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Paged Control Store Prefetch Mechanism

IP.com Disclosure Number: IPCOM000080380D
Original Publication Date: 1973-Dec-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Enger, TA: AUTHOR

Abstract

In a paged, writeable, microprogram control store (CS) system, time is lost when a required microword is not in the control store buffer. Frequently, one microinstruction will "know" in advance that another group of words will be required. Lost time is saved by prefetching a line when it is expected to be needed.

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Paged Control Store Prefetch Mechanism

In a paged, writeable, microprogram control store (CS) system, time is lost when a required microword is not in the control store buffer. Frequently, one microinstruction will "know" in advance that another group of words will be required. Lost time is saved by prefetching a line when it is expected to be needed.

In a paged control store 1, a directory 2 contains addresses corresponding to lines in the buffer. A complete address in a microstruction 3 consists of a Line portion 4 and a Word portion 5. The Line portion is compared, 6, to the entries in the directory. If the Line is found, its position within the buffer is generated by an encoder 7. This is combined with the Word portion of the address to create the complete address for the buffer 1. If the line is not found, the Line address is passed to the backing store, not shown, to be fetched.

To implement prefetch, triggers 8 are added to capture the output of the encoder 7. (Capturing the compare results would work equally well.) Thus, the triggers 8 will always point to the line containing the current microword. When the prefetch command from portion 9 is decoded, the state of the triggers 8 replaces the result of the comparisons 6 for determining the buffer line to be accessed for the next microword. The Word portion 5 functions in the usual way to select the required word within the line. The target of any word issuing a prefetch command must be constrained to be in...