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Section III
3-31_
MULTIPLE DISC PROGRAMMING.
3-32.
When a computer utilizes two or more discs,
programming procedures require only that the appropriate
command channel and data channel I/O select codes be
used for each disc.
3-33.
TRACK PROTECTION.
3-34.
To write on a protected track, the track protect
switch is set to the down (nonprotect) position. The switch
is returned to the up (protect) position to re-establish the
protect status.
3-35.
WRITE OR READ ABORT.
3-36.
A disc write or read abort can be initiated by
programmed means, or it can be the result of certain types
of equipment failure. When an abort occurs, no further data
is transferred to or from the disc.
If
the abort takes place
after initiation of a disc operation, but before the first
desired sector is reached on the disc, no data is transferred.
If
fewer than 64 words are programmed to be written or
read in the last sector, and an abort of either type occurs
after the last desired word has been transferred to or from
the disc, the data write or read operation is completed
successfully.
3-37.
An equipment-failure abort occurs when certain
types of fault occur in the disc system. In some cases, when
an abort of this type takes place bit 3 of the disc status
word is set to logic 1. (Refer to table 3-2.)
3-38.
Note that bit 3 of the status word can become
logic 1 if an equipment failure occurs after completion of
the data transfer operation. Therefore, the bit should be
checked as soon as possible after completion of data
transfer to or from the disc. Completion of transfer can be
determined by an examination of the disc busy bit (bit 0)
of the status word.
If
the disc is still busy, the LIA/B
instruction which acquires the status word does not inter-
fere with the transfer of data to or from the disc.
3-39.
A programmed abort is brought about by a CLC
instruction that addresses the data channel, or which has an
I/O select code of zero. A programmed abort also occurs if
an OT A/B instruction address the command channel. (Such
an OT A/B instruction usually is performed in order to start
another disc read or write operation.)
3-40.
DMA LOCKUP.
3-41.
If
a disc write or read operation is aborted as a
result of an equipment failure, the DMA system will lock
up. When this situation occurs, DMA waits to transfer more
words to or from the disc, but the disc system does not
send a signal to DMA to indicate that another word is
required (when writing) or is ready (when reading).
3-42.
It
should be noted that a programmed abort, using
a CLC instruction with the disc data channel I/O select
code or with a zero I/O select code, does not cause DMA
lockup.
3-4
12606B
3-43.
When DMA lockup has occurred, a programmed
check of the DMA channel, using an SFS or SFC instruc-
tion with the DMA channel I/O select code, will indicate
that the DMA channel is busy. Futhermore, a check of the
disc busy bit (bit 0) of the disc status word will indicate
that the disc is busy.
3-44.
The lockup condition continues until one of the
following actions is performed:
a. Clear the entire I/O system by programming a CLC
instruction with a zero I/O select code.
b. Start a new disc write or read operation on the same
DMA channel, using the normal disc and DMA initiation
instructions. (The 2114B Computer has only a single DMA
channel.)
c. Perform a CLF instruction with the DMA channel
I/O select code, and a CLC instruction with the disc data
channel I/O select code. These two instructions can be
programmed in any sequence.
d. Stop the program, then press the PRESET switch.
e. Turn off computer power, then restore power.
3-45.
Existence of DMA lockup is indicated by bit 3 of
the disc status word. (Refer to table 3-2.)
If
this bit is logic
1 after sufficient time for completion of the data transfer,
an equipment-failure abort has occurred. Another method
of checking for an equipment-failure abort is to perform an
LIA/B instruction with the DMA channel I/O select code.
If
sufficient time has been allowed for completion of the
transfer, the word count should be zero.
3-46.
Checking for an abort condition as described in
the preceding paragraph will not interfere with the data
transfer operation, if it is still in progress. The check should
be made as soon as possible after completion of the transfer
of data. This will avoid the possibility of an equipment
failure setting the abort flag after completion of the data
transfer, thereby erroneously indicating that the transfer
was not completed.
3-47.
If
the computer has two DMA channels, the
channel not used for the disc write or read operation is not
affected by a DlWA channel lockup.
3-48.
POWER-OFF PERIODS.
3-49.
If
ac power is removed from the computer or the
disc memory power supply and then restored, data
recorded on the disc is retained. However, if disc writing is
taking place at the time of power removal, erroneous data
may be recorded in the sector in which writing is taking
place.
If
power is removed from the disc memory power
supply but not from the computer, the disc ready flag
(bit 7 of the disc status word) is logic 0 during the
power-off period. Also, the abort flag (bit 3 of the status
word) becomes logic 1, and it remains 1 after restoration of
disc power. When power is restored to the disc after a
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