<P> In the IBM System / 360 and its successors, including the current z / Architecture machines, the boot process is known as Initial Program Load (IPL). </P> <P> IBM coined this term for the 7030 (Stretch), revived it for the design of the System / 360, and continues to use it in those environments today . 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) followed by pressing the LOAD button . On the high end System / 360 models, most 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 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 which was keyed into the input area . The 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 . </P> <P> The IPL function in the System / 360 and its successors, 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 Words (CCWs) to continue loading the startup program (the first CCW is always simulated by the CPU and consists of a Read IPL command, 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 . The IPL device is usually a disk drive, hence the special significance of the 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 02h command also causes the selected device to seek to cylinder 0000h, head 0000h, simulating a Seek cylinder and head command, 07h, and to search for record 01h, simulating a Search ID Equal command, 31h; seeks and searches are not simulated by tape and card controllers, as for these device classes an 02h command is simply a sequential read command, not a Read IPL command . </P> <P> 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 . </P>

Basic instructions for booting the computer and loading the operating system are stored in