Each IDA W is used for the transfer of up to 2,048 bytes. The IDA W specified by the CCW can
designate any location. Data is then transferred, for
read, write, control, and sense commands, to or from
successively higher storage locations or, for a read
backward command, to successively lower storage
locations, until a 2,048-byte block boundary is
reached. The control of data transfer is then passed
to the next IDAW. The second and any subsequent
IDAWs must specify, depending on the command,
the first or last byte of a 2,048-byte block. Thus, for
read, write, control, and sense commands, these IDAVls will have zeros in bit positions 21-31. For a
read backward command, these IDAWs will have
ones in bit positions 21-31. EX1;ept for the unique restrictions on the specifi­
cation of the data address by the IDA W, all other
rules for the data address, such as for protected stor­
age and invalid addresses, and the rules for data
prefetching, remain the same as when indirect data
addressing is not used.
A channel may prefetch any of the IDAWs per­
taining to the current CCW. An IDAW takes control
of the data transfer when the last byte has been
transferred for the previous IDAW for that CCW.
Errors detected in prefetched IDAWs are not indi­
cated until the IDA W takes control of the data
transfer.
Name Code
Write MMMM MM01 CD CC Read MMMM MM10 CD CC Read Backward MMMM 1100 CD CC Control MMMM MM11 CD CC Sense MMMM 0100 CD CC Transfer In Channel XXXX 1000 Explanation: CD Chain data CC Chain command SLI Suppress length indication SKIP Skip PCI Program-controlled interruption I DA Indirect data addressing
M Modifier bit
X Ignored Channel Command Codes SLI SLI SLI SLI SLI The format of the IDA Wand the significance of
its fields are as follows: 1000000001 Data Address
o 8 31
Bit positions 0-7 are reserved for future use and
must contain zeros. If any of the bits is detected to
be a one, a program-check status condition is gener­
ated, and the operation is terminated.
Bits 8-31 specify the location of the first byte to
be used in the data transfer. In the first IDAW for a
CCW, any location can be specified. For subsequent
IDAWs, depending on the command, either the first
or the last location of a 2,048-byte block located on
a 2,048-byte boundary must be specified. For read,
write, control, and sense commands, the beginning
of the block must be specified, and bits 21-31 of the
IDAW will be zeros. For a read backward command,
the end of the block must be specified, and bits 21-
31 of the IDA W will be ones. Improper data ad­
dress specification causes the program-check status
conditions to be generated and causes the operation
to be terminated.
Commands
The following table lists the command codes for the
six commands and indicates which flags are defined
for each command. The flags are ignored for all
commands for which they are not defined. Flags PCI IDA SKIP PCI IDA SKIP PCI IDA PCI IDA SKIP PCI IDA Input/ Output Operations 217

All flags have individual significance, except that
the CC and SLI flags are ignored when the CD flag
is on. The SLI flag is ignored on immediate opera­
tions, in which case the incorrect-length indication is
suppressed regardless of the setting of the flag. The PCI flag is ignored during initial program loading.
Each command is described below with an illustra­
tion of its CCW format.
Programming Note
A malfunction that affects the validity of data trans­
ferred 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
main storage should not be destroyed until the pro­
gram has verified that no malfunction affecting the
transfer and recording of data was detected.
Write
Read I MMMMMM10 I o 8
C
C SSp I D
C
L K C
D 00 I I P I A
32 [ Data Address
31
Count
63
A read operation is initiated at the I/O device, and
the sub channel is set up to transfer data from the
device to main 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 in storage is placed in an ascending order of
addresses, starting with the address specified in the
CCW.
A CCW used in a read operation is inspected for
every one of the six flags--CD, CC, SLI, SKIP, PCI, and IDA. Bit positions 0-5 of the CCW contain
modifier bits. Data Address J Read Backward
o 8
31
1 r-- M-MM-M-110-0 .------O-ata-Ad-dre-ss ---r-II :: 8 31
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 main
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 CC'W used in a write operation is inspected for
the CD, CC, SLI, PCI, and IDA flags. The setting of
the Skip nag is ignored. Bit positions 0-5 of the
CCW contain modifier bits. Progranruoodng Note
When writing on devices for which block length is
not defined, such as a magnetic tape unit or an in­
quiry station, the amount of data written is con­
trolled only by the count in the CCW. Every opera­
tion terminated under count control causes the
incorrect·-Iength indication, unless the indication is
suppressed by the SLI flag.
218 Principles of Operation C
C SSp I D C L K C D 00 I I P I A
32
Count
63
A read-backward operation is initiated at the I/O device, and the sub channel is set up to transfer data
from the device to main storage. On magnetic tape
units, read backward reading to be performed
with the tape moving backwards. 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 in the CCW. The
bits within an eight-bit byte are in the same order as
sent to the device on writing.
A CCW used in a read-backward operation is
inspected for every one of the six flags--CD, CC,