      db x
    end repeat
    copy:
  end virtual

even if I have old fasm 1 ?

26 Mar 2020, 15:40

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8434
Location: Kraków, Poland

Tomasz Grysztar 26 Mar 2020, 16:35

No, you need fasm 1.73.21 or higher.

26 Mar 2020, 16:35

Jin X

Joined: 06 Mar 2004
Posts: 133
Location: Russia

Jin X 26 Mar 2020, 17:10

Ok, thanks!

26 Mar 2020, 17:10

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