Search found 758 matches

by Nemesis
Tue Mar 04, 2008 10:09 am
Forum: Cartridge
Topic: The not so famous /YS signal
Replies: 28
Views: 17167

Ok, I've run a series of tests on the system. From what I can tell, I was correct in that the interrupt is acknowledged immediately after it is raised, even when it is masked. I started by measuring the IPL lines when interrupts were masked by the M68000, and found that the IPL lines were only asser...
by Nemesis
Mon Mar 03, 2008 10:50 pm
Forum: Cartridge
Topic: The not so famous /YS signal
Replies: 28
Views: 17167

I am not so sure about that: the 68000 manual explicitely say that if the priority of the pending interrupt is lower than the current processor priority (masked interrupt), the interrupt processing is postponed until it becomes greater. And the acknowledge cycle is described as one of the first ste...
by Nemesis
Mon Mar 03, 2008 6:21 am
Forum: Cartridge
Topic: The not so famous /YS signal
Replies: 28
Views: 17167

I'm pretty sure the M68000 acknowledges all interrupts immediately. If the interrupt is masked, it's simply discarded, but the interrupt is acknowledged by the M68000 after the current bus cycle completes, whether the interrupt is taken or not.
by Nemesis
Sun Feb 17, 2008 10:05 pm
Forum: Hardware
Topic: Workings of the Mega Drive: Bus Interactions
Replies: 10
Views: 7523

I've been studying the schematics in some detail, trying to figure out how each possible interaction between the chipsets happen, and I'm realising how complex the banked memory access to the M68000 bus is from the Z80. The only controller which receives all the data lines from the Z80 is the 315-53...
by Nemesis
Sat Feb 16, 2008 4:23 pm
Forum: Hardware
Topic: Workings of the Mega Drive: Bus Interactions
Replies: 10
Views: 7523

Thanks for the diagram HardWareMan, I'll add it to my archive.
by Nemesis
Sat Feb 16, 2008 4:10 pm
Forum: Hardware
Topic: Workings of the Mega Drive: Bus Interactions
Replies: 10
Views: 7523

The Genesis/MD chipset decodes some of the 68K address lines to generate chip selects for RAM, the cartridge port, the side expansion port etc. Peripherals on the cartridge and expansion ports can ignore these chip selects and do their own decoding, but that's not very common. Aha! That makes sense...
by Nemesis
Sat Feb 16, 2008 2:18 am
Forum: Hardware
Topic: Workings of the Mega Drive: Bus Interactions
Replies: 10
Views: 7523

Workings of the Mega Drive: Bus Interactions

Hi guys. I've noticed there a number of people here with a lot more knowledge about the nitty gritty hardware of the humble Mega Drive than myself, so I thought I'd pose a few questions which I've been unable to figure out the answers to on my own. I'm currently writing an emulator for the Mega Driv...
by Nemesis
Mon Jan 07, 2008 5:01 am
Forum: Megadrive/Genesis
Topic: Genesis Address Bus and Max RAM
Replies: 35
Views: 21703

Does someone have a copy of that image HardWareMan posted? Link seems to be dead.