Browse Prior Art Database

Directory Look Aside Table Controls

IP.com Disclosure Number: IPCOM000052898D
Original Publication Date: 1981-Jul-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 41K

Publishing Venue

IBM

Related People

Drimak, EG: AUTHOR [+2]

Abstract

In a virtual memory system, it is often desirable for performance reasons to keep protection keys associated with real address X in the Directory Look-Aside Table (DLAT) entry that translates a virtual address to real address X. The problem that this creates, however, is that somehow that DLAT entry must be invalidated should the key value be changed by programs executing within the system. A mechanism is proposed to accomplish this invalidation process by using the existing compare functions(s) already required to perform the system storage protection function.

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Directory Look Aside Table Controls

In a virtual memory system, it is often desirable for performance reasons to keep protection keys associated with real address X in the Directory Look-Aside Table (DLAT) entry that translates a virtual address to real address X. The problem that this creates, however, is that somehow that DLAT entry must be invalidated should the key value be changed by programs executing within the system. A mechanism is proposed to accomplish this invalidation process by using the existing compare functions(s) already required to perform the system storage protection function.

The figure illustrates a conventional two-way associative DLAT. Virtual addresses in the Storage Subsystem Address Register (SSAR) 5 select a one of N pairs of DLAT entries. The SSAR is compared against the virtual addresses (VAs) in each DLAT entry. If either compare is positive (a DLAT hit), the real address (RA) in that DLAT entry is used to address memory. A miscompare on both entries (a DLAT miss) causes the system to translate the address by referencing various translation tables in memory and load a valid DLAT entry via the Instruction Processor Unit (IPU) Data Bus.

Compare circuits 1 and 2 exist in the system to provide the storage protection function. During an IPU memory reference the key associated with the reference
(i.e., the current value in the IPU Key Register) is compared against the key field in each DLAT entry. Providing there is a DLAT hit, these com...