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I'm having trouble understanding this line on the wiki for the 8259 pic initialisation:

outb(PIC1_DATA, 4); // ICW3: tell Master PIC that there is a slave PIC at IRQ2 (0000 0100)

It say's that to tell the master pic that the slave is at irq2, you for some reason have to send 4 to it instead. The manual vaguely mentions a "PIC number" to send as icw3 which I assumed meant the irq number, i.e. 2 not 4. I've looked in the technical reference for the IBM AT and it doesn't mention anything that it's doing that could cause this and at this point I just have no idea what it could be

I think each bit represents an IRQ line - 100b sets the second bit (counting from zero) to represent which IRQ line the PIC2 connects to.

sandras wrote: ↑Wed Jul 09, 2025 3:22 pm I think each bit represents an IRQ line - 100b sets the second bit (counting from zero) to represent which IRQ line the PIC2 connects to.

Oh right that does make sense. Documentation was vague so I thought it meant the actual IRQ number

JohnSwag wrote: ↑Wed Jul 09, 2025 2:39 pm https://en.wikipedia.org/wiki/Binary_number

Given that I said that my struggle was with why the value was 4 and not 2 should tell you that I understand how to read binary numbers. I was looking for help about the information that the PIC was expecting due to finding the documentation difficult to read.

It is possible to have more than one slave attached to a master. It would be difficult to represent this with a single number so a bitmap is used. Much like the mask enabling/disabling individual interrupts.