how to set time and date in rtc ds12c887 RAM

I am making a digital calender using ds12c887 and at89c52. I can read the time from rtc properly. But i cannot write to rtc RAM.

code fragment i am using for writing is as follows:
I am using keil compiler.

void rtc_set()
{
    delay_ms(20);   // it provide 20ms delay
    PBYTE[0x0a]=0x20;    //make oscilator on
    PBYTE[0x0b]=0x83;   // disable updates
    PBYTE[0x04]=0x16;   //set hours
    PBYTE[0x02]=0x58;   //set mins
    PBYTE[0x00]=0x55;   // set secs
    PBYTE[0x07]=0x19;   //set date
    PBYTE[0x08]=0x10;   //set month
    PBYTE[0x09]=0x04;   //set year
    PBYTE[0x0b]=0x03;   //enable updates
}

Plz help me to find the problem in my code.
which settings are required to write to external ram in keil?