False exception for cancelled delayed requests

Details False exception for cancelled delayed requests False exception for cancelled delayed requests

1998-03-25

2001-04-17

Kim, Matthew (Department: 2186)

Electrical computers and digital processing systems: memory

Storage accessing and control

Hierarchical memories

C711S169000, C711S117000, C711S204000, C711S205000, C711S207000

Reexamination Certificate

active

06219758

ABSTRACT:

FIELD OF THE INVENTION
This invention is related to computers and computer systems and in particular to handling cases where prefetching provides a miss or a result which may be canceled.
BACKGROUND OF THE INVENTION
Instruction and operand prefetching maximize the efficiency of a pipelined machine because they keep each stage of the pipeline busy. Prefetching can be done on sequential as well as branch paths. Most of the time, the data needed are resident in the cache, translation lookaside buffer (TLB) or ART look-aside buffer (ALB) and are immediately available. However, for cases where the data are not found, more time is required to fetch them from the storage subsystem, for dynamic address translation (DAT) or for access register translation (ART).
A problem arises when the miss is due to a prefetch along predicted paths which may or may not be taken. If the data will indeed be used, then performance benefits. However, if it turns out that the data will not be used, the cache, TLB or ALB latency increases and performance is degraded. In addition, this new fetch displaces a cache line, TLB entry or ALB entry that may be needed later.
SUMMARY OF THE INVENTION
We have addressed the aforementioned problem by providing the ability to cancel translation or outstanding line fetches to the storage subsystem of data that have been determined to be of little or no use. The mechanism used is to force a false exception on the delayed request. This approach shuts down all wait states without adding much hardware because the exception logic already exists. The exception sent back to the requester is ignored because the request was already rescinded. A rescinded request means that the requester does not want the data anymore and has already forgotten about it. Any response related to that request is ignored.


REFERENCES:
patent: 4985825 (1991-01-01), Webb, Jr. et al.
patent: 5423014 (1995-06-01), Hinton et al.
patent: 5539895 (1996-07-01), Bishop et al.
patent: 5699553 (1997-12-01), Iino et al.
patent: 5829010 (1998-10-01), Cherabuddi

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