Execute Channel Program: Difference between revisions

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==Channel programs==

System/360 channel programs are a sequence of commands which are executed by the channel and the I/O device. Channel programs can be locaed anywhere in main storage. The channel program is a sequence of ''Channel Command Words'' (CCWs), which aremay normallybe executed sequentially. Each CCW is a doubleword (eight bytes) as follows:<ref>{{cite book |last1=IBM Corporation |title=IBM System/360 Principles of Operation |date=January 1967 |pages=84-121 |url=http://bitsavers.org/pdf/ibm/360/princOps/A22-6821-6_360PrincOpsJan67.pdf |accessdate=Dec 4, 2019}}</ref>

<pre>

bit 0 7 8 31

+--------+-----------------------+

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+------+---+--------+------------+

Six commands are defined, in the low-order bits of the command field (0-7). The high-order four (or six) bits are ''modifiers'' for some commands, or are ignored. The commands are:

0100 - Sense

1000 - Transfer in Channel (TIC)

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--10 - Read

--11 - Control

</pre>

The data address (8-31) is the 24-bit address of a main storage buffer area to or from which the data is to be transferred.

The flag bits (32-36) are defined as follows:

{| class="wikitable"

|-

! Bit !! ID !! Name !! Description

|-

| 32 || CD || Chain data || Use the storage area specified in the next CCW to continue this command

|-

| 33 || CC || Chain command || Execute the next sequential CCW when this command completes<br> if CC or CD are not set, the channel program terminates after executing this command

|-

| 34 || SLI || Suppress length indication || Ignore incorrect length for this command

|-

| 35 || SKIP || Skip || Suppress data transfer for this command

|-

| 36 || PCI || Program controlled interruption || Generate an interrupt when this command starts executing

|}

The count field (48-63) indicate the number of bytes to be transferred by this command.

==References==