Forums

Sega Master System / Mark III / Game Gear
SG-1000 / SC-3000 / SF-7000 / OMV
Home - Forums - Games - Scans - Maps - Cheats - Credits
Music - Videos - Development - Hacks - Translations - Homebrew

View topic - SG-1000 40K Mapping

Reply to topic
Author Message
  • Joined: 11 Mar 2018
  • Posts: 13
  • Location: New Zealand
Reply with quote
SG-1000 40K Mapping
Post Posted: Fri Nov 20, 2020 12:54 am
Hi,

I'm working on an emulator, Snepulator, and have noticed that a number of SG-1000 games have both a 16/32 KiB 'normal' rom dump available, but also a 40 KiB version.

I've done a search, but haven't managed to find any documentation for how this is actually laid out. Eg. How does the memory map look with regards to mirroring?


Cheers,
Joppy
  View user's profile Send private message Visit poster's website
  • Site Admin
  • Joined: 19 Oct 1999
  • Posts: 13435
  • Location: London
Reply with quote
Post Posted: Fri Nov 20, 2020 8:15 am
Some mention here: https://www.smspower.org/forums/post63301#63301

It is 8KB at $0000 (mirrored up to $7fff) and then another 8KB at $8000 (I guess mirrored up to $bfff). The ROM images contain the mirrored data in the “gap”, but I’m not sure if they are accessed.
  View user's profile Send private message Visit poster's website
  • Joined: 11 Mar 2018
  • Posts: 13
  • Location: New Zealand
Reply with quote
Post Posted: Sat Nov 21, 2020 1:54 am
Cheers :3

Turns out the problem when I was running into was that I was padding the ROM size up to a power-of-two (64K), which then conflicted with RAM.

Having tidied that up, and having now also adding mapper support for Loretta no Shouzou, I think I can now run all SG-1000 games :D
  View user's profile Send private message Visit poster's website
  • Site Admin
  • Joined: 08 Jul 2001
  • Posts: 8249
  • Location: Paris, France
Reply with quote
Post Posted: Sat Nov 21, 2020 10:56 am
I am pretty sure some games access the mirrored regions but I forgot which. I thought I did a write-up on that but I cannot find it right now...

When releasing those dumps I was faced with the choice of
A- Releasing them as 16 kb files and requiring every emulator to detect/support this specific mapping layout.
B- Releasing them as 40 kb files with the mirrored data.

The later was both more convenient for everyone and not destructive in the sense that with naming and looking at individual pages crc it's obvious to tell that the actual chips are storing 16 kb of data. So I made this choice of releasing them as 40 kb files.
  View user's profile Send private message Visit poster's website
  • Site Admin
  • Joined: 19 Oct 1999
  • Posts: 13435
  • Location: London
Reply with quote
Post Posted: Sat Nov 21, 2020 12:20 pm
I guess they also zip well. However I guess you should have padded to 48KB with the upper 8KB duplicated to match any mirrored accesses at high addresses?

It’s likely Emulicious disassembly will find any reachable mirrored code or data.
  View user's profile Send private message Visit poster's website
Reply to topic



Back to the top of this page

Back to SMS Power!