Re: Interrupt acknowledge Query

["Thomas J. Merritt" <tjm@codegen.com>]

|<><><><><> Original message from Saravanan A  <><><><><>
|Dear PCI Experts
|
|I have few questions ??
|
|1.When interrupt Acknowledge command is issued by pci master?? and what
|is significance of the vector 
|returned by  the interrupt controller.

The Interrupt Acknowledge command is a relatively obsolete feature of the
PCI bus.  It exists primarily because the 8259 interrupt controllers sat
on the ISA bus behind the PCI-ISA bridge.  The 80486 processor would
generate an Interrupt Acknowledge cycle that cause passed to the 8259.
The 8259 then returned the 8-bit interrupt vector that was previously
programmed into it.  The 80486 would then use that value to calculate
the address of the interrupt service routine.

In more modern systems the interrupt controller resides in our close to
the processor, and the 8259 cores are only present for software compatibility.
If you do not have an 8259 in your design it is probably best to avoid
interrupt acknowledge cycles.

|2.Can INT pins  be bi-directional?? If so , when it can be
|bi-directional pins??

The pins are generally open-drain so can be read by a device that also
controls the INT pin.  For it to be truely bi-directional you would
have to control all parts of the system.

|3.What are minimum functions that should be supported for a multi
|-function device  or 
|when we say it is a multi-function device ??

You need to implement the configuration space for the eight functions of
a multi-function device.  Any of configuration registers that are read 
only can be shared between functions.  Unimplemented functions should
return all ones for the vendor and device id.

|Your suggestions/guidance is always appreciated
|
|Thanks and rgds
|
|Saran
|

TJ Merritt
tjm@codegen.com			System Software Design and Embedded Firmware
1-415-834-9111			    Development and Consulting