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
