Re: Apple II Disk Drive Question

Bryan Parkoff <none@xxxxxxxxxx> wrote:

"David Empson" <dempson@xxxxxxxxxxxxx> wrote in message
news:1hc37c1.zm35pr1p8py2mN%dempson@xxxxxxxxxxxxxxxx
Rotation speed is about 300 rpm. Data rate is nominally 250 kbps (4
microseconds per bit), derived from the Apple II CPU clock which runs at
about 1.023 MHz, with one bit every 4 clock cycles, so about 255.75
kbps.

I have a question. If RPM is to be between 280 and 310. Can you please
tell how many KBps per second will be. For example -- 300 RPM is 250KBps.
280 RPM may be 220KBps. 310 RPM may be 280KBps. Please list KBps each RPM
like 280, 281, 282, and so forth until 310. I am curious.

It isn't as simple as that.

The data rate is fixed when writing: it is determined by the controller,
which will write one bit every four CPU cycles, no matter how fast the
disk is spinning.

The read speed is somewhat variable, to allow for differences in
rotation speed between the drive which wrote the disk and the drive
which is reading it.

According to the manual for my Apple 5.25 drive, the optimal rotation
speed is 299 rpm. Can anyone else confirm this is the actual optimal
midpoint for the rotation speed?

For the purposes of the following, I'll assume all machines are running
at exactly the same clock speed. In reality there is a noticeable small
difference between PAL and NTSC models, since PAL Apple II models have a
master clock frequency of 14.25 MHz, while NTSC is 14.31818 MHz, and
there will be minor variations between individual machines (in the order
of tens or perhaps hundreds of parts per million, well under 1%). The
following could be done experimentally with two drives on the same
computer, adjusting the speed of the two drives to create the necessary
differences.

If the writing and reading drive are operating at exactly the same
rotation speed, then one bit will be read from disk every four CPU
cycles (32 CPU cycles per byte).

If the writing drive is spinning faster than the reading drive, then the
bits will be somewhat shorter from the reading drive's point of view,
resulting in bytes taking less than 32 CPU cycles to read (on average).

If the writing drive is spinning slower than the reading drive, then the
bits will be somewhat longer from the reading drive's point of view,
resulting in bytes taking more than 32 CPU cycles to read (on average).

I assume the range you quote (280 to 310 rpm) allows for worst case,
then a disk written at 280 rpm must be readable at 310 rpm, and vice
versa. This will be determined by the amount of variability allowed for
in the disk controller read hardware. At the extremes, there might be
limits due to the worst case magnetic response from the disk and
sensitivity of the read hardware, risking interference between adjacent
bits or read errors with too much time between flux reversals (one
bits). The software might also be timing sensitive, not able to keep up
with the data if it is arriving too fast, or regarding the data as
invalid if the bytes are too short or too long.

If you want to calculate the actual duration of each bit, you need to
know the rotation speed of both the writing and reading drives, and it
is a simple proportional calculation.

At the extremes: 280/310 is about 0.90, so bits written and read at
opposite extremes will 90% of the expected duration (write at 310, read
at 280) or 110.7% of the expected duration (write at 280, read at 310).

This works out at 3.6 CPU cycles per bit (on average) in the shortest
case (284 kbps), 4.4 CPU cycles per bit (on average) in the longest case
(232 kbps).

--
David Empson
dempson@xxxxxxxxxxxxx
.

Relevant Pages

  • Re: Apple II Disk Drive Question
    ... which will write one bit every four CPU cycles, ... disk is spinning. ... adjusting the speed of the two drives to create the necessary ...
    (comp.sys.apple2)
  • Re: Recorded TV playback
    ... I still believe that the problem is that since MCE uses VMR9 decoding and ... It could be that there is some microcode support that the CPU needs that ... both in recording and playing. ... and not the USB/IEEE drives? ...
    (microsoft.public.windows.mediacenter)
  • Re: Recorded TV playback
    ... Any and ALL stuttering/ hang problems come from MCE only. ... Your CPU may be what's called a HyperThreaded or HT CPU which means it ... both in recording and playing. ... and not the USB/IEEE drives? ...
    (microsoft.public.windows.mediacenter)
  • Re: Recorded TV playback
    ... Any and ALL stuttering/ hang problems come from MCE only. ... Your CPU may be what's called a HyperThreaded or HT CPU which means it looks ... both in recording and playing. ... and not the USB/IEEE drives? ...
    (microsoft.public.windows.mediacenter)
  • Re: Any ideas - PC stopped POSTing/booting
    ... waiting the usual boot time I found it had not booted. ... wouldn't have noticed any beeps or lack of beeps. ... I have overnighted the identical mobo and CPU from NewEgg and I ... The drives are irrelevant since I cannot get anything even with just ...
    (alt.comp.periphs.mainboard.asus)