Editing Loader (computing) (section)

==Relocating loaders==
Some operating systems need [[relocating loader]]s, which adjust addresses (pointers) in the executable to compensate for variations in the address at which loading starts. The operating systems that need relocating loaders are those in which a program is not always loaded into the same location in the (virtual) address space and in which pointers are absolute addresses rather than offsets from the program's [[base address]]. Some well-known examples are IBM's [[OS/360]] for their [[System/360]] mainframes, and its descendants, including [[z/OS]] for the [[z/Architecture]] mainframes.
===OS/360 and derivatives===
In OS/360 and descendant systems, the (privileged) operating system facility is called IEWFETCH,<ref>{{cite book|last=IBM Corporation|title=IBM OS MVT Supervisor|year=1972|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/os/R21.0_Mar72/plm/GY28-6659-6_MVT_Supervisor_PLM_Release_21_Mar72.pdf}}</ref> and is an internal component of the OS Supervisor, whereas the (non-privileged) LOADER application can perform many of the same functions, plus those of the Linkage Editor, and is entirely external to the OS Supervisor (although it certainly uses many Supervisor services).

IEWFETCH utilizes highly specialized [[Channel_I/O#Channel_command_words|channel programs]], and it is theoretically possible to load and to relocate an entire executable within one revolution of the DASD media (about 16.6&nbsp;ms maximum, 8.3&nbsp;ms average, on "legacy" 3,600&nbsp;rpm drives). For load modules which exceed a track in size, it is also possible to load and to relocate the entire module without losing a revolution of the media.

IEWFETCH also incorporates facilities for so-called overlay structures, and which facilitates running potentially very large executables in a minimum memory model (as small as 44&nbsp;KB on some versions of the OS, but 88&nbsp;KB and 128&nbsp;KB are more common).

The OS's [[Kernel (operating system)|nucleus]] (the always resident portion of the Supervisor) itself is formatted in a way that is compatible with a stripped-down version of IEWFETCH. Unlike normal executables, the OS's nucleus is "scatter loaded": parts of the nucleus are loaded into different portions of memory; in particular, certain system tables are required to reside below the initial 64&nbsp;KB, while other tables and code may reside elsewhere.

The system's [[Linker (computing)#Linkage editor|Linkage Editor]] application is named IEWL.<ref>{{cite book|last=IBM Corporation|title=IBM OS Linkage Editor and Loader|year=1972|url=http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/360/os/R21.0_Mar72/GC28-6538-9_OS_Linkage_Editor_and_Loader_Release_21_Jan72.pdf}}</ref> IEWL's main function is to associate load modules (executable programs) and object modules (the output from, say, assemblers and compilers), including "automatic calls" to libraries (high-level language "built-in functions"), into a format which may be most efficiently loaded by IEWFETCH. There are a large number of editing options, but for a conventional application only a few of these are commonly employed.

The load module format includes an initial "text record", followed immediately by the "relocation and/or control record" for that text record, followed by more instances of text record and relocation and/or control record pairs, until the end of the module.

The text records are usually very large; the relocation and/or control records are small as IEWFETCH's three relocation and/or control record buffers are fixed at 260 bytes (smaller relocation and/or control records are certainly possible, but 260 bytes is the maximum possible, and IEWL ensures that this limitation is complied with, by inserting additional relocation records, as required, before the next text record, if necessary; in this special case, the sequence of records may be: ..., text record, relocation record, ..., control record, text record, ...).

A special byte within the relocation and/or control record buffer is used as a "disabled bit spin" communication area, and is initialized to a unique value. The Read CCW  for that relocation and/or control record has the Program Controlled Interrupt bit set. The processor is thereby notified when that CCW has been accessed by the channel via a special [[Input/Output Supervisor|IOS]] exit. At this point the processor enters the "disabled bit spin" loop (sometimes called "the shortest loop in the world"). Once that byte changes from its initialized value, the CPU exits the bit spin, and relocation occurs, during the "gap" within the media between the relocation and/or control record and the next text record. If relocation is finished before the next record, the NOP CCW following the Read will be changed to a TIC, and loading and relocating will proceed using the next buffer; if not, then the channel will stop at the NOP CCW, until it is restarted by IEWFETCH via another special IOS exit. The three buffers are in a continuous circular queue, each pointing to its next, and the last pointing to the first, and three buffers are constantly reused as loading and relocating proceeds.

IEWFETCH can, thereby, load and relocate a load module of any practical size, and in the minimum possible time.