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Task Switch: JMP DWORD AX:0x12345678 doesn't work
Posted: Mon Aug 18, 2008 11:21 am
by sevobal
Hey everyone!
I've tried to make a hardware based task switch. First of all, I load a selector into ax:
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mov esi,[tasking_current] ;base of a task table entry
add esi, 0x00000002 ; TSS selector for a task
mov word ax, [es:esi] ; Now the selector is in ax
Now ax holds a selector of a gdt entry which points to a tss of a task. Now, all I have to do is making a far jmp using this selector as segment. Offset can be a random value, because it will be ignored. So I tried this:
But the assembler (nasm) doesn't like it. So how can I handle it? I need ax (or it's value) to do a correct jump.
Re: Task Switch: JMP DWORD AX:0x12345678 doesn't work
Posted: Mon Aug 18, 2008 2:45 pm
by ru2aqare
Try this:
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push eax ; ax contains your TSS selector
push 0 ; offset is ignored
jmp far ptr [esp+0]
add esp, 8 ; if you switch back to this task, remove the dwords pushed onto the stack
Or you can use self-modifying code, like this:
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mov [offset dummy_label + 1], ax
dummy_label:
db 0EAh ; opcode for far jump
dw 0 ; selector will be plugged in here
dd 12345678h ; offset
Re: Task Switch: JMP DWORD AX:0x12345678 doesn't work
Posted: Mon Aug 18, 2008 6:03 pm
by Brendan
Hi,
sevobal wrote:But the assembler (nasm) doesn't like it. So how can I handle it? I need ax (or it's value) to do a correct jump.
It's the CPU that wouldn't like it...
You could try something like this:
Code: Select all
mov esi,[tasking_current] ;base of a task table entry
add esi, 0x00000002 ; TSS selector for a task
jmp far dword [es:esi - 4]
This works because the offset is ignored, so we can pretend that "[es:esi-4]" contains a 4 byte offset (that's ignored) followed by the 2 byte selector.
Cheers,
Brendan