PMON - IdentifyFlashType

["erras stefan" <stefan.erras@dallmeier-electronic.com>]

Can anybody of you great guys can explain me the folling function which
I found in PMON:

BOOL IdentifyFlashType()
{
        ULONG   mId, dId;

        WRITE_REGISTER_ULONG(FLASH_START, 0x90909090);
        mId = READ_REGISTER_ULONG(FLASH_START);
        WRITE_REGISTER_ULONG(FLASH_START, 0x90909090);
        dId = READ_REGISTER_ULONG(FLASH_START+4);
        
        if ((mId == 0x00890089) && (dId == 0x00180018)) 
        {
                FlashType = 1;          // J3 flash
                WRITE_REGISTER_ULONG(FLASH_START, 0x00ff00ff);
                printf("J3 32MB flash found on this platform\r\n");
                return TRUE;
        }       
        else if ((mId == 0x00890089) && (dId == 0x00170017)) 
        {        
                FlashType = 1;          // J3 flash
                WRITE_REGISTER_ULONG(FLASH_START, 0x00ff00ff);
                printf("J3 16MB flash found on this platform\r\n");
                return TRUE;
        }
        return FALSE;
}

I have to know what the WRITE_REGISTER_ULONG and READ_REGISTER_ULONG
functions do affect.
Why do they write 0x90909090 or 0x00ff00ff to FLASH_START?
Whats the meaning of mId and dId?

Thank you all in advance for your help!!!

Greetings
Stefan