Browse Prior Art Database

Two Level DLAT Hierarchy

IP.com Disclosure Number: IPCOM000048572D
Original Publication Date: 1982-Feb-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 2 page(s) / 72K

Publishing Venue

IBM

Related People

Ngai, CH: AUTHOR [+2]

Abstract

It is known that the larger the DLAT (Directory Look-Aside Table), the less frequent the need to translate a virtual address through a translation mechanism, hence, higher system performance. But designing such a DLAT will make the cost/performance unattractive, because more high speed array chips and more cooling are required.

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Two Level DLAT Hierarchy

It is known that the larger the DLAT (Directory Look-Aside Table), the less frequent the need to translate a virtual address through a translation mechanism, hence, higher system performance. But designing such a DLAT will make the cost/performance unattractive, because more high speed array chips and more cooling are required.

By using a two-level DLAT design, a better cost/performance ratio can be obtained. In the drawing, the DLAT is arbitrarily designed with 256 deep and 2- way associativity as opposed to the known arrangements which have 32 deep and 2-way associative DLAT. For example, this DLAT is implemented by using slow array chips. The fast DLAT is an 8-way associative array which is implemented with logic chips. This DLAT can be easily contained in a VLSI chip. One data processor uses an associative array for the DLAT and has a 95 percent DLAT hit ratio.

Assume both DLATs have valid entries. When a virtual address (VA) comes in, it is used to compare with all eight virtual addresses in the fast DLAT 3. At the same time, bits 5-12 of the virtual address are used to address the slow DLAT array 5. Bits 0-4 of the virtual address are used to compare with the 2 virtual addresses coming out from the slow DLAT in compare circuits 7 and 9. The segment table origin (STO) addresses are also compared in compare circuits 13 and 15 with the current STO on line 11 to determine a DLAT hit. If the fast DLAT has a compare, the corresponding rea...