ZiLOG System 8000 Hardware Reference Manual page 115

HRM Zilog
Table 4-14 Command and Command-Status Registers
HRM
REGISTER
ADDRESS
xxOO
xX01
xx02
xX03
xX04
xX05
xx06
xX07
xX08
xxog
NAME
Command Register
Unit Register
Head Register
Cylinder Address
Low Register
Cylinder Address
High Register
Sector Register
Reserved
Reserved
Transfer Word
Count
8i
ts 7 to
0
Transfer Word
Count
:B
its 15 to ·3
DEFINITION
The CPU sends specific commands
to the controller through this
register. This must be the last
register the CPU writes too, for
it clears the status byte to
enable handshaking between
th~
CPU and the controller and
begins the operation.
The CPU uses this register to
specify which unit (drive) it
wants to participate in I/O
activity. One of four drives
(0, 1,2, or 3) can be specified.
The CPU uses this register to
specify the read-write head it
wants for the current operation.
The C·PU stores the lo\.-.r-order 8
bits (7 to 0) of the 16-bit
cylinder address in this register.
The CPU stores the high-order
8 bits (15 to 8) of the cylinder
address in this register. With
the aurrent disk drives, only
bits 8 and 9 are used.
The CPU stores in this register
the number of the sector it
wants to read from or write to.
For expansion
For expansion
This register contains the
loworder byte of the transfer
word count.
This register contains the
highorder byte of the transfer
word count.
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Zilog
4-22