OpenEuphoria

Re: x86 VM

• Posted by Bernie <xotron at PCOM.NET> Nov 11, 2000
• 463 views

On Sat, 11 Nov 2000 12:50:02 -0500, Darth Maul, aka Matt
<uglyfish87 at HOTMAIL.COM> wrote:

>Then would you mind explaining why NASM generates code for a relative jump(
>near)?
>
>Here's some assembly code:
>
>Jmp lbl
>Jmp short lbl
>lbl:
>
>And here's NDISASM's output:
>
>[WINDOWS] C:\EUPHORIA>ndisasm test
>00000000  E90200            jmp 0x5
>00000003  EB00              jmp short 0x5


  First debug can not be told to use a certain instruction
  so it can not be used for comparison. The above code really
  would not be logical because the second instruction wouldn't
  be executed, but I know what you are getting at in
  your example.

  The first instruction uses a E9 jmp RELATIVE 16 and the 16 bit
  address following E9 is the NUMBER of bytes to junp.
  Because 0200 is BYTE SWAPPED so it represents 2 BYTES ( 0002 ).
  The PROGRAM COUNTER ALWAYS point to the next INSTRUCTION to be
  EXECUTED which is ADDRESS 0003. The 2 byte RELATIVE offset is ADDED
  to the PROGRAM COUNTER which advances to the NEXT instruction to
  be executed.

  The second instruction EB jmp RELATIVE 8 will ADD a RELATIVE offset
  of 00 to the PROGRAM COUNTER because REMEMBER it is already pointing
  to ADDRESS 5.

  Take a look at the pete's ASM.E because it ENCODES instructions and
  it might help to write your code.

  Bernie







  be ADDRESS 5.

  In the second instr