Editing Bootloader (section)

===IBM System/360 and successors===
In [[IBM System/360]] and successors, the '''LOAD''' [[IBM System/360 architecture#Operator controls|operator control]] initiates a process called ''Initial Program Load'' (IPL),<ref group=nb>For [[z/Architecture]] the details are slightly different due to 64-bit addresses.</ref> which
#Does a ''System reset''
#Sends a ''Read IPL'' (IPL) channel command ({{base|02|16}}) to the selected device in order to read 24 bytes into locations 0-23 and causes the channel to begin fetching CCWs at location 8; the effect is as if the channel had fetched a CCW from location 8 with a length of 24, an address of 0 and the flags containing Command Chaining + Suppress Length Indication.
#Stores the I/O address in the halfword at location 2
#Loads the PSW from location 0.

The operating systems for S/360 through [[z/Architecture]] reside on [[direct access storage device]]s (DASDs), e.g., disk, drum. For these devices, ''Read IPL'' does a seek to cylinder {{base|0000|16}}, head {{base|0000|16}}, and orients to record {{base|01|16}}. For all supported operating systems, record {{base|01|16}} contains a ''Read Data'' CCW to read a ''bootstrap record'' and a ''Transfer In Channel'' (TIC) CCW to the bootstrap. The channel program in the bootstrap reads the IPL program text into location 0, beginning with a PSW pointingto the first IPL program text instruction.

For [[OS/360]]<ref group=nb>The process has more complexity for successors to OS/360, and has some differences for other OS families.</ref><ref>{{cite book
 | title       = OS Release 21 - IBM System/360 Operating System - Initial Program Loader and Nucleus Initialization Program - Program Number 360S-CI-535
 | id          = GY28-6661-5
 | date        = March 1972
 | edition     = Sixth
 | section     = THE INITIAL PROGRAM LOADER
 | section-url = http://bitsavers.org/pdf/ibm/360/os/R21.0_Mar72/plm/GY28-6661-5_Inital_Program_Loader_and_Nucleus_Initialization_Program_Rel21_PLM_Mar72.pdf#page=11
 | pages       = 3-10
 | series      = Program Logic
 | url         = http://bitsavers.org/pdf/ibm/360/os/R21.0_Mar72/plm/GY28-6661-5_Inital_Program_Loader_and_Nucleus_Initialization_Program_Rel21_PLM_Mar72.pdf
 | access-date = April 16, 2023
 }}
</ref> the IPL program does some initialization, relocates itself, locates the ''Nucleus'',<ref group=nb>The nucleus contains OS components that must always be present in processor storage. Most of the code is privileged, but there are also subroutines for use by application programs.</ref><ref group=nb>The Nucleus resides in member '''IEANUC0x''' of '''SYS1.NUCLEUS''' on the IPL volume, where ''x'' is normally 1 but may be changed by the operator.</ref> loads the nucleus and transfers to the ''Nucleus Initialization Program'' (NIP) at the end of the Nucleus.