Who knows the use of TF(Trap Flag) ?

david · Post by **david** » Thu Jun 12, 2008 4:38 am

There is TF(bit 8 ) in 80X86 CPU.
I can't understand its use.
I think it maybe has relations with Int 01h.

Interrupt Jump Table wrote: Int 01 - CPU-generated - SINGLE STEP
Int 01 - CPU-generated (80386+) - DEBUGGING EXCEPTIONS

Volume 3A_ System Programming Guide wrote: TF Trap (bit 8 ) : Set to enable single-step mode for debugging; clear to
disable single-step mode. In single-step mode, the processor generates a
debug exception after each instruction. This allows the execution state of a
program to be inspected after each instruction. If an application program
sets the TF flag using a POPF, POPFD, or IRET instruction, a debug exception
is generated after the instruction that follows the POPF, POPFD, or IRET.

I wrote some code to set TF.

Code: Select all

	pushf
	pop ax
	or ax, 0100h
	push ax
	popf

But nothing happened.

AJ · Post by AJ » Thu Jun 12, 2008 5:03 am

Hi,

Have you set up an IDT handler for the debug exception (exception 3) which does more than just return to the next instruction?

Cheers,
Adam

df · Post by df » Thu Jun 12, 2008 2:04 pm

turning the TF on will tell the cpu to trigger interrupt 0x01 before the instruction is run.

david · Post by **david** » Thu Jun 12, 2008 8:28 pm

I found int 01h's ISR entry point is the same with int 03h's(F000:E819).
I disassemblyed the ISR code.
They only set 0:46B's value.

Code: Select all

push ds
push ax
push cx
mov ax, 40h
mov ds, ax
jmp 0EFBDh
.....
0EFBD:
mov ah, 0FFh
mov [6Bh], ah
pop cx
pop ax
pop ds
iret

I could not found any useful things.

df · Post by df » Fri Jun 13, 2008 9:15 am

you have to provide your own ISR routine for it to do anything usefull.

when this gets called you will have on top of stack the cs:ip of the instruction to be executed, which you can modify, etc. its good for single step debugging.

david · Post by **david** » Sun Jun 15, 2008 8:14 am

df wrote:you have to provide your own ISR routine for it to do anything usefull.

when this gets called you will have on top of stack the cs:ip of the instruction to be executed, which you can modify, etc. its good for single step debugging.

What is the number of my own ISR? int 1h or other?

suthers · Post by **suthers** » Sun Jun 15, 2008 9:28 am

You can change any Interrupt to whatever you want by changing there entry in the IVT/IDT (Depending on what mode you're in...), so change INT 1 to whatever you want, the point of the TF is to have an interrupt that can give you tailored more targeted debug info for every single operation....
Jules