North Star MDS-A-O User Manual page 33

DISK CONTROLLER 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 information is
recorded starting about 96 microseconds after the sector hole is
detected.
Single Density
Double Density
Zeros
16 bytes 32 bytes
Sync Char (FB) 1 byte 2 bytes
Data
256
bytes
512 bytes
Check Char
1 byte 1 byte
274 bytes 547 bytes
The check character is computed iteratively
by
setting
it
to zero
and then exclusive ORing each successive data byte value with the
current value of the check character and left cycling the result.
DISK CONTROLLER SOFTWARE
The basic low-level function of the controller is to transfer or
verify 256/512 byte blocks of data between a specified area on
the disk and a specified area of the RAM. The algorithm now
described accomplishes this task. The program to perform this
algorithm resides in the DOS.
1.
Start the drive motor(s) if they were off and wait 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(s) were off then select the
new drive, and wait for the index detect signal to be true.
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
track. 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. reset the step flip-flop
c. wait the specified track stepping time.
4.
Wait until the next sector pulse and test if the
the desired sector position by reading C-status.
repeat this step.
d:ts-k
is
If not
at
then
33 North Star MICRO-DISK System