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Serial - interrupt-pending-flag not being set
Posted: Wed Jul 22, 2009 11:54 pm
by alethiophile
I have written some serial-port code, hopefully as a debugging aid for my other problems. I have tried a busy-wait loop for input, as it shows in the wiki, and it worked just fine. I then wrote an IRQ handler for the serial port, and it broke. Code:
Code: Select all
void serial_irq(registers_t regs) {
if (regs.eax) {} // prevent GCC complaining about unused parameter
printf("serial irq called\n");
u8int iir = inportb(COM1_PORT + 2); // interrupt identification register
printf("%x\n", iir);
if (!(iir & 1)) { // starts here
/* The UART didn't cause this IRQ; in future, we'll probably want to handle this,
but for now just return. */
printf("not a serial irq\n");
return;
} // ends here
if ((iir & 0x0e) != 4) {
/* This isn't a received-data-available interrupt; again, just return. */
printf("not a received-data interrupt\n");
return;
}
u8int read = inportb(COM1_PORT);
// for now just print it
printf("%c", read);
}
When the code from 'starts here' to 'ends here' is commented out, it works as advertised. This code checks the interrupt-pending flag in the interrupt identification register. With the code intact, however, it returns early--and in returning early, doesn't reset the data-available interrupt in the UART, and hence no more interrupts get fired. My question is, why is the interrupt-pending flag not being set? I'm using Qemu.
Edit: syntax
Re: Serial - interrupt-pending-flag not being set
Posted: Thu Jul 23, 2009 9:36 pm
by alethiophile
OK, issue resolved. It turns out that the interrupt-pending flag is zero on true, one on false. Sigh.
Re: Serial - interrupt-pending-flag not being set
Posted: Fri Jul 24, 2009 9:26 am
by Troy Martin
Good to know you figured it out by yourself, but I might recommend that you re-format your code so it's easier to read. For example, here's a tidier formatting for your code:
Code: Select all
void serial_irq(registers_t regs)
{
if (regs.eax); // prevent GCC complaining about unused parameter
printf("serial irq called\n");
u8int iir = inportb(COM1_PORT + 2); // interrupt identification register
printf("%x\n", iir);
if (!(iir & 1))
{
/* The UART didn't cause this IRQ; in future, we'll probably want to handle this,
but for now just return. */
printf("not a serial irq\n");
return;
}
if ((iir & 0x0e) != 4)
{
/* This isn't a received-data-available interrupt; again, just return. */
printf("not a received-data interrupt\n");
return;
}
u8int read = inportb(COM1_PORT);
// for now just print it
printf("%c", read);
}
Whitespace makes everything nice
Also, I hope your IRQ stub in assembly sends the appropriate EOI acknowledgement to the PIC. If it doesn't, you're up a creek.
--Troy
Re: Serial - interrupt-pending-flag not being set
Posted: Fri Jul 24, 2009 9:58 am
by -m32
FYI, there's an "official" way to declare a parameter as "unused" in GCC
Code: Select all
void serial_irq(__attribute__((unused)) registers_t regs)
Re: Serial - interrupt-pending-flag not being set
Posted: Fri Jul 24, 2009 10:00 am
by alethiophile
What happens if you use an unused param?
Re: Serial - interrupt-pending-flag not being set
Posted: Fri Jul 24, 2009 10:01 am
by -m32
alethiophile wrote:What happens if you use an unused param?
The planet will ASPLODE! Zomg!
