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Hi everyone, I am learning OS development and come across a questsion.

When handling interrupts the book I choose tells me to use 8295A and IDT. I wrote some code according to the book, init 8295A and IDT. The code runs well. (The interrupt can be triggered and the handler is calleld) But the point is I think the code should report error.

The environment is bochs 2.6.7 with Pentium 4 cpu. The cpu uses xAPIC instead of 8295A. So I think the code I wrote for 8295A should not work on the platform, but it does work. I have searched a lot and it seems that they are not compatible.

So why my code for 8295A can run on the xAPIC platform? Does this mean Pentium 4 cpu still have 8295A inside, and how about the newer cpus?

Note: From the Intel manual I know APIC can be disabled but in my code I don't do this and I don't even know what will happen if APIC is disabled. And the code I wrote is compiled into a com file running on freedos since I have not written a complete boot loader. The book tells me right now it is OK to treat it as a com file on freedos.

Sorry if the question is stupid and thanks everyone in advance.

Hi,

angwer wrote:The environment is bochs 2.6.7 with Pentium 4 cpu. The cpu uses xAPIC instead of 8295A. So I think the code I wrote for 8295A should not work on the platform, but it does work. I have searched a lot and it seems that they are not compatible.

It doesn't support xAPIC instead of 8295A - it supports xAPIC/IO APIC in addition to 8295A/PIC chips. When you first boot, the system is using PIC chips (for backward compatibility). To use IO APIC you need to disable the PIC chips and enable/configure the IO APIC.

angwer wrote:Does this mean Pentium 4 cpu still have 8295A inside, and how about the newer cpus?

The IO APIC and/or PIC chip is in the chipset, not in the CPU. Almost all newer chipsets still provide the PIC chips; however there are some (designed for UEFI only with none of the old stuff needed for backward compatibility) which do not have PIC chips.

angwer wrote:Note: From the Intel manual I know APIC can be disabled but in my code I don't do this and I don't even know what will happen if APIC is disabled. And the code I wrote is compiled into a com file running on freedos since I have not written a complete boot loader. The book tells me right now it is OK to treat it as a com file on freedos.

Don't forget that there's 2 separate pieces - the IO APIC (which mostly does what the PIC chip did, but has more IRQ lines and supports multi-CPU properly) in the chipset; plus the local APIC in the CPU (which receives interrupts from the IO APIC, and allows you to send "inter-processor interrupts" from one CPU to another, and has other "miscellaneous" stuff built into it like the local APIC timer).


Cheers,

Brendan

Thanks to Brendan. You do solve my problem. Thanks!!!!!!!!!!!!!!!

By the way, it is the first time to use osdev forum. When I want to reply I treat the report this post as reply this post. Shamed!!! And sorry to Brendan, I report the answer that helps me....