BIOS protection by Holly

[OzOnE]

Ok, some more thoughts (hope this is OK to post?)...

Код:

    /* initialize ROM checksum */
    *HW32(0xa05f74e4) = 0x001fffff;

    /* switch to double precision (for 64bit copying below) */
    reg[FPSCR] = 0x00140001;

    /* Copy various parts from ROM to RAM */
    /* This is done for two reasons:
     * First, code and data must be available to be executed from RAM
     * secondly, the data read from ROM are passed through some type
     * of "checksum". Unless a specific value is computed, a flag is
     * set to disable the GDROM drive. Sega must have put this in there
     * to "discourage" people from replacing the BootROM with their custom
     * versions!
     */
    src = (uint64_t *)0xa0000000;
    dst = (uint64_t *)0x8c000000;
    for (i = 0; i < 32; i++) {    /* skip the system variables */
        (void)*src++;        
        dst++;
    }
    for (i = 0; i < 2016; i++)     /* copy a0000100-a0004000 to 8c000100 */
        *dst++ = *src++;

    for (i = 0; i < 2048; i++) {    /* skip a0004000 to a0007fff */
        (void)*(src++);
        dst++;
    }
    for (i = 0; i < 258048; i++)     /* copy a0008000-a0200000 to 8c008000 */
        *(dst++) = *(src++);

    /* back to single precision */
    reg[FPSCR] = 0x00040001;

    boot2(reg[DBR]);

The above code was taken from the bottom of this page...
http://www.ludd.luth.se/~jlo/dc/

Could it simply be that the HOLLY chip is checking for something in the chunk of "License Screen" code in the BIOS, from 0x8000 to 0xB3FF?

This is the same chunk of code that it checks against the code in IP.BIN from 0x300 to 0x36FF...
http://mc.pp.se/dc/ip.bin.html

"Note that the code in this area can not be modified. The ROM checks every byte of it against a copy in the ROM, and will not boot the disk if there is a difference."

SWAT - have you tried just putting that chunk of code back in the ROM, then doing the GD activation check by copying only from 0x8000 to 0xB3FF?

I'll try some tests on that to see if it still passes the checksum.

OzOnE