CeDOS - Commit 076b961a

Time: Switch to RTC interrupt for ticks Previously, the tick counter was increased by the scheduler interrupt. However, this causes changing tick durations when the yield function is called. Instead, we can use the previously unused RTC interrupt, which should as an added benefit have consistent timing across devices.
Celina Sophie Kalus
hello@celinakalus.de
Tue, 02 Jan 2024 19:51:25 +0100
5 files changed, 73 insertions(+), 11 deletions(-)
M kernel/main.c → kernel/main.c 
@@ -2,6 +2,7 @@ #include "drivers/console.h"
 #include "drivers/keyboard.h"
 #include "drivers/graphics.h"
 
+#include "time.h"
 
 #include "sched/sched.h"
 #include "sched/process.h"

@@ -61,6 +62,10 @@ printk("done.\n");
 
     printk("Installing syscalls...");
     syscall_init();
+    printk("done.\n");
+
+    printk("Initializing RTC...");
+    time_init();
     printk("done.\n");
 
     printk("Initializing scheduler...");
M kernel/sched/sched.h → kernel/sched/sched.h 
@@ -11,7 +11,7 @@
 #include "mm/paging.h"
 
 // 11928 ~ 10ms per interval
-#define SCHED_INTERVAL (1193)
+#define SCHED_INTERVAL (11928)
 
 /*!
  * Structure of the process stack when the scheduler is executed.
M kernel/sched/sched.s → kernel/sched/sched.s 
@@ -16,7 +16,6 @@ // pass process stack as arguments
   push %ebx
   push %eax
   xor %eax, %eax
-  call time_tick
   call sched_interrupt_c
   pop %eax
   pop %ebx
M kernel/time.c → kernel/time.c 
@@ -1,5 +1,8 @@
 #include "time.h"
 #include "assembly.h"
+#include "interrupts.h"
+#include "pic.h"
+#include "core.h"
 
 #define RTC_COMMAND  			0x70
 #define RTC_DATA 				0x71

@@ -9,6 +12,7 @@
 #define RTC_REGISTER_A			0x0A
 #define RTC_REGISTER_B			0x0B
 #define RTC_REGISTER_C			0x0C
+#define RTC_REGISTER_D			0x0D
 
 #define RTC_GET_SECONDS			0x00
 #define RTC_GET_MINUTES			0x02

@@ -17,6 +21,11 @@
 #define RTC_GET_DAY 			0x07
 #define RTC_GET_MONTH			0x08
 #define RTC_GET_YEAR			0x09
+
+#define RTC_DISABLE_NMI         0x80
+#define RTC_ENABLE_NMI          0x00
+
+#define RTC_ENABLE_INT8         0x40
 
 int ticks = 0;
 

@@ -30,22 +39,65 @@ }
 
 static uint8_t rtc_get(uint8_t reg) {
     outb(reg, RTC_COMMAND);
+    /*nop();
     nop();
     nop();
+    nop();*/
+    uint8_t res = inb(RTC_DATA);
+    return res;
+}
+
+static uint8_t from_BCD(uint8_t bcd_value) {
+    return (bcd_value >> 4) * 10 + (bcd_value & 0x0F);
+}
+
+static uint8_t rtc_set(uint8_t reg, uint8_t value) {
+    outb(reg, RTC_COMMAND);
+    /*nop();
     nop();
     nop();
-    uint8_t res = inb(RTC_DATA);
-    return (res >> 4) * 10 + (res & 0x0F);
+    nop();*/
+    outb(value, RTC_DATA);
 }
 
 int time_now(datetime_t *buffer) {
-    buffer->year = 2000 + rtc_get(RTC_GET_YEAR);
-    buffer->month = rtc_get(RTC_GET_MONTH);
-    buffer->day = rtc_get(RTC_GET_DAY);
+    buffer->year = 2000 + from_BCD(rtc_get(RTC_GET_YEAR));
+    buffer->month = from_BCD(rtc_get(RTC_GET_MONTH));
+    buffer->day = from_BCD(rtc_get(RTC_GET_DAY));
 
-    buffer->hour = rtc_get(RTC_GET_HOURS);
-    buffer->minute = rtc_get(RTC_GET_MINUTES);
-    buffer->second = rtc_get(RTC_GET_SECONDS);
+    buffer->hour = from_BCD(rtc_get(RTC_GET_HOURS));
+    buffer->minute = from_BCD(rtc_get(RTC_GET_MINUTES));
+    buffer->second = from_BCD(rtc_get(RTC_GET_SECONDS));
 
     return 0;
-}+}
+
+INTERRUPT(rtc_interrupt, frame) {
+    time_tick();
+
+    rtc_get(RTC_REGISTER_C);
+
+    pic2_eoi();
+}
+
+int time_init(void) {
+    install_interrupt(PIC2_IRQ(0x00), rtc_interrupt, 0x18, INT_GATE);
+
+    // disable all interrupts
+    cli();
+
+    uint8_t reg_a = rtc_get(RTC_REGISTER_A | RTC_DISABLE_NMI);
+    rtc_set(RTC_REGISTER_A | RTC_DISABLE_NMI, (reg_a & 0xF0) | RTC_INT_RATE);
+
+    uint8_t reg_b = rtc_get(RTC_REGISTER_B | RTC_DISABLE_NMI);
+    rtc_set(RTC_REGISTER_B | RTC_DISABLE_NMI, reg_b | RTC_ENABLE_INT8);
+
+    reg_b = rtc_get(RTC_REGISTER_B);
+
+    rtc_get(RTC_REGISTER_C);
+    rtc_get(RTC_REGISTER_D);
+    // enable interrupts again
+    sti();
+
+    pic_unmask_interrupt(8);
+}
M kernel/time.h → kernel/time.h 
@@ -1,6 +1,10 @@
 #ifndef __TIME_H
 #define __TIME_H
 
+// frequency =  32768 >> (rate-1);
+// rate at least 2 (~8kHz)
+#define RTC_INT_RATE 2
+
 typedef struct {
     int year;
     int month;

@@ -10,6 +14,8 @@ int hour;
     int minute;
     int second;
 } datetime_t;
+
+int time_init(void);
 
 void time_tick(void);