Timer related problems
Posted: Sun Jan 22, 2012 12:52 am
Hi all,
I have some firmware that I want to run. But, unfortunately, there is no original loader for it. So I decided to write my own wrapper (lets say loaded via BIOS) that would make required initialization and load/start firmware.
I solved problems of loading firmware, setting up GDT/IDT and starting firmware. But faced problems inside firmware itself, that are caused of missing initialization I think.
This some sort of timer calibration loop (or something like this):
Before this piece of code timer initialized to 1000Hz, square wave, channel 0.
Currently if we disable interrupts (cli) - this code will loop at loop2 forever. If I put empty IRQ0 handler and enable interrupts - then this code will loop forever on loop1 (because my handler just 'out 0x20, 0x20' - so IRQ0 marked as serviced, and read from 0x20 port will return bit0=0)
I suspect that IRQ0 handler and whole system should be somehow dependent on 'out 0x80, 0x61'. So the IRQ will be serviced only after this command block.
But in Intel ICHx documentation this mostly a read-only port
One thing that I found, that could be related to operation:
If someone had seen such code before - please help to understand.
I have some firmware that I want to run. But, unfortunately, there is no original loader for it. So I decided to write my own wrapper (lets say loaded via BIOS) that would make required initialization and load/start firmware.
I solved problems of loading firmware, setting up GDT/IDT and starting firmware. But faced problems inside firmware itself, that are caused of missing initialization I think.
This some sort of timer calibration loop (or something like this):
Code: Select all
xor ecx, ecx
xor ebx, ebx
jmp short loop1_entry
loop1:
inc ecx
loop1_entry:
mov edx, 20h
in al, dx
test al, 1
jz short loop1
mov eax, 80h
mov dl, 61h
out dx, al
jmp short loop2_entry
loop2:
inc ecx
loop2_entry:
mov edx, 20h
in al, dx
test al, 1
jnz short loop2
inc ebx
cmp ebx, 99
jle short loop1_entry
mov dx, 10000
lea eax, [ecx+5000]
mov ecx, edx
xor edx, edx
div ecx
Currently if we disable interrupts (cli) - this code will loop at loop2 forever. If I put empty IRQ0 handler and enable interrupts - then this code will loop forever on loop1 (because my handler just 'out 0x20, 0x20' - so IRQ0 marked as serviced, and read from 0x20 port will return bit0=0)
I suspect that IRQ0 handler and whole system should be somehow dependent on 'out 0x80, 0x61'. So the IRQ will be serviced only after this command block.
But in Intel ICHx documentation this mostly a read-only port
One thing that I found, that could be related to operation:
So writing 0x80 to 0x61 should "Reset Timer 0 Output Latch"... But I have no clue how system and handler should be initialized to honor this operation.Port 061 - 8255A-5 Port B
1xxx xxxx 0=Keyboard enable, 1=Keyboard acknowledge
PS/2: Write: Reset Timer 0 Output Latch (IRQ0)
Read: Parity check (1=parity check occured).
If someone had seen such code before - please help to understand.