NorthStar HORIZON Manual page 60

DISK CONTROLLER COMMANDS
Commands to the disk controller are specified bv memory read
references to addresses within a particular lK byte block of the
CPU address space. The lK byte block subdivides into four cases.
In all cases the high order 6 address bits (BS) are used to
determine if the lK block is addressed (i.e. the controller board
is selected). The next two bits determine the subcase and the
use of the low order eight bits depends on the subcase.
CASE 0
PROM addressing
BS
PROM Address
CASE 1
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Read byte from the
256
bytes of PROM.
Write byte of data
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BS
1
Data
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Write a byte of data to
register is not empty.
byte to be written.
the disk. Wait if the write shift
The low order 8 bits specify the
CASE 2
Controller Orders
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BS 2 IDOISSIOP!STI
OS
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Load 8-bit order register from low order 8 address bits.
DO Controls density on write 00=1 for double density and
00=0 for single density.
SS Specifies the side of a double-sided diskette. The
bottom side (and only side of a single-sided diskette)
is selected when S5=0. The second (top) side is
selected when 55=1.
North Star HORIZON Computer
60