Re: What's the IIe's--Arghh!--MHz Speed?

[kentd@cup.hp.com (Kent Dickey)]

In article <36a9564a.19517955@news.bconnex.net>,
Jeff Blakeney <CUTjefbla@bconnex.net> wrote:
>On Sun, 17 Jan 1999 11:16:58 GMT, kaimbridge@my-dejanews.com wrote:
>
>> Alright, I'll ask what has probably already been asked 10^1000 time:  What is
>>the Mhz processor speed of a regular, 128K, 80-column Apple IIe?
>
>Well, I've seen a lot of people answer this question but I don't think
>I've seen one reply get the speed correct.  :-)
>
>Unfortunately I couldn't find the reference that I first found the
>speed listed (it has probably been 15 years or more since I read it)
>or I'd list it to confirm my statement but I'm 99.99% sure that the a
>stock Apple IIe computer runs at 1.023 MHz.

Here's some detail:

The nominal 1MHz clock is derived from a base crystal 14MHz = 14318180.
(Crystals aren't that accurate, but it's the nominal frequency).
(It's 28MHz in a //gs, at exactly twice that frequency, and divided
by 28 to get the 1MHz clock and divided by 10 to get the 2.8MHz clock).

The clock is divided by 14, giving 1.022727MHz, often rounded
to 1.023MHz.  But wait, there's more.  A complexity involving the fact
that the base Apple II clock and the video clock are strongly related
means a quirk of NTSC video requires some adjustment to this clock
circuit.  In short, every 65th 1MHz clock is stretched an extra 140ns
(two 14MHz clocks) to make the video signal work correctly.

The effective MHz is thus: (14318180/14)*(65*14/(65*14+2)) = 1.020MHz.

Apple almost always uses 1.023MHz since designers must design to
1.023MHz to make timing, but effective average speed is lower.

This formula is taken from KEGS, an Apple //gs emulator for Unix,
available at http://www.crosswinds.net/san-jose/~kentd/kegs/.

Steve Wozniak wrote an article for Byte in '81 or '82 where he gives a
program to calculate e to 20,000 decimal places or so on an Apple II.
He gives some more details and I believe gives something equivalent
to the above formula.

Kent Dickey
kentd@cup.hp.com