long jump failure in MASM5.0 of windows XP OS

[Jerry]

Supposing there is a section code at 0x00600h, I want long jump to there in my programme as follows. But in fact, I can't do that. Why?

Code: Select all

code	segment
	org	100h
	assume	cs: code
start:
	jmp	begin
dest	dw	600h
begin:
	xor	ax, ax
	mov	ds, ax
	mov	si, dest
	jmp	dword ptr [si]
code	ends
end	start

I take a screenshot as follows.

[Combuster]

Because you are using colors, and you are not using code tags, so I can't possibly see what your code is?

Also, you are not debugging the code you wrote. The last few lines assemble to:

Code: Select all

0000000A  BE0401            mov si,0x104  ; not that exact number, but close
0000000D  66FF24            jmp dword near [si]

[b.zaar]

begin:
xor ax, ax
mov ds, ax
mov si, dest
jmp dword ptr [si]

You set DS to 0 (zero) before reading your data at SI so unless your code is loaded at 0x0000:0100 you will read garbage.

[Casm]

For a far jump (which is what dword implies), you need to give both an offset and a segment address - for example,

dest dw 600h, 0

But you have only specified an offset. If you don't know what the seg address should be at compile time, you will need to load it dynamically. For example:

xor ax,ax
mov dest[2], ax

An assume for the ds register probably wouldn't be out of place either: assume ds:code

Code: Select all

code   segment
   org   100h
   assume   cs: code, ds:code

start:
 jmp   begin

dest   dw   600h, 0     ;or whatever seg address you want

begin:
xor   ax, ax
mov   ds, ax
mov   si, offset dest    ;(or alternatively lea si, dest)
jmp   dword ptr [si]

code   ends
end   start

[iLewis]

MASM of ANY version does not natively support long jumps. you must either hard code them or do this

Code: Select all

.data
FarDesc dd 0 ; this is the address you want to jump to
dw 10h ; this is the CODE SEGMENT... if your in userland, best of to set this to the current CS value

.code
----- snip -----

jmp 	fword ptr FarDesc