Editing Booting (section)

==== IBM System/360 and successors ====
{{Anchor|IPL|Initial Program Load}}
In the [[IBM System/360]] and its successors, including the current [[z/Architecture]] machines, the boot process is known as [[IBM System/360 architecture#Initial Program Load|''Initial Program Load'']] (IPL).

IBM coined this term for the [[IBM 7030 Stretch|7030 (Stretch)]],<ref name="stretch-ipl"/> revived it for the design of the System/360, and continues to use it in those environments today.<ref name="zAPoP">{{cite book |url=http://publibz.boulder.ibm.com/epubs/pdf/a2278324.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://publibz.boulder.ibm.com/epubs/pdf/a2278324.pdf |archive-date=2022-10-09 |url-status=live |title=z/Architecture Principles of Operation |pages=Chapter 17 |publisher=[[IBM]] |date=September 2005 |access-date=2007-04-14}}</ref> In the System/360 processors, an IPL is initiated by the computer operator by selecting the three hexadecimal digit device address (CUU; C=I/O Channel address, UU=Control unit and Device address<ref group="nb">UU was often of the form Uu, U=Control unit address, u=Device address, but some control units attached only 8 devices; some attached more than 16. Indeed, the 3830 DASD controller offered 32-drive-addressing as an option.</ref>) followed by pressing the ''LOAD'' button. On the high end [[System/360]] models, most<ref group="nb">Excluding the 370/145 and 370/155, which used a 3210 or 3215 console typewriter.</ref> [[System/370]] and some later systems, the functions of the switches and the LOAD button are simulated using selectable areas on the screen of a graphics console, often<ref group="nb">Only the S/360 used the 2250; the [[360/85]], [[370/165]] and [[370/168]] used a keyboard/display device compatible with nothing else.</ref> an [[IBM 2250]]-like device or an [[IBM 3270]]-like device. For example, on the System/370 Model 158, the keyboard sequence 0-7-X (zero, seven and X, in that order) results in an IPL from the device address that was keyed into the input area. The [[Amdahl Corporation|Amdahl]] 470V/6 and related CPUs supported four hexadecimal digits on those CPUs which had the optional second channel unit installed, for a total of 32 channels. Later, IBM would also support more than 16 channels.

The IPL function in the System/360 and its successors prior to [[IBM Z]], and its compatibles such as Amdahl's, reads 24 bytes from an operator-specified device into main storage starting at real address zero. The second and third groups of eight bytes are treated as [[Channel Command Word]]s (CCWs) to continue loading the startup program (the first CCW is always simulated by the CPU and consists of a Read IPL command, {{Mono|02h}}, with command chaining and suppress incorrect length indication being enforced). When the I/O channel commands are complete, the first group of eight bytes is then loaded into the processor's [[Program Status Word]] (PSW) and the startup program begins execution at the location designated by that PSW.<ref name="zAPoP"/> The IPL device is usually a disk drive, hence the special significance of the {{Mono|02h}} read-type command, but exactly the same procedure is also used to IPL from other input-type devices, such as tape drives, or even card readers, in a device-independent manner, allowing, for example, the installation of an operating system on a brand-new computer from an OS initial distribution magnetic tape. For disk controllers, the {{Mono|02h}} command also causes the selected device to seek to cylinder {{Mono|0000h}}, head {{Mono|0000h}}, simulating a Seek cylinder and head command, {{Mono|07h}}, and to search for record {{Mono|01h}}, simulating a Search ID Equal command, {{Mono|31h}}; seeks and searches are not simulated by tape and card controllers, as for these device classes a Read IPL command is simply a sequential read command.

The disk, tape or card deck must contain a special program to load the actual operating system or standalone utility into main storage, and for this specific purpose "IPL Text" is placed on the disk by the stand-alone DASDI (Direct Access Storage Device Initialization) program or an equivalent program running under an operating system, e.g., ICKDSF, but IPL-able tapes and card decks are usually distributed with this "IPL Text" already present.

IBM introduced some evolutionary changes in the IPL process, changing some details for System/370 Extended Architecture (S/370-XA) and later, and adding a new type of IPL for z/Architecture.