Kernel: Fix BIOS date/time on hardware

It turns out some BIOS vendors don't support non-BCD date/time mode, but
we were relying on it being available. We no longer do this, but instead
check whether the BIOS claims to provide BCD or regular binary values for
its date/time data.

Kernel/RTC.cpp

@@ -8,11 +8,6 @@ static time_t s_boot_time;
 
 void initialize()
 {
-    u8 cmos_mode = CMOS::read(0x0b);
-    cmos_mode |= 2; // 24 hour mode
-    cmos_mode |= 4; // No BCD mode
-    CMOS::write(0x0b, cmos_mode);
-
     s_boot_time = now();
 }
 
@@ -87,17 +82,39 @@ static unsigned days_in_years_since_epoch(unsigned year)
     return days;
 }
 
+u8 bcd_to_binary(u8 bcd)
+{
+    return (bcd & 0x0F) + ((bcd >> 4) * 10);
+}
+
 void read_registers(unsigned& year, unsigned& month, unsigned& day, unsigned& hour, unsigned& minute, unsigned& second)
 {
     while (update_in_progress())
         ;
 
-    year = (CMOS::read(0x32) * 100) + CMOS::read(0x09);
-    month = CMOS::read(0x08);
-    day = CMOS::read(0x07);
-    hour = CMOS::read(0x04);
-    minute = CMOS::read(0x02);
+    u8 status_b = CMOS::read(0x0b);
+
     second = CMOS::read(0x00);
+    minute = CMOS::read(0x02);
+    hour = CMOS::read(0x04);
+    day = CMOS::read(0x07);
+    month = CMOS::read(0x08);
+    year = CMOS::read(0x09);
+
+    if (!(status_b & 0x04)) {
+        second = bcd_to_binary(second);
+        minute = bcd_to_binary(minute);
+        hour = bcd_to_binary(hour & 0x70);
+        day = bcd_to_binary(day);
+        month = bcd_to_binary(month);
+        year = bcd_to_binary(year);
+    }
+
+    if (!(status_b & 0x02) && (hour & 0x80)) {
+        hour = ((hour & 0x7F) + 12) % 24;
+    }
+
+    year += 2000;
 }
 
 time_t now()