Method and Apparatus of Low Power Cache Operation With a Tag Hit Enablement

Copyright .COPYRGT., Texas Instruments Incorporated, 1993. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office, patent file or records, but otherwise reserves all rights in its copyright whatsoever.

This invention relates to processing systems with a cache, and more particularly to a method for low power cache operation with a tag hit enablement.

In the prior art, processing systems including large cache consume large amounts of power. With the advent of portable systems there has been an increased need to reduce power consumption particularly in segmentable areas of a processing system which remain idle. Such an area is the cache.

The present invention provides a method for low power operation of a cache, the cache having a data RAM having output buffers, a tag RAM, a translation lookaside buffer and at least two comparators, the comparators comparing a tag address from the translation lookaside buffer with the contents of an address stored in the tag ram and generating a tag hit signal when a match occurs and providing a tag miss signal when no match occurs, the tag hit signal activating the output buffers of the data RAM when provided thereto for the reading of data therefrom. The method includes the steps of inactivating all of the output buffers; generating a tag hit enable signal that goes active after a predetermined time interval at least long enough to allow the comparators to generate the tag hit signal; generating a tag hit signal; and enabling the providing of the tag hit signal to the output buffers with the tag hit enable signal.

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram view of a cache with tag circuitry in which the method of the preferred embodiment is performed.

FIG. 2 is a schematic view of the tag circuitry of the preferred embodiment.

FIG. 3 is a schematic view of the tag circuitry of the preferred embodiment.

FIG. 4 is a schematic view of the tag circuitry with a dummy comparator.

FIG. 5 is a schematic view of the tag circuitry.

FIG. 6 is a schematic view showing the tags and multiple lines.

FIG. 7 is a schematic view showing the cache with tag circuitry.

FIG. 8 is a schematic view showing a processor and system.

FIG. 9 is a flow chart showing the method of the preferred embodiment.

Referring generally to FIG. 8, the preferred embodiment of a microprocessor 11 with a low power cache 13 and tag assembly 15 is shown.

The method of low power operation of the cache includes: all output buffers during all cache operations to be inactivated except when a valid data operation is performed, a tag hit enable signal is generated, to activate the data RAM output buffers with the tag signal, data RAM output buffers which are receiving a hit state from a tag RAM are activated in order to transfer data from data RAM to a CPU data bus, tag hit signals are pre-charged to miss state, all data RAM output buffers with tag miss signals are disabled.

Referring to FIG. 1, the method of operation of cache 13 includes pre-charging all tag and tag complement lines (FIG. 3) to a same state; sense amp 17 activates a tag and tag complement line pair to differential states, and activating comparators 19 for determining hit or miss signals, selecting a tag signal with a slowest signal path for activating a dummy comparator 37 (FIG. 3 and FIG. 4) to generate a tag hit enable signal, enabling a tag comparator output 21 with the tag hit enable signal 23, and generating a correct tag hit state.

The method further includes generating a self-time pre-charge signal for tag comparators and tag hit circuitry using the tag hit enable signal.

The method of operation of cache 13 also includes a line invalidation control circuit (FIG. 5) and a tag including multiple lines; each of the multiple lines has an identical block address; at least one multiple line including block valid bits; each of the lines has local valid bits; when writing a new tag, the block valid bits and the local valid bits are reset; a control circuit to provide a line invalidation control signal for resetting local valid bits upon receiving a tag miss signal; comparing any pending read miss requests with a read miss request corresponding to the tag miss signal; aborting sending of the line invalidation control signal upon finding a match up the step comparing.

Cache Organization

The microprocessor on-chip cache 13 is a unified code and data cache. The cache is used for both instruction and data accesses and acts on physical addresses. The on-chip cache is 8 Kbytes in size. The cache is organized as two-way set 25, 27 associative and each line is 4 bytes wide. The 8 Kbytes of cache memory are logically organized as 1024 sets, each containing 2 lines.

