z/VSE internals: z/Architecture mode and low core - what is special in z/VSE ?
Ingolf24 120000DRN3 Visits (1960)
Processors like MP3000 support ESA/390 mode only. In ESA/390 mode you can address up to 2 GB (Gigabyte) real and virtual storage.. All System z processors (z800, z890, z9, z10 and zEnterprise) support ESA/390 as well as z/Architecture mode. In z/Architecture mode you can access more than 2 GB real and virtual storage - in z/VSE up to 32 GB real and up to 90 GB virtual storage.
z/VSE runs in z/Architecture mode since z/VSE 4.1. To hold the larger addresses (8 byte addresses instead of 4 byte addresses) the location of the interrupt information changed, such as the location for I/O, program, supervisor call and external interrupt information. The interrupt information is stored in low core (prefix area - starting at address zero), which is 4 KB (Kilobyte) in ESA/390 mode and 8 KB in z/Architecture mode.
To avoid the adaption of system routines and vendor applications to the new z/Architecture layout, z/VSE emulates the z/Architecture interrupt information to the corresponding ESA/390 locations. That is most of the system routines and vendor applications still use the ESA/390 interrupt information executing in z/Archetecture mode. No adaption was necessary in most cases. That reduces the effort to support z/Architecure mode significantly - or even better - no change was required for nearly all applications.
This is just an example how z/VSE always tries to make it easy even for system application to migrate to new releases.