Editing IBM System/370 (section)

===Extended real addressing===
The initial System/370 architecture has a 24-bit limit on physical addresses, limiting physical memory to 16 MB.  Page table entries have 12 bits of page frame address with 4 KB pages and 13 bits of page frame address with 2 KB pages, so combining a 12-bit page frame address with a 12-bit offset within the page or a 13-bit page frame address with an 11-bit offset within the page produces a 24-bit physical address.{{sfn|S370|loc=[http://bitsavers.org/pdf/ibm/370/princOps/GA22-7000-10_370_Principles_of_Operation_Sep87.pdf#page=51 Page-Table Entries]|pp=3{{hyp}}26}}

The extended real addressing feature in System/370 raises this limit to 26 bits, increasing the physical memory limit to 64 MB.  Two reserved bits in the page table entry for 4 KB pages were used to extend the page frame address.  The extended real addressing is only available with address translation enabled and with 4 KB pages.{{sfn|S370|loc=[http://bitsavers.org/pdf/ibm/370/princOps/GA22-7000-10_370_Principles_of_Operation_Sep87.pdf#page=51 Page-Table Entries]|pp=3{{hyp}}26}}