The cache memory is physically split into two 4-Kbyte blocks each containing 1024 lines (see FIG. 6). Associated with each 4-Kbyte block are 256 20-bit tags. This implies there are 4 lines in a block which are associated with the same tag. These 4 lines are consecutive at 16-Byte boundaries. There is a valid bit associated with every byte in a line. Hence there are 4 valid bits per line indicating which of the four data bytes actually contain valid data. There is also a valid bit associated with each block (of 4 lines), which when reset (to zero) indicates that none of the 16 bytes in the 4 lines of that block are valid. Note that when block valid bit is reset (to zero) the 16 valid bits in the block no longer imply validity of data bytes in the block.

The allocation strategy of the cache is read allocate. The data space is allocated to the cache only on read misses, and not on write misses. Whenever a new tag is written into the cache, a block of data space is allocated although fewer bytes of valid data may be actually written. The write strategy of the cache is write through. All writes will drive an external write bus cycle in addition to writing to the cache if the write was a cache hit (which may be partial). This implies that a tag hit is a necessary and sufficient condition for updating the cache during a write cycle. The cache uses a LRU replacement algorithm. Note that writes where both tag and byte valid bits match are always cacheable.

Cache Low Power Issue

A large size cache such as the one used in a microprocessor consumes a large amount of power when clocking at a high rate. Many design efforts were devoted to reduce the cache active power, one of the key design implementations in a microprocessor on-chip cache is preventing bus signals from switching except when true data have arrived to cut down the unnecessary power dissipation. The cache data RAM output bus is the target for the new circuit technique, since its switching power is particularly high. In the microprocessor on-chip cache design 32 output buffers and long metal lines are subjected to switching every time the buffers are enabled. The new circuit technique only enables the data RAM cache output buffers when the correct hit signal arrives. A four way cache architecture block diagram is shown in FIG. 1. When a tag match occurs in the tag RAM, the hit signals from the tag RAM hit logic block turn on one set of 32 from 128 data RAM output buffer in data RAM Waysel circuit and allows the data RAM data to be driven onto the data bus. Only one out of four this signals can be on at a time.

Low Power Circuit Technique

During a miss or non-cache cycle, the data bus is completely disconnected from the data RAM to keep the data bus quiet and save the data bus switching power. To be able to achieve this, all hit signal from the tag RAM have to be kept low (inactive) during all operations. This is accomplished by pre-charging all hit signals to miss condition, thus disconnecting data bus from all data RAM output buffers. Only when a tag hit occurs, the output buffers which received the hit states from the tag RAM are turned on to transfer the data from the data RAM to the CPU data bus. When data transfer operations are completed, all hit signals are again pre-charged to miss condition.

To accomplish the function described above, all tags and their complement (tagx) lines of the tag RAM are pre-charged to a same state. When the tag sense amps activate, then each tag and its tagx line are driven to differential states. The tag comparators are designed to determine hit or miss output when the tag lines are complemented. FIG. 2 shows an example of the comparator 19. Both tag inputs 29, 31 are pre-charged to low, and the comparator output 33 are pre-charged to high if it is a hit, and will be pulled low if it is a miss. Since this is reversed of what is needed for the circuit, (the output need to pre-charge to a miss) one more logic layer is needed to convert the output to the correct state.

Referring to FIG. 4, all comparator outputs are enabled by an enable signal 35 (TCOMP). This is the key for the new circuit. This signal is derived by using tag signals from a slowest tag block which has the slowest signal path. These signals are connected to a dummy comparator (FIG. 3) which is constructed similarly to a regular comparator for maximum signal tracking. The dummy comparator 37 are wired as such its output is switched to enable state when the tag signals arrive. The dummy comparator enable all comparator output when it fires. The tracking mechanism of the dummy comparator keeps the incorrect state (hit) of the regular comparator from propagation until the tag comparisons have been completed. This is important for high speed and low power cache operation, because a slow TCOMP signal hurts speed and too fast TCOMP increases power by allowing incorrect hit states to propagate to the output buffers.

In addition, the TCOMP signal is also used to generate a self-time pre-charge signal to pre-charge the tag comparators and the tag hit circuitry. This improves the maximum operation frequency since the comparator pre-charge period is self timed and independent to the main pre-charge clock, thus it is unlikely to be in the critical path. The block diagram of the new technique is shown in FIG. 4.

