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Table 11. Operational Status Byte t
"
Function
Value
Significance
Transmission error
, 1
One of the conditions I isted under "Transmission Error Conditions
II
has occurred since
~
the previous order was received.
Incorrect length
1
An incorrect length condition has occurred since the previous order was received.
-
Chanr~p.1
end
1
The controller has terminated the operation for one of the reasons listed under
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"Channel End Conditions."
Unusual end
1
The controller has terminated the operation for one of the reasons listed under
"Unusual End Conditions."
t For the bit positions of these functions in the Operational Status, Byte, see the applicable Xerox computer reference
manual.
ADDITIONAL PROGRAMMING CONSIDERATioNS
INFORMATION PROTECTION
The contents of a disk are protected in the event of primary
power failure - the recorded information will not be lost or
altered. In addition, the Write-protect feature prevents
writing by disabling the write amplifier. Write-protect is
set when the LOAD/RUN switch on the operator control
panel is moved from LOAD to RU N. It is reset when the
operator momentarily depresses the PROTECT switch.
A Write-protect violation will occur if the INITIALIZE
switch on the multiple-drive selection unit is left on after
initializing a new disk.
COMMAND CHAINING AND DATA CHAINING
The Cartridge Disk System is designed to permit track switch-
ing and order modification (read to write and vice versa)
during an intersector gap.
The command chaining feature
must be used. (Command chaining is not available on the
Xerox 530 and Sigma 3 computers.)
Frequent data chaining (small byte counts), or frequent use
of test instruction loops (TIOs and TDVs) reduces the transfer
rate because of the additional communication required be-
tween the I/O system and the CPU, and because of the mem-
ory required for either task.
This can reduce the transfer
rate as much as 50 percent and can cause data overruns.
'A'hen "immediate" mode transfer techniques are used - data
transmission at the next available sector - the program must
add 1 to the sector address received from the Sense order.
This ensures one sector time (1.56 msec) for the program to
prepare the command I ist for the next data transfer. It is
recommended that command chaining be used between the
ensuing Seek order and the related operation - Read, Write,
or Check-write.
Vlhen
command chaining is not used,
2 must be added to the sector address received from the
Sense order or the time of one disk revolution will be lost
before the data transfer is initiated.
20
Additional Programming Considerations
I/O
INTERRUPT ENVIRONMENT
The I/O interrupt environment is established by setting flag]
in lOCOs and by using specified time-out delays in
softwa~
time-out routines.
The recommended software time-out
delay for disk operations is a minimum of one second.
lOCO FLAG: XEROX 530 AND SIGMA 3 COMPUTERS
Set the Interrupt flag to 1 in each lOCO unless data chain-
ing is specified.
This permits an I/o interrupt to be re-
quested whenever a channel end or unusual end condition
occurs. When data chaining is specified, set the Interrupt
flag to 1 only in the last lOCO of the command list.
lOCO FLAGS:' XEROX SIGMA 5/6/1/8 COMPUTERS
For the Xerox Sigma 5/6/1/8 computers, the following flagl
must be set:
ICE
Interrupt at Channel End. Set th is flag to 1
only in the last lOCO of a command list.
IUE
Interrupt at Unusual End. Set this flag to 1 in
all lOCOs.
HTE
Halt on Transmission Error. Set this flag to 1
in all lOCOs.
SIL
Suppress Incorrect Length. Set this flag to 1
whenever an incorrect length indication
should inhibit an lOP Halt and the subsequent
unusual end I/O interrupt.
See "Key Events" for definitions of the foregoing conditions.
SEQUENCE OF ACnVlTY
Figures 8 through 13 illustrate the sequential relationship
of the principal events that occur during disk operations.
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