Little new to bootloader development. Quick initial question

[devsau]

I read through the wiki entries and I just had a few questions that I don't think were covered anywhere.

First is, upon execution of loader at 7c00, the IF flag is clear correct? I would assume so based upon the fact that we need to setup stack segment, but just to be clear.

Second question. For PIC mode (or virtual wire for backward compatibility) I see that the BIOS sets the vector offset of the master PIC to 0x8, but would that IRQ not conflict with the double fault exception as well? Should I just remap it again myself?

3rd and final I am familiar with APIC programming due to several past projects I worked on, one thing I didn't find covered with the 8259 PIC is interrupt priority. Obviously interrupts are masked until we write to EOI, but is there a priority level based upon the vector number?

Thanks.

[Yoda]

First is, upon execution of loader at 7c00, the IF flag is clear correct?

You should not make assumptions about IF flag in this case.

I would assume so based upon the fact that we need to setup stack segment, but just to be clear.

You don't need to disable interrupts to setup stack. Just load SS, then SP. Every loading of SS automatically disables interrupts for the next instruction.

[Mikemk]

I read through the wiki entries and I just had a few questions that I don't think were covered anywhere.

Actually all three of these are easy to find.

First is, upon execution of loader at 7c00, the IF flag is clear correct? I would assume so based upon the fact that we need to setup stack segment, but just to be clear.

You shouldn't make any assumptions about hardware whatsoever.

Second question. For PIC mode (or virtual wire for backward compatibility) I see that the BIOS sets the vector offset of the master PIC to 0x8, but would that IRQ not conflict with the double fault exception as well? Should I just remap it again myself?

Yes, that's exactly what you do. Why do you think you disable interrupts when entering protected mode?

3rd and final I am familiar with APIC programming due to several past projects I worked on, one thing I didn't find covered with the 8259 PIC is interrupt priority. Obviously interrupts are masked until we write to EOI, but is there a priority level based upon the vector number?

Yes. In real/pmode, it's irq 0 first, and I don't remember or feel like looking up the rest. In long mode, it starts as that but is changeable.

Thanks.

You're welcome

[Griwes]

Second question. For PIC mode (or virtual wire for backward compatibility) I see that the BIOS sets the vector offset of the master PIC to 0x8, but would that IRQ not conflict with the double fault exception as well? Should I just remap it again myself?

Yes, that's exactly what you do. Why do you think you disable interrupts when entering protected mode?

That's the far less relevant part of the problem. See differences between IVT and IDT - specifically, firing *any* interrupt before IDT is set up will cause #GP, because IDT limit is 0, which will cause #GP causing #DF because IDT limit is 0, which will cause #GP causing #TF, because IDT limit is 0.

Get your facts straight before answering next time.

Second question. For PIC mode (or virtual wire for backward compatibility) I see that the BIOS sets the vector offset of the master PIC to 0x8, but would that IRQ not conflict with the double fault exception as well? Should I just remap it again myself?

Yes, you should, it is mentioned in about every tutorial or resource that mentions PIC and interrupts.

[Brendan]

Hi,

I read through the wiki entries and I just had a few questions that I don't think were covered anywhere.

First is, upon execution of loader at 7c00, the IF flag is clear correct? I would assume so based upon the fact that we need to setup stack segment, but just to be clear.

The IF flag should actually be set (IRQs enabled); and there should be a stack. The problem is that you don't know how large the stack is or where the stack is. For a (deliberately "worst case") example; the BIOS might use exactly 1234 bytes for its IRQ handlers and might have a 1234 byte stack just above your boot code, so if you push anything on the stack your code gets trashed as soon as an IRQ occurs. Of course a far more likely problem is that your boot code wants to load something (e.g. a kernel) from disk and trashes its own stack.

To fix the "unknown stack" problem you have to setup a known stack.

You don't need to disable interrupts to setup stack. Just load SS, then SP. Every loading of SS automatically disables interrupts for the next instruction.

For 8086 the CPU doesn't disable IRQs for the instruction following a load of SS (only later CPUs do that). My normal approach is to assume the CPU might be an ancient 8086 (and explicitly disable IRQs when setting up a known stack); then (later) check if the CPU is too old for the OS (and display an error and refuse to boot if it is). Of course this may be considered "excessively cautious".

3rd and final I am familiar with APIC programming due to several past projects I worked on, one thing I didn't find covered with the 8259 PIC is interrupt priority. Obviously interrupts are masked until we write to EOI, but is there a priority level based upon the vector number?

The IRQ priority is based on "PIC input number", and is fixed/hard-wired; and due to the way the slave PIC is connected to the master PIC (via. "PIC input number 2") it's a little strange. The priority of IRQs (from highest priority IRQ to lowest priority IRQ) is: IRQ 0, 1, 8, 9, 10, 11, 12, 13, 14, 15, 3, 4, 5, 6, 7.


Cheers,

Brendan

[Yoda]

Of course this may be considered "excessively cautious".

Yes, this is really excessive caution because, first, you will hardly find working PC/XT, second, even if you find it, you won't blame modern OS if it will just hang instead of spitting error message about your ancient hardware, and third, it is even too difficult to write good first stage bootloader without using 386 code.

[Brendan]

Hi,

Of course this may be considered "excessively cautious".

Yes, this is really excessive caution because, first, you will hardly find working PC/XT, second, even if you find it, you won't blame modern OS if it will just hang instead of spitting error message about your ancient hardware, and third, it is even too difficult to write good first stage bootloader without using 386 code.

I've been doing "assume 8086 until initial CPU detection" for a long time. It's not too difficult; but if you're going to use 80386+ code anyway, then you should be using the LSS instruction.


Cheers,

Brendan

[Casm]

First is, upon execution of loader at 7c00, the IF flag is clear correct? I would assume so based upon the fact that we need to setup stack segment, but just to be clear.

What does that matter? Clear it anyway - and a cld might be a good idea as well.

Second question. For PIC mode (or virtual wire for backward compatibility) I see that the BIOS sets the vector offset of the master PIC to 0x8, but would that IRQ not conflict with the double fault exception as well? Should I just remap it again myself?

When the 8086 was first launched Intel reserved the first 32 interrupts for their own use, but IBM decided they could ignore that, and for many years they got away with it - until protected mode OSes came along. Nowadays you should reprogram the PIC if you are going to have a protected mode OS.

3rd and final I am familiar with APIC programming due to several past projects I worked on, one thing I didn't find covered with the 8259 PIC is interrupt priority. Obviously interrupts are masked until we write to EOI, but is there a priority level based upon the vector number?

IRQ0 has the highest priority, and IRQ7 the lowest.

[devsau]

thanks for all the responses guys