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946261-9701
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2
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5
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8
9
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1 1
12
13
14
15
0
0
0
0
TIH
DISK
SPARE
HEAD/SURFACE ADDRESS
COMMANDS
(A) 133993
Figure 3-3. Control Word 1
Bit 1, Not Ready (NRJ. When bit 1 is set, it indicates that the selected disk drive is in the process
of performing a seek operation, or that the heads are not loaded.
Bit
2,
Write Protect (WP).
When bit 2 is set, it indicates that the write protect status
(WRITE/PROTECT CART switch or WRITE/PROTECT FIXED switch) of the selected disk
drive is on. When this switch is on, it inhibits the write logic within the disk drive.
Bit
3,
Unsafe (US).
When bit 3 is set, it indicates that the selected disk drive is unsafe. This means
that the safety circuits that protect the recorded information are unsafe, and the unsafe condition
must be removed before any more commands are attempted. If an unsafe condition occurs, the
F AUL T switch on the selected disk drive can be used to clear the fault condition provided the condi-
tion has ceased. A restore command can also be issued to clear the fault condition.
Bit
5,
Seek Incomplete (SI).
When bit 5 is set, it indicates that the selected disk drive has failed to
complete a seek operation because of a disk malfunction or detection of an illegal cylinder address
(illegal address interlock). When this status is detected, the software must initiate a restore com-
mand before attempting to execute any other command.
Bits 8 through 11. Bits 8 through 11 are not used and are forced to logic 1 because overlapped
seeking of logical units is prohibited.
Bits
12
through
15,
Attention Interrupt Mask (0-3).
These bits cause an interrupt when set to logic 1.
The TILINE interrupt is set when the attention interrupt mask is on and the controller is idle. Soft-
ware can disable the interrupt source by clearing the interrupt mask bits.
3.5.2 COMMAND CONTROL, CONTROL "'ORD 1. Control Word 1 is used for disk
cOllunands
and surface selection. Figure 3-3 shows the bits of control word 1. The bits of control word 1 are
defined as follows:
Bit
4,
Transfer Inhibit (TIH).
When bit 4 is set, the controller will not transfer any disk words into
memory when a read data command is specified. The purpose of this bit is to allow data verification
by relying on the cyclical redundancy character (CRC) check without having to provide a buffer in
memory for the specified block. After the read data command is completed, this bit is reset by the
computer.
Bits
5
through
7,
Command.
These three bits are a 3-bit code that designates a command (one of the
eight commands illustrated in Figure 3-1). The bits and the commands they specify are listed in
Table 3-1 and described as follows:
NOTE
Details of the contents, format, and programming procedures for
the disk commands are provided later in this section. Refer to
paragraph 3.5.9.
3-4
Digital Systems Division
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