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Associative Search Bubble Devices for Content Addressable Memories

IP.com Disclosure Number: IPCOM000083816D
Original Publication Date: 1975-Jul-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 4 page(s) / 78K

Publishing Venue

IBM

Related People

Chang, H: AUTHOR [+2]

Abstract

In an associative memory, a word can be accessed on the basis of what it holds, rather than on the basis of where it is. To access a word or words, a part or the whole of the word is searched for a match with a specified content (called a tag). If the tag of an associative memory extends over the entire length of a word, the memory is called fully associative; otherwise partially associative.

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Associative Search Bubble Devices for Content Addressable Memories

In an associative memory, a word can be accessed on the basis of what it holds, rather than on the basis of where it is. To access a word or words, a part or the whole of the word is searched for a match with a specified content (called a tag). If the tag of an associative memory extends over the entire length of a word, the memory is called fully associative; otherwise partially associative.

To achieve fast search in an associative memory, all the words or a group of words in the memory are compared simultaneously with the tag. If all the bits of the words are compared with the respective bits of the tag simultaneously, it is called word-parallel search. If single bits in the words are compared successively with the corresponding tag bit on a bit-by-bit basis, then it is called bit-parallel search. Parts of the tag can also be masked for those parts of content which will not be used for comparison.

As an example, consider a bit-parallel search memory which contains four words. Let these words be: A(1) = 1011 A(2) = 1101 A(3) = 0000 A(4) = 0101 and the tag be T = 0101.

Search for words which have contents identical to the tag. When searching for the rightmost bit (1) of the tag, the third word is winnowed since its rightmost bit is 0. By continuing the winnowing process bit-by-bit, the first word, no word, and then the second word are successively winnowed. As a result, the fourth word is selected.

In general, the bit-parallel search is slower than the word-parallel search, but it requires less circuitry and hence lower cost. However, due to the serial nature of magnetic-bubble propagation, actually there is no loss of time if the bit-parallel search is used. Loadable-Disk Latches for Passage Sealing:

The serial nature and switching capability of magnetic bubbles can be used to implement a simple associative memory using a bit-parallel search. In this section, the basic element - a loadable-disk latch - will be described, and the procedure for executing associative search will be outlined.

A bubble switch provides two bubble paths, and the selection of a path can be effected by a residence bubble control. Actually, a bubble bubble-steered switch is a switch with memory capability, i.e., a latch. The following will describe how to use bubble latches to seal off unwanted bubble streams.

A bubble memory consists of shift registers, each of which stores a word. Assume that a bubble represents 1 while a void 0. In the passage between each shift register and the read station, a loadable bubble latch can be inserted, which is controlled by a common transfer/clear line. A loadable latch is designed such that when unloaded it will not affect the propagation of bubbles through the passage (see Fig. 1a). It can be loaded with a bubble which is contained in the incoming bubble stream, by activating the transfer current (Fig. 1b). Once a latch is loaded, the bubble residing in...