lesson 19, IRQs

19-interrupts-irqs/README.md

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+*Concepts you may want to Google beforehand: IRQs, PIC, polling*
+
+**Goal: Finish the interrupts implementation and CPU timer**
+
+Remap the PIC
+-------------
+
+When the CPU boots, the PIC maps IRQs 0-7 to INT 0x8-0xF
+and IRQs 8-15 to INT 0x70-0x77. This conflicts with the ISRs
+we programmed last lesson. Since we programmed ISRs 0-31, 
+it is standard to remap the IRQs to ISRs 32-47.
+
+The PICs are communicated with via I/O ports (see lesson 15).
+The Master PIC has command 0x20 and data 0x21, while the slave has
+command 0xA0 and data 0xA1.
+
+The code for remapping the PICs is obfuscated, so check 
+[this article](http://www.osdev.org/wiki/PIC) if you're curious.
+Otherwise, just look at `cpu/isr.c`, new code after we set the IDT
+gates for the ISRs. After that, we add the IDT gates for IRQs.
+
+Now we jump to assembler, at `interrupt.asm`. The first task is to
+add global definitions for the IRQ symbols we just used in the C code. 
+Look at the end of the `global` statements.
+
+Then, add the IRQ handlers. Same `interrupt.asm`, at the bottom. Notice
+how they jump to a new common stub: `irq_common_stub` (next step)
+
+We then create this `irq_common_stub` which is very similar to the ISR one.
+It is located at the top of `interrupt.asm`, and it also defines
+a new `[extern irq_handler]`
+
+Now back to C code.
+- Write the `irq_handler` in C: