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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.
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