I/O Channel Commands, Ccw's, And Ending Status - IBM 2265 Manual

I/O CHANNEL COMMANDS, CCW'S, AND ENDING
STATUS AND SENSE BYTES
To perform information interchanges between the central
processor (via the I/O channel and 2701 Data Adapter Unit)
and the 2845 Display Control, only two I/O channel com-
mands are required: Read and Write. These commands
to the I/O channel are issued under the Start I/O instruc-
tion by the central processor program. Table 7 shows the
settings or contents of significant bits of the CCW's (chan-
nel control words) that direct the I/O channel operations
during interchange of information between the 2701 and
2845. Command-chaining (CC bit set to 1) is specified to
maintain continuity of sequences and responses. The data
address portion of the CCW specifies the address of the
first byte of a multiple-byte transmission or the address of
the single byte when only one byte is to be transmitted.
The byte count does not include the LRC byte. The LRC
byte is used by the 2845 and 2701 for checking purposes
only and is not transferred to or from the I/O channel.
The Chain Data (CD), Skip, and Program-Controlled
Interruption (PCI) flags are not shown because their
settings are optional. However, if the CD flag is on, the
CC andSLI flags are ignored.
Reference to Table 7 may be helpful to the programmer
when reviewing the command sequence/response diagrams
shown in Figures 6 through 13. The table is not intended
to show all possible responses. Its purpose is to show a
typical program sequence that may be used to execute a
particular operation. For example, when executing a
specific poll to a display station, the Write command is
issued under a Start I/O instruction to the I/O channel.
As shown in Table 7, the data address of the CCW specifies
the location of the first byte of the four-byte addressing
sequence. In this example, the first byte is the EOT con-
trol character. A byte count of four is indicated because
the addressing sequence contains four bytes.
The Write command is chained to a Read command to
enable the I/O channel to receive the response from the
2845. A maximum of 960 characters can be displayed on
a 2265 screen; therefore, to accommodate a maximum-
46
length message from the Display Station, the CCW byte
count must be 964. This permits receiving STX, the 2265
address, ETX, and. a possible CAN character in addition to
the complete message text. The fITst byte of the message
must be STX and will be stored at the location specified
by the data address of the CCW. If the 2845 has no mes-
sage to send, it responds with EOT. EDT will break the
command chain. Subsequent action is determined by the
program. Table 7 assumes a message is sent by the 2845,
and therefore indicates that the Read command is chained
to send the I/O channel response to the 2845. The table
indicates an STX/EOT response, but it could be any of the
responses shown in Figure 6, such as EDT, ACK, SOH,
etc. If it is STX/EOT, for example, then STX must be
located at the address specified by the data address.
Note that the 2845 commands, such as General Poll or
Write Printer, have no meaning for the I/O channel or 2701.
A 2845 command byte is treated as any other text charac-
ter by the I/O channel and 2701. Only when a 2845 com-
mand is received and decoded by the 2845 does the com-
mand assume control functions and then only to the 2845.
When a Read or Write command operation (initiated by
the Start I/O instruction) is terminated, status and sense
bytes are generated to indicate conditions under which the
operation ended. The Write command is terminated with
Channel End and Device End status when the CCW byte
count reaches zero. On the other hand, the Read com-
mand is ended with Channel End and Device End status
either when the CCW byte count reaches zero or when the
ETX character is received from the 2845. Other conditions
present at the time a Read operation is ended result in the
setting ·of additional bits
ill
the status and sense bytes of
the I/O channel and further defme ending conditions.
Table 8 lists the control characters that may terminate the
Read command and the resulting status and sense bits that
are set in the I/O channel status and sense bytes.
Figure 19 in Appendix D contains information useful in
computing total time required to perform an I/O opera-
tion.