Browse Prior Art Database

Extended Branch Condition Selection

IP.com Disclosure Number: IPCOM000110544D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 2 page(s) / 73K

Publishing Venue

IBM

Related People

Flurry, GA: AUTHOR [+2]

Abstract

This technique integrates the ability to select special branch conditions into an existing processor architecture. The RISC/CISC is a processor intended to run its own native instruction set and, using that instruction set, emulate another processor. In the processor, it is necessary to allow branching on conditions and special processor conditions unique to the other processor. The native conditional branch instruction, the format of which is shown in Fig. 1, allows choosing the condition based on a bit in the Condition Register (CR) and the content of the Count Register (CTR). The BO instruction field selects a particular combination for conditions, and the BI instruction field selects a particular bit in CR, as shown in Fig. 2.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 72% of the total text.

Extended Branch Condition Selection

       This technique integrates the ability to select special
branch conditions into an existing processor architecture.  The
RISC/CISC is a processor intended to run its own native instruction
set and, using that instruction set, emulate another processor.  In
the processor, it is necessary to allow branching on conditions and
special processor conditions unique to the other processor.  The
native conditional branch instruction, the format of which is shown
in Fig. 1, allows choosing the condition based on a bit in the
Condition Register (CR) and the content of the Count Register (CTR).
The BO instruction field selects a particular combination for
conditions, and the BI instruction field selects a particular bit in
CR, as shown in Fig. 2.

      The capability depicted in Fig. 2 is inadequate for high
performance emulation.  The present technique first requires a new
conditional branch instruction.  This new instruction format is
identical to the native conditional branch instruction, but has a new
primary operation code to distinguish it from the native instruction.
The BO field still selects a combination of conditions, but now the
BI field selects a special condition, as shown in Fig. 3.

      The first six rows (where the third and sixth row have y=0) in
Fig. 3 now select from a first set of conditions.  The branch if true
and branch if false rows (with y=1) allow a second set of conditions
to be selected.

   ...