IBM System/360 architecture: Difference between revisions

Revision as of 23:51, 19 March 2025

Latest revision as of 11:51, 27 July 2025

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|+ Floating Point Registers 0-, 2, 4 and 6

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|+ style="text-align: left; font-size:95%" | {{nowrap|Program Mask}}

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! Meaning

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* {{Anchor|Channel end|CE}}'''[[#CSW_Channel_end|Channel end]]'''{{sfn|PoOps|p=115}} indicates that the device has completed the data transfer for a channel command. There may also be an [[#Incorrect length|Incorrect length]] indication if the Count field of the CCW is exhausted, depending on the value of the [[#CCW-SuppressLengthIndication|Suppress-Length-Indication]] bit.

* {{Anchor|Device end|DE}}'''[[#CSW_Device_end|Device end]]'''{{sfn|PoOps|p=115}} indicates that the device has completed an operation and is ready to accept another. DE may be signalled concurrently with [[#Channel en|CE]] or may be delayed.

* {{Anchor|Unit check|UC}}'''[[#CSW_Unit_check|Unit check]]'''{{sfn|PoOps|pp=115–116}} indicates that the device or control unit has detected an unusual conditionscondition and that details may be obtained by issuing a Sense command.

* {{Anchor|Unit exception|UE}}'''[[#CSW_Unit_exception|Unit exception]]'''{{sfn|PoOps|p=116}} indicates that the device has detected an unusual condition, e.g., end of file.

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* '''{{anchor|Initial_Program_Load}}Initial Program Load''' (IPL){{sfn|PoOps|p=123}} is a process for loading a program when there isn't a loader available in storage, usually because the machine was just powered on or to load an alternative operating system.<ref name=A22-6821-7/>{{rp|page=123}} This process is sometimes known as [[Booting]].

:: As part of the IPL facility the operator has a means of specifying a 12-bit{{efn|name=ChanNum}} device address, typically with three dials as shown in the operator controls drawing. When the operator{{efn|Or an equivalent automated facility.}} selects the ''Load'' function, the system performs a ''System Reset'', sends a Read IPL{{efn|Read with all modifier bits zero}} channel command to the selected device in order to read 24 bytes into locations 0-23 and causes the channel to begin fetching ''CCW''s at ___location 8; the effect is as if the channel had fetched a CCW from ___location 0 with a length of 24, an address of 0 and the flags containing Command Chaining + Suppress Length Indication. At the completion of the operation, the system stores the I/O address in the halfword at ___location 2 and loads the PSW from ___location 0.

:: Initial program loading is typically done from a tape, a card reader, or a disk drive. Generally, the operating system was loaded from a disk drive; IPL from tape or cards was used only for diagnostics or for installing an operating system on a new computer.