Something that confused me for weeks was the lack of chipselect on the VIC chip. The datasheet helped once I realized it has a fixed address of $1000. That can offer some explanation of VA13 being inverted during cpu phase. On May 29, 2016 12:14 AM, "Jim Brain" <email@example.com> wrote: > On 5/28/2016 3:23 PM, David Wood wrote: > >> AFAIK the firmware roms (basic, kernal) don't get gated; >> > You are correct, they do not. > > they're on the bus with the /BLK(1,2,3,5,6,7) areas. The char rom and >> internal ram get gated because the nmos 6502 doesn't have the ability to >> tristate its buses, and the VIC chip needs the buses to get video data. >> Since the character bitmaps and internal ram are necessary to provide a >> working display they have to be on the VIC side of the gates. >> > I understand, I had the use of UD9 backwards. I notice now that all of > the areas that can't be reached by VIC-I are noted on the UD9 input list > (RAM1,2,3, BLK1,2,3,5, etc.) > > > >> Why the I/O space is included is something I'm unsure about. Likely a >> cost-saving method since the VIC chip does have to be on the gated side of >> the bus and it's also an I/O device in itself. >> > Yep, that is interesting. IO0 is not included in the list, and the VIAs > use BDX lines. It must just be a cost saving reason, as the VIA address > lines use CAXX lines, not VAXX lines. > > Jim > > Message was sent through the cbm-hackers mailing list > Message was sent through the cbm-hackers mailing listReceived on 2016-05-29 21:00:07
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