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I am trying to work around a problem with the different types of jmp and call instructions. My basic goal is to be able to implement an intersegmentry call of jmp. However it has turned out to be a little more complicated than I thought.... If I have the code:


jmp FWORD PTR cs:@F
@@:

masm generates the code:


00401000 2EFF2D07104000  jmp FWORD PTR cs:[401007h]

This is perfectly correct, except the far pointer cs:[401007h] is not the new address of the instruction pointer!!! thus it causes an exception. Instead it is a pointer to an FWORD (as described by the operand size in the instruction) in memory which in turn contains the pointer to the address. So if I had:


jmp FWORD PTR cs:@F
@@:
DWORD jmpaddress
WORD 015fh ;The CS descriptor
jmpaddress:

The code would work perfectly. With this I can perform a far jmp to a different code segment. But this is messy, I am using a pointer to a pointer. Is there a way under masm, to jmp to the location (Imaginary) 0987h:0287614h with the following method:


mov gs,0987h
mov ebx,0287614h
jmp DWORD PTR gs:

I think this is the opcode 09ah, CALL ptr16:32 Call far, absolute, address given in operand

Posted on 2001-05-15 04:05:00 by George

FAR jmps & calls with Masm


 .data
  address dw ?

 .code
  mov dword ptr , 0987h
  mov dword ptr , 0287614h
  jmp 

  end

 try that. i'm not sure if that will work with window, but with
 dos, that's how it is.

Posted on 2001-05-15 21:40:00 by disease_2000

FAR jmps & calls with Masm

Thanks, thats what I was trying to highlight before, that you supply a memory location which contains the address to jmp to. I dont see it as unreasonable, can you do a far jmp, as you would do a short jmp? The address of the jump supplyied in the operand? as such with:


mov gs,0987h           ;Segment to jmp to
mov ebx,0287614h       ;Address in segment to jmp to
jmp DWORD PTR gs: ;

Posted on 2001-05-15 23:04:00 by George

FAR jmps & calls with Masm


 in dos, the obvious solution doesn't work.
 jmp dword far  will generate error. for window, eh..
 i'm not really sure. ;) i think someone with more experience
 with windows can help you. ;) (specially addressing memory).

Posted on 2001-05-16 00:47:00 by disease_2000

FAR jmps & calls with Masm

I suppose one might try push myseg ;16 bits push myaddr ;32 bits, e.g. push dword ptr retf but that looks a bit silly. When masm is too ambiguous I sometimes make up my own opcodes in the form of macros such as (for DOS) jmp32 macro arg db 0EAh dd arg endm --Larry

Posted on 2001-05-17 00:53:00 by Larry Hammick

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