[atari7800] Re: Some questions..
- From: "Dan Boris" <dboris@xxxxxxxxxxx>
- To: <atari7800@xxxxxxxxxxxxx>
- Date: Tue, 14 Sep 2004 18:14:57 -0400
I have never worked with a real 7800 so I can't give you definitive
answers but I will take some guesses...
It may be possible to reset the DLL by some tricky manipulation of the
registers but you really shouldn't need to. I don't think they intended
for someone to do a DL on every scanline, you usually assign DL's to
groups of 8 or 16 scanlines. This is really the way the system is setup
to work.
As for the timing, the Maria Specs document from atari-history.com has
different timings then the ones you showed which are from the 7800
Software Guide. The ones in the Maria Specs actually make more sense:
Short Header: 8 cycles
Long Header: 10 cycles
That comes out to 2 cycles per byte.
Graphics, per byte: 3 cycles
Indirect Map Fetch: 3 cycles (plus 1 or 2 graphics fetches)
The Maria Spec doesn't list the Character Map Access sperately, so I am
guessing the the 6 and 9 cycles specified in the Software Guide already
include the Character Map Access. This makes sense:
Direct graphics read: 3 cycles
Indirect/1 Byte: (Character read) 3 cycles + (Graphics read) 3 cycles =
6 cycles
Indirect/2 Bytes: (Character read) 3 cycles + (2 Graphics reads) 6
cycles = 9 cycles.
Hopefully someone with a real 7800 development environment can test
this.
Dan
> -----Original Message-----
> From: atari7800-bounce@xxxxxxxxxxxxx
> [mailto:atari7800-bounce@xxxxxxxxxxxxx] On Behalf Of Andy Mucho
> Sent: Tuesday, September 14, 2004 5:23 AM
> To: atari7800@xxxxxxxxxxxxx
> Subject: [atari7800] Some questions..
>
>
> Newbie hat firmly in place, so bare with me :)
>
> I've started tinkering with the 7800 now I've got some time
> on my hands, and it's a lovely machine, but I've got a few
> questions, that I could probably answer myself if I could run
> on a real 7800, but I can't ;) I've just bought a PAL 7800,
> but now need to obtain something to program the bugger with
> and missed out on the CC2 by a mile, but that and my path to
> making a dev kit are another story..
>
> Firstly..
> Is there anyway at all I can force a DLL to restart from a
> new address, even through using DLIs ? For some little tests
> I've reached the point where I've got DLs for every scanline,
> and I've reached the limit of the DLL not being able to be
> longer than 512 bytes.. Is there anyway I can force the DLL
> to restart ? Switching the DMA off, point it to new one, then
> back on, or something ? Ideally being able to have 2 DLLs,
> one for the top half of the screen, one for the bottom ?
>
> Also, CharMap access.. I'm confused with the timings ?
> Am I correct in understanding that for example:
>
> 2 byte wide chars of 20 characters:
> 12 Cycles - 5 byte header
> 20 * 3 Cycles - Character Map Access
> 9 * 20 Cycles - Character Data fetch
> ---
> 252 Cycles
>
> 1 byte wide chars of 40 characters:
> 12 Cycles - 5 byte Header
> 40 * 3 Cycles - Character Map Access
> 6 * 40 Cycles - Character Data fetch
> ---
> 372 Cycles
>
> These figure just don't seem right to me ? Though I guess
> they are, but I'm just looking for clarification..
>
> I've been looking through emulator sources but even then the
> timings seem to be inconsistent ?
>
> In general sprite usage I'm in love with this machine and the
> way it renders
> :)
>
> One last thing.. I recall seeing somewhere on Erics page that
> he found the timings for Direct Graphic read from RAM were 2
> cycles instead of the documented 3 cycles ? I can't find this
> now, but I'm wondering if this is correct ? Does this apply
> only to the internal RAM, or does it apply to cart RAM in the
> case of a SuperCart with RAM ? Just curious..
>
> Regards
> AndyM00
>
> PS: On freelists.org trying to find the address to send mail
> to the list, when you've not received yet any is a bit tricky
> ;) Nowhere does it simply say atari7800@xxxxxxxxxxxxx! I'm
> guessing this is the right address..
>
>
>
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