Using slow ROM for new games?

Discussion of hardware and software development for Super NES and Super Famicom.

Moderator: Moderators

Forum rules
  • For making cartridges of your Super NES games, see Reproduction.
User avatar
Señor Ventura
Posts: 174
Joined: Sat Aug 20, 2016 3:58 am

Re: Using slow ROM for new games?

Post by Señor Ventura » Tue Jan 05, 2021 7:23 pm

lidnariq wrote:
Tue Jan 05, 2021 5:10 pm
Uh. The amount of time to read or write a single byte can be as little as 1÷(3.6MHz), but DMA is the only thing that can sustain anything close to that.
So, something is happening here... i ialways believed that the reason why the DMA runs at 2.68 mhz was the WRAM (cause it ran supposedly at 2.68mhz).

But now the problem seems to be totally the DMA by itself. Better thought, at least maybe the cpu could read from WRAM at 3.58mhz, but writing at 2.68mhz. It has sense to me.


In this point, the DMA constraints could be a hardware problem, or a software problem?. How boots the snes?.
The rest of the areas of WRAM doesn't allow to write that Byte at 3.58mhz?, only the one marked of blue?.

lidnariq
Posts: 10118
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Using slow ROM for new games?

Post by lidnariq » Tue Jan 05, 2021 7:30 pm

Señor Ventura wrote:
Tue Jan 05, 2021 7:23 pm
In this point, the DMA constraints could be a hardware problem, or a software problem?. How boots the snes?.
Hardware. We don't really know where the timing problem comes from.

There's lots of other weird gotchas in here. We don't have obvious reasons for many of them.
The rest of the areas of WRAM doesn't allow to write that Byte at 3.58mhz?, only the one marked of blue?.
It literally says on the image you have there, on the right side.

Code: Select all

2000 - 3FFF 3.58 M PPU, etc.
0000 - 1FFF 2.68 M WRAM (8K - BYTE)
and then at the bottom

Code: Select all

RAM 1 ----Fix 2.68 MHz 8K byte
RAM 2 ----Fix 2.68 MHz 120K byte

tepples
Posts: 22235
Joined: Sun Sep 19, 2004 11:12 pm
Location: NE Indiana, USA (NTSC)
Contact:

Re: Using slow ROM for new games?

Post by tepples » Tue Jan 05, 2021 7:42 pm

Just a conjecture:

WRAM's B bus port can be written with a fast cycle because there are guaranteed not to be fast writes in consecutive CPU cycles.
WRAM's B bus port can be read with a fast cycle because there are guaranteed not to be fast reads in consecutive CPU cycles, and setting the address presumably preloads the value into a register in the DRAM's controller.

creaothceann
Posts: 275
Joined: Mon Jan 23, 2006 7:47 am
Location: Germany
Contact:

Re: Using slow ROM for new games?

Post by creaothceann » Tue Jan 05, 2021 11:27 pm

Señor Ventura wrote:
Tue Jan 05, 2021 7:23 pm
i always believed that the reason why the DMA runs at 2.68 mhz was the WRAM (cause it ran supposedly at 2.68mhz).
There also might be slow RAM/ROM chips in the cartridge.
My current setup:
Super Famicom ("2/1/3" SNS-CPU-GPM-02) → SCART → OSSC → StarTech USB3HDCAP → AmaRecTV 3.10

User avatar
Señor Ventura
Posts: 174
Joined: Sat Aug 20, 2016 3:58 am

Re: Using slow ROM for new games?

Post by Señor Ventura » Wed Jan 13, 2021 5:55 pm

I'm looking at the rom info of the snes9x, What is the difference between "30/fastrom" and "31/fastrom"?.

lidnariq
Posts: 10118
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Using slow ROM for new games?

Post by lidnariq » Wed Jan 13, 2021 5:57 pm

first digit 2 = slow
first digit 3 = fast

second digit 0 = lorom
second digit 1 = hirom

User avatar
Señor Ventura
Posts: 174
Joined: Sat Aug 20, 2016 3:58 am

Re: Using slow ROM for new games?

Post by Señor Ventura » Fri Jan 15, 2021 5:29 am

lidnariq wrote:
Wed Jan 13, 2021 5:57 pm
first digit 2 = slow
first digit 3 = fast

second digit 0 = lorom
second digit 1 = hirom
Thank you ^^

Post Reply