Embedded systems typically do not have loaders, and instead, the code executes directly from ROM or similar.
IEWFETCH utilizes highly specialized 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 ms maximum, 8.3 ms average, on "legacy" 3,600 rpm drives).
The OS's 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 KB, while other tables and code may reside elsewhere.
[3] 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.
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.
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.