Input/Output Operation Initiation All I/O operations are initiated by START I/O. If the
channel facilities are free, START I/O is accepted and
the CPU continues its program. The channel independ­
ently selects the I/O device specified by the instruction.
Channel Address Word
Successful execution of START I/O causes the channel
to fetch a channel address word ( CAW) from the
main-storage location 72. The CAW specifies the byte
location in main storage where the channel program
Figure 18 shows the format for the CAW. Bits 0-3 specify the storage-protection key that will govern the I/O operation. Bits 4-7 must contain zeros. Bits 8-31
specify the location of the first channel command
word (Cc:W). 1 Key 1
Command Address
o 34 7 8 31
Figure 18. Channel Address Word Format
Channel Command Word
The byte location specified by the CAW is the first of
eight bytes of information that the channel fetches
from main storage. These 64 bits of information are
called a channel command word (CCw). One or more ccw's make up the channel program
that directs channel operations. If more than one
ccw is to be used, each ccw points to the next ccw
to be fetched, except for the last ccw in the chain,
which identifies itself as the last in the chain. Figure
19 shows the format for ccw's. Six channel commands are provided:
Read Backward
Transfer In Channel Input / OILJtput Commands Read
The read command causes a read operation from the
selected I/O device and defines the area in main
storage to be used. 20 I Command
Data Address
Code 0 7 8 L Flags Count
3637 3940 4748
Bits 0-7 specify the command code.
Bits 8-31 specify the location of a byte in main storage.
Bits 32-36 are flag bits. J 63
Bit 32 causes the address portion of the next CCW to be used a
Bit 33 causes the command code and data address in the next CCW to be used.
Bit 34 causes a possible incorrect length indication to be
Bit 35 suppresses the transfer of information to main storage.
Bit 36 causes an interruption as
Bits 37-39 must contain zeros.
Bits 40-47 are ignored
Bits 48-63 specify the number of bytes in the operation a
Figure 19. Channel Command Word Format
The write command causes a write operation on the
selected I/O device and defines the data in main stor­
age to be written.
Read Backward
The read-backward command causes a read operation
in which the external document is moved in a back­
ward direction. Bytes read backward are placed in
descending main storage locations.
The control command contains information used to
control the selected I/O device. This control informa­
tion is caned an order. Orders are peculiar to the par­
ticular I/O device in use; orders can specify such func­
tions as rewinding a tape unit, searching for a par­
ticular track in disk storage, or line skipping on a
printer. The relationship of I/O instructions, com­
mands, and orders is shown in Figure 20. CPU Channels Control Units
(Executes I/O Devices I/O (Execute f..--- Instructions) Commands) (Execute Orders) Figure 20. Relationship of 110 Instructions, Commands, and
Orders Sense The sense command specifies the beginning main
storage location to which status information is trans-
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
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