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Mega CD bios dumper

fabiodl

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Hello,
I developed a mega cd bios dumper.

Code is here
https://github.com/fabiodl/mcdBiosDumper
Rom is here

The program dumps the bios content over controller port 2, pin 6 (connect gnd, pin 8, as well). You may use any ttl serial - usb converter like this to dump it to a PC. The github repository includes a simple sample python script to write the file to disk. You need to terminate with ctrl-c when the dump is finished.

I found an undumped bios, labeled epr-14088d with CRC32 b0ba193f
 

MetalliC

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as far as I know, you can't properly dump Sega/MegaCD BIOS ROMs in a "software way", because small part of ROM overlaped by other hardware. so, huh, everything read in this way technically will be bad dumps.

epr-14088d was dumped already, it's CRC32 should be dfa95ee9, you may check it and compare with your readout data.
 

fabiodl

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Do you know exactly where? Having a look at the sega cd documentation, it seems only the ROM is mapped in the range 0x400000~0x420000. (The megadrive manual, however, only reports "sega reserved" for that portion of the address space, so who knows).
 

MetalliC

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afair it is 4bytes at 0x70 (H-INT vector), usually bytes at 0x72 and 0x73 is 0xFF if dumped in software way, but physical ROM chip have other values there.
 

Bad_Ad84

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fabiodl

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Thank you guys. In my case, I start with a flash cart (everdrive) and the mega cd bios is mapped to 0x400000~0x420000.
The vector table should not be a problem, or does the megadrive mirror the vector table to 0x4000XX ?
 

MetalliC

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does the megadrive mirror the vector table to 0x4000XX ?
looks so
I've grabbed epr-14088d dump (CRC32 dfa95ee9), then changed bytes at 72-73 to FF, then byteswapped file - and I got CRC32 b0ba193f binary, same as your's dump.
 

fabiodl

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Thank you, it really seems so. The same happens with mpr-14088h
I now wonder whether it is possible to read those bytes from the sub-cpu then.
 
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