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Cybernator
Registered: Jun 2002
Posts: 154 |
VIC features at 2MHz
I'm curious as to how 2MHz change things relating the VIC features/timing. As the DOT clock is exactly 8 times the phi-2 clock, two things can happen:
1. Increase the DOT clock to 2*2MHz (~16 MHz), so that the nr. of cycles per frame doesn't change, but we have faster screen refresh.
2. Leave the DOT clock at ~8 MHz, and have more CPU cycles per rasterline.
Now that I have access to a C128 (:-DDDD) I can see the first case is incorrect. (*)
Since on C64 I can _move_ the write access in 8-pixel steps, that means on C128 I can do that in 4-pixel steps. On the other hand, VIC's I/O accesses are synced to the phi-2 clock (which is now at 2 MHz), not the DOT clock. That would mean there're still 63 cycles per rasterline, i.e. the second case is wrong as well, but then the first case would have to be correct. ;P
Another possibility is to leave VIC at 1Mhz, while the CPU is at 2MHz. But then there would be access-conflict between VIC and CPU, I guess.
Hope you understand what I'm thinking of. ;)
Btw, the CPU clock on C64 is not exactly 1 MHz, but 0.985 MHz (IIRC). Thus I guess the CPU clock on C128 is not exactly 2 MHz. So what's the exact frequency then?
Also, are there any VDC features?
Would be cool to do something with this monster sometime. ;)
Btw2, how can I use CPM? Is there a disk image available for download, and can it work with a 1541? Is there anything cool I can do with Z80 that I can't do with 8502?
--
(*) Though I'm not exactly sure
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Ninja
Registered: Jan 2002
Posts: 407 |
Unlike VDC, the MMU is not available in C64-Mode.
And for the extended keys, check register $D02F (http://unusedino.de/ec64/technical/aay/c64/vic47.htm). Couldn't you imagine, that these extra-keys are accessible through the VIC? ;)) |
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Oswald
Registered: Apr 2002
Posts: 5031 |
also the test bit in d030 does wild things, you can get 60hz screen, and have the normal screen displayed in the top /bottom borders. Check risen from oblivion if you have a 128. |
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Cybernator
Registered: Jun 2002
Posts: 154 |
> Unlike VDC, the MMU is not available in C64-Mode.
Yes, but somehow you have to make it unavailable when you switch to C64 mode. That's what I was curious about, how do I switch to C64 mode? I mean, all you'd need to do is to call the C64 reset routine and voila, but in 64 mode you cannot control VIC's images and colorram banks. Or?
> And for the extended keys, check register $D02F
> (http://unusedino.de/ec64/technical/aay/c64/vic47.htm).
> Couldn't you imagine, that these extra-keys are accessible
> through the VIC? ;))
Nope, and I couldn't imagine that AAY64 has 128 stuff, either. ;)
> also the test bit in d030 does wild things, you can get
> 60hz screen, and have the normal screen displayed in the
> top /bottom borders.
Hope AAY64 has some stuff on that.
> Check risen from oblivion if you have a 128.
Yes, I checked it. On one TV it worked fine, on another one it flickered (slided vertically).
Wish I could see the VDC part of RFO. ;)
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Raf
Registered: Nov 2003
Posts: 343 |
you can watch VDC on TV too ;-) there is composite mono output (just like in amiga 500) , and you can make color-composite-cable too with some basic electronix. anyway... better use a dedicated monitor.
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Oswald
Registered: Apr 2002
Posts: 5031 |
the c64 mode is 99,9% compatible with the c64, there are 2 differencies. You have 2 extra registers in the vic, d02f to read the numeric keyboard, and d030 for 2mhz mode and its test bit. Then you have the VDC registers also mapped in.
You change to c64 mode with the mmu (afair). simply there's no way to have the mmu visible in c64 mode. but the only difference between c64 and 128 mode is the mmu visibility and different rom set imho. |
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Ninja
Registered: Jan 2002
Posts: 407 |
Cybernator: Can't recall right now, what it exactly takes to switch from C128 to C64 mode; I have it in a book somewhere, but I would expect it to be on the net, too.
And you are right, you can't switch the ColorRAM in C64-mode (at least it is not known yet ;)).
AAY64 has the C128-stuff, which can be used in 64-mode (Hmm, VDC is missing...) So, $D030, Bit 1, is in there, too. With this bit, you can play with some hardcoded VIC-limits (e.g. 60 Hz mode by reaching vertical retrace earlier than expected)
Oswald: At least on German C128, you also have Bit 6 in $00/$01 as a difference (ASCII/DIN charset switch). This is why some programs fail, relying on $01 being $3x as it is usually $7x on those machines. |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
@Oswald: There's another thing which is different. The SID mirrors at $D500-$D7FF do not exist on a C128. SID tunes which use these mirrors will not work. An example for this is the Frantic Freddie tune. But that's about it, Frantic Freddie is the only game I ever encountered who used the SID mirrors. |
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Ninja
Registered: Jan 2002
Posts: 407 |
Yeah, I remember some tunes by Moppe/Oneway which don't work on C128. The editor is called 6581-something, IIRC. Wonder why they used a mirror? |
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Graham
Account closed
Registered: Dec 2002
Posts: 990 |
Because it is more h4xx0r to use mirror registers instead of real registers... |
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JackAsser
Registered: Jun 2002
Posts: 1995 |
Or maybe simply good ol' Moppe disliked C128 owners? |
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