Browse Prior Art Database

Real To Virtual Address Translation

IP.com Disclosure Number: IPCOM000077013D
Original Publication Date: 1972-May-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Kelley, WJ: AUTHOR [+2]

Abstract

The table look-up instruction which has been used for converting virtual addresses to real addresses in a relocate environment can be used to convert real addresses to virtual addresses, the addressing exception logic being removed. Hardware paging tables (in main storage) permit conversion of a real address to a virtual address, provided that the virtual-address space does not overlap real-address space. The hardware page tables are the same tables which are used for virtual-to-real address translation.

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Real To Virtual Address Translation

The table look-up instruction which has been used for converting virtual addresses to real addresses in a relocate environment can be used to convert real addresses to virtual addresses, the addressing exception logic being removed. Hardware paging tables (in main storage) permit conversion of a real address to a virtual address, provided that the virtual-address space does not overlap real-address space. The hardware page tables are the same tables which are used for virtual-to-real address translation.

When a page is read from the external storage device, the page is made valid in both the Virtual Page table entry and the Real Storage page table entry. The real-storage address of the page is inserted as a Virtual Page table entry and the virtual-storage address of the page is inserted as a Real Storage page table entry.

This new method enables address conversion using only one instruction; the former software method required up to twenty instructions. The new method is up to six times faster. It is dependent on hardware page tables only and not upon internal paging tables. The figure shows an example of the page table entries. Issuing the instruction LRA 20,000 yields a result of 2,000 as the real address. LRA 2,000 yields 20,000 as the corresponding virtual address.

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