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STATUS INFORMATION
As previously stated, an I/O error does not stop the
computer. At the termination of each command,
either a software or a user routine determines if the
command was completed successfully. The Control
Electronics of each channel maintains a STANDARD
DEVICE BYTE. This byte contains info rmation about
the current status of the channel.
STANDARD DEVICE BYTE
BIT
20
ILLEGAL OPERATION - Improper command
code for this device.
i. e., Read from Card
Punch.
21
INOPERABLE - The device is unusable until
condition is cleared. i. e. , No power, jammed,
interlock open, etc.
22
SECONDARY INDICATOR SET - Indicates that a
bit in the "SENSE BYTE" is set. An I/O SENSE
operation must be executed to determine the
particular condition.
2 3
DEVICE END - Set when device terminates.
Indicates that device is available.
24
CONTROL BUSY - Channel is engaged in pre-
viously initiated operation.
2
5
DEVICE BUSY - Device is engaged in previously
initiated operation.
2 6
TERMINATION INTERRUPT -
27
MANUAL INTERRUPT PENDING - Interrupt
button on Interrogating Typewriter depressed.
When an I/O instruction has been completed, the bit
configuration of the Standard Byte indicates how the
command was terminated.
- If the
22
bit (Secondary Indicator) is set, an
error or exceptional condition developed dur-
ing execution.
- If the 2 6 bit is set, a normal termination is
indicated. The instruction was completed suc-
cessfully.
- The
27
bit (Manual Interrupt) diffe rentiates
between a termination interrupt and an exter-
nal interruption initated by the operator at
the console typewriter.
The Standard Device Byte is transferred from the
Control Electronics into HSM using one of two
methods.
53
One - If I/O Interrupt is not inhibited, hardware
automatically stores the Standard Byte in
reserved memory location (46)10
when
interrupts occurs.
Two- If I/O Interrupt
~
inhibited, it is neces-
sary to execute a Post Status (PS) instruc-
tion. This command stores the Standard
Byte in one of eight reserved locations,
depending on which channel is addressed.
If the instruction:
OPERATION
OPERAND
PS
2(1)
is executed, the Standard Device Byte for Channel
2,
device 1 is stored in reserved HSM Location 10.
The Post Status Command does not transfer the Stan-
dard Byte into memory, until an I/O instruction has
reached some type oftermination. Ifwe immediately
follow a read or write with a Post Status, the main-
frame is "held-off", and simultaneous overlap is
lost.
SENSING EXCEPTIONAL CONDITIONS
Once the Standard Device Byte has been stored in
HSM, the Secondary Indicator (22) can be checked.
If the bit is set, it is necessary to transfer a second
byte, the Peripheral Unit Sense Byte, from the con-
trol Electronics to memory. The format ofthis byte
differs from one peripheral unit to the next, and in-
dicates specifically what error or exceptional con-
dition set the Secondary Indicator. The chart (below)
summarizes the meaning of each bit of the Sense
Byte for the Card Reader, Card Punch, Magnetic
Tapes, Interrogating Typewriter, and Printer.
The Input/Output Sense instruction transfers the
"sense" byte into a HSM location determined by the
operands of the instruction.
OPERATION
OPERAND
lOS
There the byte is analyzed by the Test Under Mask
instruction.
Example
of
110 Coding
The following sample I/O routine illustrates how the
Branch on Condition, Post Status, and I/O Sense
instructions facilitate input-output logic.
It
is not
intended to be a complete program; nor is simultan-
eous logic included. Assume that I/O interrupt is
inhibited.
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