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3.4.1 COMMAND COMPLETION WITHOUT INTERRUPTS. If interrupts are not used, the
computer program can determine if the controller has completed a command by reading bit 0 of
control word 7. If bit 0 (idle) is zero, the disk controller is busy. When a command is complete, the
idle bit and either the error bit or the complete bit is set. If lockout is being used, it is set after con-
trol word 7 is read, and if the error bit was set, an error status bit is set.
3.4.2 COMMAND COMPLETION WITH INTERRUPTS. The controller issues two kinds of in-
terrupts to the computer. One interrupt is issued when the controller completes a command, and one
when the disk is issued a seek or restore command (interrupt given immediately after return to idle
state with interrupt mask bit set).
3.4.2.1 Command Completion Interrupts. In order to have the controller signal the completion of a
command by issuing an interrupt to the computer, the computer program sets the interrupt bit of
control word 7 (bit 3) at the same time the command is started by resetting the idle bit (bit O). When
the controller returns to idle, an interrupt is issued when the controller detects either the complete or
error bit set with interrupt enabled. The interrupt is cleared by resetting interrupt enable in R7.
3.4.2.2 Seek and Restore Completion Interrupts. Control word 0 contains four attention bits and
four attention mask bits. The attention bits are not used by this disk controller. They are, instead,
forced to logic one whenever register RO is read. The programmer can set or reset the mask bits by
using any of the computer memory instructions.
The correct way to use the control word 0 interrupts is to issue the seek or restore command to the
controller; then, after the controller reports completion, set the correct mask bit. When the disk
drive finished the dummy seek operation, an interupt is issued to the computer. The interrupt can
then be cleared by resetting the mask bit. The TILINE interrupt is always reset when the controller
goes from an idle to a busy condition. The controller will not execute an independent seek com-
mand. It accepts the command, goes through initialization and self diagnostics, then terminates with
operation complete so that interrupts are handled the same.
3.5 CONTROL WORD CONTENT AND FORMATS
The eight control words shown in Figure 3-1 are used by the programmer to direct the activities of
the disk drive. The format, content, and activities of each of these control words are described in the
following paragraphs.
3.5.1 DISK STATUS, CONTROL WORD O. When the controller is not busy, the disk status of
control word 0 can be used. Figure 3-2 shows the bits of control word 0 (disk status).
Bits 0 through 5 of control word 0 contain individual status indicators from the selected disk drive.
Bits 8 through 11 are forced to logic 1. The bits are defined as follows.
Bit 0, Ojjline (OL). When bit 0 is set, it indicates that there is no power to the selected disk drive, the
drive is not at the proper speed, or not loaded with a disk cartridge, or that an unsafe condition
exists.
o
2
3
4
5
6
7
8
11
12
15
OL
NR
WP
US
SPARE
SI
SPARE
1
1
1
1
ATTN INTERRUPT
MASK (0-3)
(A) 133992A
Figure 3-2. Control Word 0
3-3
Digital Systems Division
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