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DATA FORMAT
Each diskette has 35 tracks of data.
Each track is divided
into
10
sectors.
The rotational position of the beginning of the
sectors is marked by sector holes in the diskette.
Each sector
is recorded using the following format.
This ihformation is
recorded starting about
96
microseconds after the sector hole is
detected.
Zeros
16 bytes
Sync Char(FB)
1 byte
Data
256 bytes
Check Char
1 byte
274 bytes
The check character is computed iteratively by setting it
to zero and then eiclusive ORing each successive data byte value
with the current value of the check character and left cycling
the result.
SOFTv~ARE
The basic low-level function of the controller is to
transfer or verify 256 byte blocks of data between a specified
area on the disk and a specified
ar~a
of the RAM.
The algorithm
now described accomplishes this task.
The program to perform
this algorithm resides partly in the on-board PROM and pattly in
the DOS.
1. Start the drive motor(s)
if they were off and wait 1 second
(i.e. 5 sector times)
for the motor to get up to speed.
2. If the disk drive to be selected is different than the drive
currently selected or if the motor was off then select the new
dr i ve.
3 .• If the disk read/write heads are not already at the desired
track then the heads must be stepped in or out to the correct
irack.
First set the step direction flip-flop and determine
the number of tracks to step.
Each step is accomplished by
the following sequence:
a. set the step flip-flop
b. wait at least
10
microseconds and then reset the step flip-
flop
c. wait 2 sector times (40ms).
4. Wait until the next sector pulse and test if the disk is at
13
North Star MICRO-DISK System
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