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the flag. The PCI flag is· ignored during initial
program loading.
Each command is described below, and the
format is illustrated.
Programming Note
A malfunction that affects the validity of data
transferred in an I/O operation is signaled at the
end of the operation by means of unit check or
channel-data check, depending on whether the
device (control unit) or the channel detected the
error. In order to make use of the checking
facilities provided in the system, data read in an
input operation should not be used until the end of
the operation has been reached and the validity of
the data has been checked. Similarly, on writing,
the copy of data in storage should not be destroyed
until the program has verified that no malfunction
affecting the transfer and recording of data was
detected.
Write
MMMMMM01
Data Address
o
8
31
C C
S
P
D
C
L
I
C
000
IIIIIIII
Count
I
I
32
35
40
48
63
A write operation is initiated at the I/O device,
and the subchannel is set up to transfer data from
storage to the I/O device. Data in storage is
fetched in an ascending order of addresses, starting
with the address specified in the CCW.
A CCW used in a write operation is inspected
for the CD, CC, SLI, andPCI flags. The setting of
the skip flag is ignored. Bit positions 0-5 of the
CCW contain modifier bits.
Programming Note
When writing on devices for which block length is
not defined, such as a magnetic-tape unit or an
inquiry station, the amount of data written is
controlled only by the count in the CCW. Every
operation terminated under count control causes
the incorrect.;.length indication, unless the
indication is suppressed by the SLI flag.
12-36
IBM 4300 Processors Principles of Operation
Read
MMMMMM10
Data Address
o
8
31
s
C C
S
K
P
D
C
L
I C
000
IIIIIIII
Count
I
p
I
32
40
48
63
A read operation is initiated at the I/O device, and
the subchannel is set up to transfer data from the
device to storage. For devices such as magnetic-
tape units, disk storage, and card equipment, the
bytes of data within a block are provided in the
same sequence as written by means of a write
command. Data is placed in storage in an
ascending order of addresses, starting with the
address specified in the CCW.
A CCW used ina read operation is inspected for
every flag-CD, CC, SLI, SKIP, and PCl. Bit
positions 0-5 of the CCW contain modifier bits.
Read Backward
MMMM 1100
Data Address
o
8
31
s
C C
S
K P
D
C
L
I C
000
IIIIIIII
Count
I P I
32
40
48
63
A read-backward operation is initiated at the I/O
device, and the subchannel is set up to transfer
data from the device to storage. On magnetic-tape
units, read backward causes reading to be
performed with the tape moving backward. The
bytes of data within a block are sent to the channel
in a sequence opposite to that on writing. The
channel places the bytes in storage in a descending
order of address, starting with the address specified
(
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