Re: Q: Timing: How many clock-cycles to print one char on screen?

[Holger Picker <pickerh@uni-muenster.de>]

Thanks a lot for your help. That was useful information indeed. If you don't mind I'd like 
to add a question concerning the clock-stretching mentioned by David Empson.
If I understood it right, then the videocontroller needs 65 "clock-cycles" compared to the 
processor to draw a line on the monitor. Then 262.5 lines per frame * 65 clock-cycles = 
17062.5 clock-cycles per frame * 60 frames per second (NTSC) = 1023750 clock-cycles per 
second ~= about 1.0227 MHz with a master clock frequency of 14.31818 MHz.
1023750 clock-cycles : 4 (LDA/STA absolute) = maximum store of ~255937.5 chars a second.
> There is a complication: to comply with NTSC video timing, the Apple ][
> stretches every 65th CPU clock cycle by two cycles of the 14 MHz clock.
Ah, I didn't know that (never heard of it)! What does it mean to the processor? Would it 
mean that after 65 clock-cycles the processor does one/two (complete) wait-cycle? Or is the 
difference too small to be noticeable (hearable? What about processor loops that access 
$c030? Could you hear a "background-humming"?) I've always thought that the 6502 in an 
AppleII runs at constant speed. Now it seems it was only true for PAL-Apples.
And, if I may ask, what happens on an AppleIIc+(4Mhz) or an accelerated AppleIIe if you 
access the disk-drive (especially writes)? The wait-routines (loops and NOPs) seem to depend 
on a constant 1Mhz-timing. How do you use the disk drive on an accelerated AppleII?

Cheers

Holger

P.S.: Thanks also for information about the AppleIIGS. Unfortunately I've never come across 
a IIGS in Germany. Now I'm getting more and more interested in this machine. But before 
bothering you with questions about it (good emulator for DOS/Windows or 65816-opcodes and 
the like) I better search the internet first. ;-)
P.P.S.: Hmm, I think I had better put up another (earlier) post also in comp.sys.apple first 
so to show you what I'm driving at (emulator programming). Sorry, I'll do that at once.
On 3 Oct 2000, David Wilson wrote:

P.P.P.S.:
I've just found Mr Wilson's reply:
> It does however have the stretched cycle. See Jim Sather's Understanding
> the Apple //e page 3-5. The only timing difference is the 0.48% increase
> in the duration of signals on a PAL //e (and the increase from 262 to 312
> horizontal scans in a 60/50Hz vertical scan).
Is there any way to find this kind of information somewhere on the internet?
As I live in Germany I probably will have no access to these books at all. (All
I've got is a very old manual on how to use Applesoft on an AppleII with
16kb (very nostalgic but absolutely useless), Jeffrey Stanton's book about
game programming (in German) and another one (in German) about DOS 3.3 in
detail (very good one), but nothing else :-( )