68000 ram access from z80 ?

TmEE co.(TM) · Post by **TmEE co.(TM)** » Wed Mar 19, 2008 10:54 pm

thanks.... that would give the max overclock you can get... I have 90ns flashes in my flashcart... and 10ns SRAMs on my SRAM cart.

Jorge Nuno · Post by **Jorge Nuno** » Wed Mar 19, 2008 11:06 pm

Not on the 68000, don't forget about the DTACK, the CPU will wait forever for it, and its duration doens't change with the clock, because it's generated by other chips (I think the VDP and the bus arbiter generate it)

Useful for accessing slower memories...

TmEE co.(TM) · Post by **TmEE co.(TM)** » Wed Mar 19, 2008 11:52 pm

ROM speed plays nice part in OC too... if you go over it, you will get system hang... I do think people would get better results using some flashcart than using official carts.

Eke · Post by **Eke** » Fri Oct 17, 2008 3:18 pm

Stef wrote:Ok, i done all possible tests and i can confirm it is *not* possible to access 68k RAM from Z80. Any read from the Z80 return $FF and write doesn't work, they are simply ignored...
I tested on each possible areas ($E00000 to $FF0000) and it gave always the same result. I tested on a PAL model 1 and a PAL model 2, same result. Charles Mc Donald infos were totally right
I'm a bit dissapointed, i want to have 68000 <--> Z80 communication without interrupt Z80. I guess i'll have to investigate the VDP solution, a lot less pratical :-/

for the record, I recently found that the game "Mamono Hunter Youko" (don't ask me how I ended up testing this game) also have Z80 writing into WRAM

what becomes interesting is that when you ignore that write in an emulator, the game will hang during the intro (z80 set some kind of flag in 68k RAM, checked later by the 68k program) so this was obviously done on purpose

maybe it was a "japanese model-only" feature ?

TmEE co.(TM) · Post by **TmEE co.(TM)** » Fri Oct 17, 2008 3:49 pm

Japanese only MD1 models are VA0 to VA2, VA3 is also present in other world, but in small quantities AFAIK. The MD2s are all same in all regions.

Stef · Post by **Stef** » Fri Oct 17, 2008 8:33 pm

Eke wrote:
Stef wrote:Ok, i done all possible tests and i can confirm it is *not* possible to access 68k RAM from Z80. Any read from the Z80 return $FF and write doesn't work, they are simply ignored...
I tested on each possible areas ($E00000 to $FF0000) and it gave always the same result. I tested on a PAL model 1 and a PAL model 2, same result. Charles Mc Donald infos were totally right
I'm a bit dissapointed, i want to have 68000 <--> Z80 communication without interrupt Z80. I guess i'll have to investigate the VDP solution, a lot less pratical :-/
for the record, I recently found that the game "Mamono Hunter Youko" (don't ask me how I ended up testing this game) also have Z80 writing into WRAM

what becomes interesting is that when you ignore that write in an emulator, the game will hang during the intro (z80 set some kind of flag in 68k RAM, checked later by the 68k program) so this was obviously done on purpose

maybe it was a "japanese model-only" feature ?

too bad, i had one of the earlier japanese MD but now it's just broken :-/
I also remember a japanese game needing some interaction between Z80 and 68000 RAM (777 Casino or something like that) but i never figured exactly what happened with this game.

AamirM · Post by **AamirM** » Sat Oct 18, 2008 7:06 am

Hi,

I thought writing worked but reading didn't. So, Mamono Hunter Youko doesn't seem to be doing anything wrong.

stay safe,

AamirM

Stef · Post by **Stef** » Sat Oct 18, 2008 5:40 pm

AamirM wrote:Hi,

I thought writing worked but reading didn't. So, Mamono Hunter Youko doesn't seem to be doing anything wrong.

stay safe,

AamirM

I did many tests on my systems and both writings as readings were impossible... i didn't tested on a japanese system though.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Oct 18, 2008 6:31 pm

Only thing that matters is the board revision... VA0 to VA3 might allow it, but I'm not sure as I only have VA4 MD1.

Snake · Post by **Snake** » Sun Oct 19, 2008 11:46 am

AamirM wrote:Hi,

I thought writing worked but reading didn't. So, Mamono Hunter Youko doesn't seem to be doing anything wrong.

stay safe,

AamirM

No, it's completely blocked in both directions.

I think Sega did this on purpose. It definitely *did* work on some early hardware. But given that it was probably a very dangerous thing to do, and yet something that most programmers would WANT to do (for the same reason Stef wants to!) they fixed the hardware so it was simply ignored.

But some games do indeed use it.

Chilly Willy · Post by **Chilly Willy** » Mon Oct 20, 2008 8:56 pm

So what areas are valid for the Z80? Just the cart?

HardWareMan · Post by **HardWareMan** » Tue Oct 21, 2008 3:55 am

I think that the reason for this phenomenon is that DevKits (and possibly the first console version) had a DRAM rather than SRAM. And the use of DRAM imposes certain limitations. Since buffer was intended for M68K, the controller and sharpened by a M68K bus cycle, and no one is going to unnecessarily complicate the scheme, to allow Z80 too could apply to the RAM. Installing the SRAM immediately fixed these problems.