FIG. 9 is a flowchart summarizing the method of the preferred embodiment of the present invention. In a first step 40, the output buffers are inactivated. The tag hit signals, on line 33 (FIG. 2) are then precharged to a miss state 41. The tag hit signal is then generated 42. However, the tag hit signal is not provided to the output buffers immediately. Rather, a tag hit enable signal, T.sub.-- COMP, is generated 43, using the dummy comparator (FIG. 3). The tag hit enable signal goes active after a predetermined time interval at least long enough to allow the comparators to complete their comparisons. The tag hit signal is enabled with this tag hit enable signal 44.

Circuit for selectively powering-up an on-chip microprocessor cache memory.

A processor accesses the cache when it needs to either read/write/instructions/data from/to the cache. The data or instruction fetch does not happen in every processor cycle, but is interspersed with decode, execute and write-back cycles. On every cycle, the access to the cache rams is controlled by the cache controller. The cache controller issues the cycle address and control signals to the cache rams. At the beginning of a cache read cycle (which may be instruction or data), the cache controller issues a "read.sub.-- access" signal to the cache ram indicating the beginning of a valid cache access cycle. The cache rams can use this signal to initiate a cache rams access cycle. Whenever the `read.sub.-- access` signal is not asserted, the cache rams remain inactive and thereby do not dissipate any active power. This `read.sub.-- access` signal is inclusive-or of instruction and data read access signals.

When the timing of the `read.sub.-- access` signal is critical and it needs to be available at the beginning of a cycle, it can be made to be a superset of the signal which accurately signals a valid cache access cycle. This less optimal superset signal will meet the cycle timing requirements, but will turn on the cache access cycle more than actually required.

A case of the superset `read.sub.-- access` signal is as follows: In a single cycle cache read operation, a valid cache read operation is initiated at the beginning of a cycle. The cache control logic supports the cache to be accessed even if there are previous read miss requests that have not yet been serviced. In that case, the current cache access cycle line and tag address are compared with all of the read miss pending requests. If there is a match it indicates that there is already a request ahead of the current request which is in process of accessing the same data. In that case, the current cycle is not initiated and the cache is not accessed. This optimal `read.sub.-- access` signal can be a little too late to signal a valid cycle to the cache. The alternative is to use a superset signal which excludes address match detection but does meet the timing requirements.

Master-Slave Tag RAM Design for Tag Duplicate Fault.

The tag duplicate fault occurs when the same tag is stored in two different tag arrays. This can generate an unwanted multiple tag hit fault: a catastrophic failure that can destroy the device, or cause reliability problems. Although it is unlikely to happen during normal tag operation mode, the problem can occur during the power-up or from a bad tag testing procedure.

The microprocessor on-chip cache uses the Master-Slave tag RAM design technique to prevent the tag duplicate fault. Tag ram #1 is the master and tag RAM #0 is the slave. The tag #1 comparator output, which is pre-charged to a hit condition, is used to inhibit tag #0 hit signal. The slave hit signal only enables when the master comparator outputs a miss condition.

The technique can be applied to the tag system with multiple tag RAMs other than two. for example: Cascading multiple levels of masters and a slave such as Master1--Master2--Master3-- . . . --Slave. Master2 is the Master1's slave and Master3 is Master2's slave and so on. A hit on the upper level master inhibits all subsequent hits. Lower level hits are only enabled by all upper level misses.

Extended Single Cycle Read/Write RAM Array.

Referring to FIG. 7, the microprocessor on-chip cache LRU array is incorporated into the tag array and uses the self-timing signals from the tag for its read operation. However, the LRU RAM requires the write operation to be executed right after the hit signal is generated from the tag RAM. This happens toward the end of the cycle. The LRU write operation therefore needs to be extended after the RAM read cycle, the array needs to be pre-charged before the next RAM cycle begins. This will determine the cycle time performance of the LRU array operation.

The LRU clock circuit determines when the array pre-charge occurs:

1. Tag write or tag miss: normal tag operation, tag pre-charge signal is used for the LRU array pre-charge.

2. Tag hit: use extended cycle operation. The LRU clock generates write signals to the LRU array and also to a LRU dummy cell. A pre-charge signal is generated based on the completion of the dummy cell write operation. This is used to pre-charge LRU array and dummy cell circuitry independently to the tag ram pre-charge operation. The next cycle can begin as soon as the LRU array completes the pre-charge.