Editing Z/OS (section)

===64-bit memory support===
z/OS supports 64-bit addresses for both virtual<ref>{{cite book
 | title       = z/OS 2.5 MVS Programming: Extended Addressability Guide
 | id          = SA23-1394-50
 | date        = 2021-09-30
 | section     = Chapter 4. Using the 64-bit address space
 | section-url = https://www.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R5sa231394/$file/ieaa500_v2r5.pdf#page=79
 | url         = https://www.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R5sa231394/$file/ieaa500_v2r5.pdf
 | pages       = 59–91
 | publisher   = [[IBM]]
 | access-date = January 6, 2023
 }}
</ref> and real addresses.
Within each address space, z/OS typically permits the placement of only data, not code, above the 2&nbsp;GB "bar". z/OS enforces this distinction primarily for performance reasons. There are no architectural impediments to allowing more than 2&nbsp;GB of application code per address space. IBM has started to allow Java code running on z/OS to execute above the 2&nbsp;GB bar, again for performance reasons.

Starting with z/OS version 2 release 3, code may be placed and executed above the 2&nbsp;GB "bar". However, very few z/OS services may be invoked from above the "bar".

Memory is obtained as "Large Memory Objects" in multiples of 1&nbsp;MB (with the expectation that applications and middleware will manage memory allocation within these large pieces). There are three types of large memory objects:

* Unshared – where only the creating address space can access the memory.
* Shared – where the creating address space can give access to specific other address spaces.
* Common – where all address spaces can access the memory. (This type was introduced in z/OS Release 10.)