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Re: Boarding pass: a work in progress.

Roger Johnstone wrote:

In <MaSdnZZpefhvTizYnZ2dnUVZ_riknZ2d@comcast.com> Michael J. Mahon wrote:


Linards Ticmanis wrote:


BLuRry wrote:



Like the CPU, the video core is a timer-based object that works at a
pre-determined interval (1.023mhz).



Why 1.023? The real Apple runs at 1.020484-ish


Thank you, Linards!  The erroneous "urban mythic" clock speed of
1.023MHz or 1.024MHz for the Apple II has always bothered me.

But it's clearly a case of cognitive dissonance--2^10-1 or 2^10
are common numbers.



Apple II Reference Manual (1981): "1,023,000 machine cycles per second"
That's presumably the result of someone rounding it to 1.023 MHz, then writing it in hertz. 

Apple IIe Technical Reference Manual (1986):
In various places it says 1.02 MHz, 1.023 MHz, and 1.02273 MHz. The first two are obviously rounded versions of the last number. 1.02273 is the main crystal frequency of 14.31818 MHz divided by 14, so the Apple II does run at 1.023 MHz rounded to three decimal places. Unless I've missied something? 

The big problem is that the Apple clock is not a simple thing.

Every 65 cycles, there is a "long cycle" that is extended by one
14.31818MHz cycle.

For 64 out of every 65 cycles, the clock rate is 1.022727MHz,
then for the 65th cycle, the inverse of the clock period is
only 0.894886MHz.

So the effective (steady) clock rate of an NTSC Apple II is:

   1.02048432MHz

which, you'll note, rounds to 1.020MHz or 1.0205MHz.  (BTW, the
"composite" rate must be computed by adding time periods and
inverting, not by "averaging" frequencies.)

The 1.023 number is bogus and, I suggest, the result of cognitive
dissonance, in which something is confused with a more familiar
thing.

-michael

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Home page:  http://members.aol.com/MJMahon/

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