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A technique is described whereby an instruction opcode and mask is used to modify hashing, when employed in a decode history table (DHT), so as to improve computer operating performance.
English (United States)
This text was extracted from a PDF file.
This is the abbreviated version, containing approximately
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Improved Decode History Table Hashing
A technique is described whereby an instruction opcode and mask is used to
modify hashing, when employed in a decode history table (DHT), so as to
improve computer operating performance.
Typically, DHTs use the low-order bits of the half word address of the branch
to access the DHT when a branch that is decoded is conditional. For example, in
the operation of a computer system, such as the IBM 3090, conditional means
that the condition codes are not available to the decoder at decode time.
Depending on the geometry of the array used to implement the DHT, it is
possible to effectively use a number of smaller DHTs with the appropriate DHT
selected by a function of the opcode and mask. For example, if the DHT array is
4 x 1024, then the determination of which of the four bits to examine/ modify
could be selected after the bits are read from the array. This is equivalent to
using the function of opcode and mask as a replacement for two bits of the half-
word address in the DHT hash.
The concept described herein concentrates on the branches that are
predicted by a DHT. It separates the branches into different classes based on
some combination of opcode and mask so that: a) Each class has approximately
equal frequency in the instruction mix, and b) The likelihood that a branch is
taken (on the average) varies significantly between the classes, then the
performance of the DHT will be improved by using such a classification instead o...