ferred from the selected control unit. This sense data
may be one or more bytes long. It provides detailed
information concerning the selected I/O device, such
as a stacker-full condition of a card reader or a fiIe­
protected condition of a reel of magnetic tape on a
tape unit. Sense data have a significance peculiar to
the I/O device involved.
Transfer In Channel
The transfer-in-channel command specifies the loca­
tion of the next ccw to be used by the channel when­
ever the programmer desires to break the existing
chain of ccw's and cause the channel to begin fetch­
ing a new chain of ccw's from a different area in
main storage.
External documents, such as punched cards or mag­
netic tape, may carry ccw's that the channel can use
to govern reading of the external document being
read. Input / Output Termination
Input/output operations normally terminate with de­
vice-end signal and channel-end conditions and an in­
terruption signal to the CPU. A command can be rejected during execution of
START I/O, however, by a busy condition, program
check, etc. The rejection of the command is indicated
in the condition code in the psw, and the details of
the conditions that precluded initiation of the I/O op­
eration are provided in the channel status word stored
when the command is rejected.
Channel Status Word
The channel status word (csw) provides information
about the termination of an I/O operation. It is formed
or reformed by START I/O, TEST I/O, or by an I/O in­
terruption. Figure 21 shows the csw format.
Command Address
o 34 7 8 31
Status Count
47 48 63 Bits 0-3 contains the storage-protection key used in the
Bit 4-7 contain zeros.
Bits 8-31 specify the location of the last CCW used.
Bits 32-47 contain an I/O device-status byte and a channel­
status byte. The status bytes provide such information as data­
check, chaining check, control-unit end, etc.
Bits 48-63 contain the residual count of the last CCW used.
Figure 21. Channel Status Word Format Input / Output Interruptions
Input/output interruptions are caused by termination
of an I/O operation or by operator intervention at the I/O device. Input/output interruptions enable the CPU to provide appropriate programmed response to
conditions that occur in I/O devices or channels.
Input/output interruptions have two priority se­
quences, one for the I/O devices attached to a chan­
nel, and another for channel interruptions. A channel
establishes interruption priority for its associated I/O devices before initiating an I/O interruption signal to
the cpu. Conditions responsible for I/O interruption
requests are preserved in the I/O devices or channels
until they are accepted by the CPU. System Control Panel
The system control panel provides the switches, keys,
and lights necessary to operate and control the system.
The need for operator manipulation of manual con­
trols is held to a minimum by the system design and
the governing supervisory program. The result is few­
er and less serious operator errors. System Control Panel Functions
The main functions provided by the system control
panel are the ability to: reset the system; store and
display information in main storage, in registers, and
in the psw; and load initial program information.
System Reset
The system-reset function resets the CPU, the channels,
and online control units and I/O devices. In general,
the system is placed in such a state that processing
can be initiated without the occurrence of machine
checks, except those caused by subsequent machine
malfunction. Store and Display
The store-and-display function permits manual inter­
vention in the progress of a program. The function
may be provided by a supervisory program in con­
junction with proper I/O equipment and the interrupt
key. Or, the system-control-panel facilities may be
used to place the CPU in the stopped state, and then
to store and display information in main storage, in
general and floating-point registers, and in instruction­
address portion of the psw.
Initial Program Loading
The initial-program-Ioading (IPL) procedure is used
to begin or renew system operation. The load key is
pressed after an input device is selected with the load­
unit switches. This causes a read operation at the
selected input device. Six words of information are System Structure 21
read into main storage and used as channel control
words and as a psw that controls subsequent system
The system controls are divided into three sections:
operator control, operator intervention, and customer
engineering control.
Operator Control Section
This section of the system control panel contains the
operator controls required when the CPU is operating
under supervisory program control.
The main functions provided are the control and
indication of power, the indication of system status,
and operator-to-machine communication. These in­
Emergency power-off pull switch
Power-on back-lighted key
Power-off key
Interrupt key
Wait light
Manual light
System light
Test light
Load light
Load-unit switches T.oad key
Operator Intervention Section
This section of the system control panel provides
controls required for operator intervention into normal
programmed operation. These include:
System reset key
Stop key
Start key
Rate switch (single cycle or normal processing)
Storage-select switches
Address switches
Data switches Store key
Display key
Set Ie key
Address compare switches Customer Engineering Section
This section of the system control panel provides the
controls intended only for customer engineering use.
Customer engineering controls are also available on
some storage, channel, and control-unit equipment.
Previous Page Next Page