Cache memory employing dynamically controlled data array...

Details Cache memory employing dynamically controlled data array... Cache memory employing dynamically controlled data array...

2000-04-25

2002-05-14

Ellis, Kevin L. (Department: 2185)

Electrical computers and digital processing systems: memory

Storage accessing and control

Access timing

Reexamination Certificate

active

06389523

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a cache memory and, more particularly, to a cache memory suitable for use as incorporated in a microprocessor.
The cache memory is smaller than the main memory in storage capacity but faster in access. Therefore, the cache memory is located very close to the central processing unit (CPU) for the purpose of supplying data held in the main memory to the CPU. A variety of problems about the cache memory are discussed in the ACM, Computing Surveys, Vol. 14, No. 3, 1994, pp. 473-530 and “Computer Organization & Design—The Hardware/Software Interface,” Morgan Kaufmann Publishers, pp. 454-527, 1994, for example. The main problems of the cache memory are access time and power consumption.
An example of a conventional cache memory of relatively small power consumption is shown in the NIKKEI Electronics, Feb. 14, 1994, pp. 79-92 (this cache memory is hereinafter referred to as the first prior-art technology).
FIG. 2
shows a block diagram of the first prior-art technology.
As shown, the cache memory according to the first prior-art technology is a four-way set-associative cache memory. The set-associative memory is provided as follows. Namely, a plurality of areas that can hold data in a size of blocks in the cache memory are divided into a plurality of rows and a plurality of columns. Each of areas in main memory (not shown) that can hold a data block is divided into a plurality of columns corresponding to the above-mentioned plurality of columns. Block storage areas in the same column in main memory are associated with a given block storage area in the cache memory column corresponding to that same column.
To be more specific, as shown in
FIG. 2
, in the prior-art cache memory, an address array
200
is composed of four memory mats (also called ways)
206
(namely, way
0
, way
1
, way
2
, and way
3
), a decoder
205
commonly provided for these ways, and a precharge and equalize circuit
207
, a sense amplifier
208
, and a comparator
209
provided for each of the ways. Likewise, a data array
201
is composed of four memory mats
218
(namely, way
0
, way
1
, way
2
, and way
3
) and an address decoder
217
, a precharge and equalize circuit
219
, a sense amplifier
220
, and an output buffer
221
provided for each of the ways.
The above-mentioned prior-art cache memory operates as follows. First, access to the four ways
206
is started according to a middle address Am entered from a line
204
. Addresses registered in the way
0
, the way
1
, the way
2
, and the way
3
are read and are outputted from the sense amplifiers
208
provided for respective ways (these addresses are also referred to as tags). In the comparator
209
provided for each way, an upper address Au entered from a line
210
is compared with the address read from each way. If a match is found, namely if the cache memory has hit, the comparator
209
asserts a corresponding hit line
211
,
212
,
213
or
214
. Conversely, if a mismatch is found, namely if the cache memory has not hit, the comparator
209
leaves the corresponding hit line negated.
Of the four ways of the data array
200
, only one way for which the address array
100
has hit, is activated by the corresponding hit line.
Consequently, the above-mentioned prior-art technology is advantageous in power saving. However, the access time of the entire cache memory is a sum of the access time of the address array
200
, the time required for the comparison operation in the comparator
209
, and the access time of the data array
201
, resulting in a relatively large value. This makes it difficult to enhance the operating frequency of the cache memory.
To overcome such a problem, the present inventors considered a method in which the address array is activated at the same time the data array is activated.
FIG. 3
shows a block diagram of a four-way set-associative cache memory
3000
that operates in this method (this cache memory is called a reference technology hereinafter). In
FIG. 3
, the structures of an address array
300
and a data array
301
are generally the same as those of FIG.
2
. The difference between the prior-art technology of FIG.
2
and the reference technology of
FIG. 3
lies in that, when the address array
300
is activated, the data array
301
is activated at the same time. The data held in an output buffer
321
of one way among the four ways of the data array
301
corresponding to a way in which hit occurred in the address array
300
may only be outputted to a data line
322
. In this method, the address array
300
and the data array
301
are accessed simultaneously, so that the access time of the entire cache memory
3000
is approximately equal to the access time of the data array
301
. Thus, the access time of the entire cache memory is relatively short. In this method, however, a way in the data array corresponding to a way in which no hit occurred in the address array is also accessed, so that the power consumption of the data array increases significantly. Further, even if the operating frequency of the cache memory is lowered, the data array operates in the same manner as mentioned above, and therefore, the power consumption is not reduced.
The NIKKEI Electronics, Mar. 27, 1995, pp. 13-20 introduces a new RISC (Reduced Instruction Set Computer) processor (a second prior-art technology hereinafter) developed by the assignee hereof and others. Especially, page 16 of the same publication describes a technology for suppressing cache power consumption that follows. Namely, SH7708 employed three methods of suppressing cache power consumption. In the first method, only a way in which hit occurred in the address array is driven. This method was also employed in SH7604, but it is impossible to drive the data array after address array hit determination at high-speed operations, because of the limitation of circuit speed in SH7708. Hence, a circuit constitution for dynamically determining a drive timing of a data array was provided and, if hit determination cannot be made in time, all four ways of the data array are driven. The limit of the frequency for selectively driving one way of the data array is about 40 MHz.
SUMMARY OF THE INVENTION
As mentioned above, the cache memory according to the first prior-art technology can operate with somewhat small power consumption but is it difficult to enhance an access speed of this cache memory. The second prior-art technology does not describe how concretely power consumption was reduced.
It is therefore an object of the present invention to provide a cache memory that can operate at a relatively high speed and consumes a somewhat small amount of power at least in a low-speed operation.
It is another object of the present invention to provide a cache memory that can reduce power consumption at a high-speed operation and further reduce power consumption at a low-speed operation.
It is still another object of the present invention to provide a cache memory that can operate at a considerably high frequency, reduce power consumption in an operation at a relatively low frequency, and also reduce power consumption in an operation at a relatively high frequency located between the above-mentioned considerably high and low frequencies.
In attaining the above-mentioned objects, a cache memory according to the present invention has, in addition to a first start circuit for activating an address array in response to a read request which requests readout of data from another memory, a second start circuit for activating a data array after activating the address array. The second start circuit has a start execution circuit for dynamically selecting and executing one of a first start operation for activating the data array before completion of a hit check operation after the start of the address array and a second start operation for activating the data array after the hit check operation completes and it is determined that the address array has hit. As the first start operation realizes a high-speed operation because

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