lesson 21, keyboard input and shell

21-shell/cpu/timer.c

@@ -0,0 +1,23 @@
+#include "timer.h"
+#include "isr.h"
+
+u32 tick = 0;
+
+static void timer_callback(registers_t regs) {
+    tick++;
+}
+
+void init_timer(u32 freq) {
+    /* Install the function we just wrote */
+    register_interrupt_handler(IRQ0, timer_callback);
+
+    /* Get the PIT value: hardware clock at 1193180 Hz */
+    u32 divisor = 1193180 / freq;
+    u8 low  = (u8)(divisor & 0xFF);
+    u8 high = (u8)( (divisor >> 8) & 0xFF);
+    /* Send the command */
+    port_byte_out(0x43, 0x36); /* Command port */
+    port_byte_out(0x40, low);
+    port_byte_out(0x40, high);
+}
+