Initial Program Load

IBM Initial Program Load

In current IBM mainframe systems, the boot process is known as Initial Program Load (IPL).

This term was coined by IBM for the design of the System/360 (ca, 1965) and continues to be used in those environments today.^[17] 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^{[NB 1]}) followed by pressing the LOAD button. On 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. 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, and its compatibles such as Amdahl's, reads 24 bytes from an operator-specified device into memory starting at location 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, 0x02, with command chaining and suppress incorrect length implied). When the I/O channel commands are complete, the first group of eight bytes is then loaded into the Program Status Word (PSW) and the startup program begins execution at the designated location.^[17] The IPL device is usually a disk drive, 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 0x02 command also causes the selected device to seek to cylinder 0x0000, head 0x0000, and to search for record 0x01, thereby also simulating a stand-alone seek command, 0x07, and a search ID equal command, 0x31; seeks and searches are not simulated by tape and card controllers).

The disk, tape or card deck must contain a special program to load the actual operating system into memory, 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 distributed with this "IPL Text" already present.