<snip> there's a little-known "mini-bar" at the high-end of the 31-bit address space </snip>
True. The x'7FFFF000' page of the address space is intentionally not mapped in virtual and accessing that location in an address space (assuming AMODE 31 or 64) will always program check. Maybe some day we'll do the same for the x'00FFF000' page to cover the AMODE 24 case too. Note that the not-mapped page does not apply to a data space. X'7FFFF000' is a value that can be used to initialize a pointer that you expect not to be used (and if you do use it, it's an error). That value is better than 0 because (aside from ZAD) the usage of 0 might well "work" whereas an AMODE 31/64 reference to x'7FFFF000' will not. While not covering as much, we sometimes use x'7FFFFBAD' because it's visually catchy. Peter Relson z/OS Core Technology Design
