lesson 20, preliminar

Author: Carlos Fenollosa

19-interrupts-irqs/README.md

@@ -2,9 +2,6 @@
 
 **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, 
@@ -14,7 +11,8 @@ 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 
+The code for remapping the PICs is weird and includes
+some masks, 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.
@@ -30,5 +28,10 @@ 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: 
+Now back to C code, to write the `irq_handler()` in `isr.c`. It sends some
+EOIs to the PICs and calls the appropriate handler, which is stored in an array
+named `interrupt_handlers` and defined at the top of the file. The new structs
+are defined in `isr.h`. We will also use a simple function to register 
+the interrupt handlers.
+
+That was a lot of work, but now we can define our first IRQ handler!

20-interrupts-timer/Makefile

@@ -0,0 +1 @@
+../19-interrupts-irqs/Makefile

20-interrupts-timer/README.md

@@ -0,0 +1,21 @@
+
+Everything is now ready to test our hardware interrupts.
+
+Timer
+-----
+
+The timer is easy to configure. First we'll declare an `init_timer()` on `cpu/timer.h` and
+implement it on `cpu/timer.c`. It is just a matter of computing the clock frequency and
+sending the bytes to the appropriate ports.
+
+## se printa gibberish, pq? mirar primero si se arregla con un kprint_int
+## yo tenia una funcion que printaba enteros??!!! pero al rever. hacerla ahora bien y
+## en el proximo episodio limpiar codigo y crear una libc
+
+
+Finally, go back to the `kernel/kernel.c` and do two things. Enable interrupts again
+(very important!) and then initialize the timer interrupt.
+
+Go `make run` and you'll see the clock ticking! Unfortunately we are not printing the correct values
+on screen, so we'll go ahead to `drivers/screen.c` and add a new `kprint_int()` method, also declaring
+it on `drivers/screen.h`

20-interrupts-timer/boot

@@ -0,0 +1 @@
+../19-interrupts-irqs/boot

20-interrupts-timer/cpu/idt.c

@@ -0,0 +1,17 @@
+#include "idt.h"
+#include "../kernel/util.h"
+
+void set_idt_gate(int n, u32 handler) {
+    idt[n].low_offset = low_16(handler);
+    idt[n].sel = KERNEL_CS;
+    idt[n].always0 = 0;
+    idt[n].flags = 0x8E; 
+    idt[n].high_offset = high_16(handler);
+}
+
+void set_idt() {
+    idt_reg.base = (u32) &idt;
+    idt_reg.limit = IDT_ENTRIES * sizeof(idt_gate_t) - 1;
+    /* Don't make the mistake of loading &idt -- always load &idt_reg */
+    __asm__ __volatile__("lidtl (%0)" : : "r" (&idt_reg));
+}

20-interrupts-timer/cpu/idt.h

@@ -0,0 +1,39 @@
+#ifndef IDT_H
+#define IDT_H
+
+#include "types.h"
+
+/* Segment selectors */
+#define KERNEL_CS 0x08
+
+/* How every interrupt gate (handler) is defined */
+typedef struct {
+    u16 low_offset; /* Lower 16 bits of handler function address */
+    u16 sel; /* Kernel segment selector */
+    u8 always0;
+    /* First byte
+     * Bit 7: "Interrupt is present"
+     * Bits 6-5: Privilege level of caller (0=kernel..3=user)
+     * Bit 4: Set to 0 for interrupt gates
+     * Bits 3-0: bits 1110 = decimal 14 = "32 bit interrupt gate" */
+    u8 flags; 
+    u16 high_offset; /* Higher 16 bits of handler function address */
+} __attribute__((packed)) idt_gate_t ;
+
+/* A pointer to the array of interrupt handlers.
+ * Assembly instruction 'lidt' will read it */
+typedef struct {
+    u16 limit;
+    u32 base;
+} __attribute__((packed)) idt_register_t;
+
+#define IDT_ENTRIES 256
+idt_gate_t idt[IDT_ENTRIES];
+idt_register_t idt_reg;
+
+
+/* Functions implemented in idt.c */
+void set_idt_gate(int n, u32 handler);
+void set_idt();
+
+#endif