Browse Prior Art Database

Condition Code Field Renaming

IP.com Disclosure Number: IPCOM000105379D
Original Publication Date: 1993-Jul-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 2 page(s) / 60K

Publishing Venue

IBM

Related People

Karim, FO: AUTHOR [+3]

Abstract

In most microprocessors, execution units set condition code fields to determine if conditional branches will be taken. In a pipelined, superscalar microprocessor, multiple execution units may be writing these conditions at the same time, out of order, and into many different condition code fields.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Condition Code Field Renaming

      In most microprocessors, execution units set condition code
fields to determine if conditional branches will be taken.  In a
pipelined, superscalar microprocessor, multiple execution units may
be writing these conditions at the same time, out of order, and into
many different condition code fields.

      A method to allow multiple execution units to write to the same
architected condition code field without overwriting each others'
data is disclosed.

      Condition code field renaming is accomplished by having more
physical condition code fields than architected condition code
fields.  An example machine may have 8 architected condition code
fields and 32 physical condition code fields.  The architected
condition code fields are those specified by the architecture and
instruction set.  The physical condition code fields are those
actually implemented in the hardware.

      \enaming on each condition code field is performed using
internal tables.  The renaming hardware assigns a physical condition
code field to each architected condition code field specified by the
instruction.  When an instruction completes, it writes its condition
results to the physical field.  When future branch instructions
reference this condition, the correct physical condition field must
be checked.

      When processing conditional branches, the branch unit will
receive the architected-to-physical mapping information from the
renaming hardware.  It uses this mapping to determine which physical
condition code field to check to determine if the branch will be
taken.

      Fig. 1 shows the renaming process for four consecutive
instructions that write the same architected condition field.  These
will be renamed to distinct physical condition fields, as shown.
With renaming of the con...