Execute Channel Program: Difference between revisions

Revision as of 21:29, 5 December 2020

Revision as of 16:34, 3 January 2021

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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 located anywhere in main storage. The channel program is a sequence of ''Channel Command Words'' (CCWs), which may be executed sequentially unless a branch, called ''Transfer In Channel'' (TIC), is executed, or the channel returns a [[IBM_System/360 architecture#CSW Status modifier|status modifier]]. 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 |accessdateaccess-date=Dec 4, 2019}}</ref>

<pre>

bit 0 7 8 31

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

All eight bits of commands that initiate I/O operations are passed to the device. "The modifier bits specify to the device how the command is to be executed." For example, for the [[IBM 2305]] DASD the ''write'' command ('MMMMMM01'BX) may have the following values:<ref>{{cite book |last1=IBM Corporation |title=Reference Manual for IBM 2835 Storage Control and IBM 2305 Fixed Head Storage Module |date=August 1971 |page=17 |url=http://bitsavers.org/pdf/ibm/dasd/2305/GA26-1589-2_2835_Storage_Control_and_2305_Fixed_Head_Storage_Module_Aug71.pdf |accessdateaccess-date=Dec 5, 2019}}</ref>

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