Myth seems a bit strong... it was true early on (as you mentioned). Regardless, good summary.
There's still no reason to get into the 2G to 4G (now 32G) area unless you have the will and skill and need to implement something like the shifted-addressing-mode that Java does. Normally, it's useful to see some bits in the high-half when going above-the-bar, and avoid those pesky are-they-or-aren't-they addresses with bit 32 set. ISTR my first IARV64 returning an address of 00000001_00000000. Can't remember what release, but that's been a long time now. Those GiB references just make me think of NCIS. Sorry :-) On Fri, Mar 31, 2017 at 8:47 AM, Tom Marchant < [email protected]> wrote: > 4G bar? > > There is no such thing. > > A 31-bit address can address 2 GiB of memory, using just one > segment table, and that is the location of the bar. > > Anything beyond that requires a larger address. While you could > look at addresses of any number of bits, it makes little sense to > think of 32-bit addresses. The next logical increment in > z/Architecture is 42 bits, or 4 TB, the amount of storage > addressable by one Region-Third-Table > > It seems that some confusion was created by the fact that early > releases of z/OS would not allocate virtual storage in the range > from 2 GiB to 4 Gib, but that is no longer true, and hasn't been for > a long time. The range from 2 GiB to 32 GiB was "reserved for Java" > starting with z/OS 1.8, and that was extended to 64 GiB with > z/OS 2.1. There are parameters on the IARV64 macro to allow the > allocation of these virtual addresses. > > The myth that the bar has a "thickness" of 2 GiB seems to stem > from presentations that were made to describe the early restriction > to VSM that prevented the allocation of storage from 2 GiB to 4 Gib. > It is no longer useful to think of it that way. > > -- > Tom Marchant > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to [email protected] with the message: INFO IBM-MAIN > -- sas ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
