NorthStar HORIZON Manual page 61

DP has shared use. During stepping operations, DP=0
specifies a step out and DP=l specifies a step in.
During write operations, write precompensation is
invoked if and only if DP=l.
ST controls the level of the head step signal to the disk
drives.
DS is the drive select field, encoded as follows
0=no drive selected
l=drive 1 selected
2=drive 2 selected
4=drive 3 selected
B=drive 4 selected
CASE 3 Controller Commands
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BS
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DM CC
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Perform a disk controller command. The commands are
specified by the B low order address bits.
DM The DM field controls what gets mUltiplexed onto the
DI bus during the command.
l=A-status
2=B-status
3=C-status
4=Read data (may enter wait state)
CC Command code.
0=no operation
l=reset sector flag
2=disarm interrupt
3=arm interrupt
4=set body (diagnostic)
5=turn on drive motors
6=begin write
7=reset controller, de-select drives, stop motors
North Star HORIZON Computer