Dolphin OS timing initialization
--------------------------------


BOOTROM:
=-=-=-=-

Bootrom will call __SyncTime() in its main() before gfx menu.


typedef s64     OSTime;
typedef s32     OSTick;

OS timer clock period is 1/12 of Gekko CPU core clock :
486 mhz / 12 = 40.5 mhz


// update time-base registers by RTC (real-time clock) value
// RTC is number of seconds, since 01 January 00:00:00 2000 (current epoch)
void __SyncTime()
{
    OSTick  rtcValue;
    OSSram* sram = __OSLockSram();

    if( __OSGetRTC(&rtcValue) == TRUE )
    {
        // counterBias can be negative value
        rtcValue += (OSTick)sram->counterBias;
        __OSSetTime( (OSTime)rtcValue * OS_TIMER_CLOCK );
    }

    __OSUnlockSram(0);
}

// here is SRAM structure
typedef struct
{
    u16     checkSum;           // SRAM byte checksum (discovered by Costis)
    u16     checkSumInv;        // inverted checksum (see YAGCD)
    u32     ead0;
    u32     ead1;
    s32     counterBias;        // now we already know meaning of this one too :)
    s8      displayOffsetH;     // used by VI library
    u8      ntd;
    u8      language;
    u8      flags;
    u8      dummy[44];
} SRAM;


OS:
=-=

__OSSetTime is used only in bootrom, to set time-base registers according
to battery-backed RTC value.


// OS low-memory variable
OSTime __OSSystemTime AT_ADDRESS(OS_BASE_CACHED | 0x30D8);

void __OSSetTime(OSTime newValue)
{
    BOOL enabled = OSDisableInterrupts();

    // system time keep its value during soft resets (?)
    __OSSystemTime += newValue - OSGetTime();   // I'm unsure of that
                                                // 64 bit math is hard to disasm

    __SetTime(newValue);

    EXIProbeReset();
    OSRestoreInterrupts(enabled);
}

// use undocumented SPR registers, to set time-base
// SPR[284] = TBL, to write
// SPR[285] = TBU, to write
void __fastcall __SetTime(OSTime time)
{
    __asm   li      r5, 0
    __asm   mtspr   284, r5     // clear TBL value (to avoid TBU++)
    __asm   mtspr   285, r3     // set TBU
    __asm   mtspr   284, r4     // set TBL
    __asm   blr
}


Emulation issues : Games dont like real TBR value. At least they work better,
when TBR is set to 0 before game start. Unemulated TBR is only affect on memcard
save date, AFAIK.

To convert TBR value to real-time I use Windows file time utility :

// covert GC time to human-usable time string;
// example output : "30 Jun 2004 3:06:14:127"
char * OSTimeFormat(u64 tbr)
{
    // FILETIME - number of 1/10000000 intervals, since Jan 1 1601
    // GC time  - number of 1/40500000 sec intervals, since Jan 1 2000
    // To convert GCTIME -> FILETIME :
    //      1: adjust GCTIME by number of 1/10000000 intervals
    //         between Jan 1 1601 and Jan 1 2000.
    //      2: assume X - 1/10000000 sec, Y - 1/40500000 sec,
    //         FILETIME = (GCTIME * Y) / X

    // coversion GCTIME -> FILETIME
    #define MAGIK 0x0713AD7857941000
    f64 x = 1.0 / 10000000.0, y = 1.0 / 40500000.0;
    tbr += MAGIK;
    u64 ft = (u64)( ((f64)(s64)tbr * y) / x );
    FILETIME fileTime; SYSTEMTIME sysTime;
    fileTime.dwHighDateTime = (u32)(ft >> 32);
    fileTime.dwLowDateTime  = (u32)(ft & 0x00000000ffffffff);
    FileTimeToSystemTime(&fileTime, &sysTime);

    // format string
    static char *mnstr[12] =
        { "Jan", "Feb", "Mar", "Apr",
          "May", "Jun", "Jul", "Aug",
          "Sep", "Oct", "Nov", "Dec"
        };
    static char gcTime[256];
    sprintf( gcTime, "%i %s %i %02i:%02i:%02i:%03i",
             sysTime.wDay, mnstr[sysTime.wMonth - 1], sysTime.wYear,
             sysTime.wHour, sysTime.wMinute, sysTime.wSecond, sysTime.wMilliseconds );
    return gcTime;
}

Although this conversion is not perfect :O) Play with MAGIK value to find
suitable bias adjustment.

Tell me please, if you find better way to format GC time.




Document by org <ogamespec@gmail.com>
RTC: 11 Dec 2004 :)