Browse Prior Art Database

Fast Compare-Branch Mechanism

IP.com Disclosure Number: IPCOM000104397D
Original Publication Date: 1993-Apr-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Muhich, JS: AUTHOR [+3]

Abstract

Disclosed is a method for improving conditional branch performance in a general condition code based branch instruction architecture. By forwarding condition code updates to the branch unit, outstanding conditional branchescan be resolved a cycle earlier than by only accessing the output of the condition code register (CR).

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 76% of the total text.

Fast Compare-Branch Mechanism

      Disclosed is a method for improving conditional branch
performance in a general condition code based branch instruction
architecture.  By forwarding condition code updates to the branch
unit, outstanding conditional branchescan be resolved a cycle earlier
than by only accessing the output of the condition code register
(CR).

      In pipelined processors, performance is heavily dependent on
branch delays caused by redirecting the instruction stream and
subsequently refilling the pipeline.  The sooner a branch is
resolved, the fewer instructions must be purged from the pipeline and
the smaller the branch delay.

      In addition, a common programming structure with high
concentrations of branches is a branch tree in which compare
instructions set condition codes which determine the next
compare-branch pair to be executed.  Thus, the compare-branch is an
important construct to make fast and efficient.

      It is generally possible to determine the results of a compare
instruction in less time than is required by some other
arithmetic-logic-unit (ALU) instructions, so that the results of the
compare instruction are known before the end of the cycle.  The
remainder of the cycle can be used to forward the compare results (in
the form of CR update information) to the branch unit in order to
resolve any outstanding branches which depend on this information.
Simultaneously, the CR is updated as it would be for any other
in...