add lab answers

labcodes_answer/lab6_result/kern/driver/clock.c

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+#include <x86.h>
+#include <trap.h>
+#include <stdio.h>
+#include <picirq.h>
+
+/* *
+ * Support for time-related hardware gadgets - the 8253 timer,
+ * which generates interruptes on IRQ-0.
+ * */
+
+#define IO_TIMER1           0x040               // 8253 Timer #1
+
+/* *
+ * Frequency of all three count-down timers; (TIMER_FREQ/freq)
+ * is the appropriate count to generate a frequency of freq Hz.
+ * */
+
+#define TIMER_FREQ      1193182
+#define TIMER_DIV(x)    ((TIMER_FREQ + (x) / 2) / (x))
+
+#define TIMER_MODE      (IO_TIMER1 + 3)         // timer mode port
+#define TIMER_SEL0      0x00                    // select counter 0
+#define TIMER_RATEGEN   0x04                    // mode 2, rate generator
+#define TIMER_16BIT     0x30                    // r/w counter 16 bits, LSB first
+
+volatile size_t ticks;
+
+/* *
+ * clock_init - initialize 8253 clock to interrupt 100 times per second,
+ * and then enable IRQ_TIMER.
+ * */
+void
+clock_init(void) {
+    // set 8253 timer-chip
+    outb(TIMER_MODE, TIMER_SEL0 | TIMER_RATEGEN | TIMER_16BIT);
+    outb(IO_TIMER1, TIMER_DIV(100) % 256);
+    outb(IO_TIMER1, TIMER_DIV(100) / 256);
+
+    // initialize time counter 'ticks' to zero
+    ticks = 0;
+
+    cprintf("++ setup timer interrupts\n");
+    pic_enable(IRQ_TIMER);
+}
+