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3.6
1791 CONTROLLER
The 1791 Controller is an
r~os
LSI device that contains logic to select
a disk track, then reco!o data on that track or read data already
recorded.
The 1791 device (Figure 3-2) consists cf a buffered Data Register,
Data Shift Register, Sector Register, Command Register, Track-Number
Register,
Status Register,
CRC Logic,
Address-Detect Logic
Arithmetic/Logic Unit (ALU), Computer-Interface Logic, and Drive-
Interface Logic.
3.6.1
DATA REGISTER
This bidirectional register receives data or commands from the CPU and
transmits data to the CPU.
3.6.2
DATA SHIF7 REGISTER
This register receives eignt bits of serial data from the drive, then
outputs it as one byte to the Dcta
Regis~er;
or receives a byte from
the Data Register and transmits it as serial data to the drive.
3.6.3
SECTOR REGISTER
This eight-bit register holes the address of the desired sector.
3.6.4
COMMAND REGISTER
This eight-bit register holds the command currently being executed by
the 1791 device. The device has 11 cOffiffiands of four types (Section
IV) •
3.6.5
TRACK-NUMBER REGISTER
This eight-bit register holds the number of the track over which the
Read/Write head is currently positioned.
Step and Direction commands
from the Drive-Interface logic increment or decrement the register.
3.6.6
STA?US REGISTER
This eight-bit register holds drive status such as ready, write-
protected, etc.; and coremand-result status.
Refer to Appendix A.
3-5
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