Editing Memory management unit (section)

=== IBM System/360 Model 67, IBM System/370, and successors ===
The [[IBM System/360 Model 67]], which was introduced August, 1965, included an MMU called a dynamic address translation (DAT) box.<ref>{{cite web|url=http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_FS360.html |archive-url=https://web.archive.org/web/20050116150036/http://www-03.ibm.com/ibm/history/exhibits/mainframe/mainframe_FS360.html |url-status=dead |archive-date=January 16, 2005 |title=IBM Archives: System/360 Dates and characteristics |website=03.ibm.com |date=23 January 2003 |access-date=2017-05-03}}</ref><ref name=IBM-S360-67-FuncChar>{{cite web|url=http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/GA27-2719-2_360-67_funcChar.pdf|title=IBM System/360 Model 67 Functional Characteristics, Third Edition|id=GA27-2719-2|date=February 1972|access-date=October 29, 2021}}</ref>  It has the unusual feature of storing accessed and dirty bits outside of the page table (along with the four bit [[Memory_protection#Protection_keys|protection key]] for all S/360 processors).  They refer to physical memory rather than virtual memory, and are accessed by special-purpose instructions.<ref name=IBM-S360-67-FuncChar/>  This reduces overhead for the OS, which would otherwise need to propagate accessed and dirty bits from the page tables to a more physically oriented data structure.  This makes [[OS-level virtualization]], later called [[paravirtualization]], easier.

Starting in August, 1972, the [[IBM System/370]] has a similar MMU, although it initially supported only a 24-bit virtual address space rather than the 32-bit virtual address space of the System/360 Model 67.  It also stores the accessed and dirty bits outside the page table.  In early 1983, the System/370-XA architecture expanded the virtual address space to 31 bits, and in 2000, the [[64-bit]] [[z/Architecture]] was introduced, with the address space expanded to 64 bits; those continue to store the accessed and dirty bits outside the page table.