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IEEE Floating-Point Guess Underflow/Overflow

IP.com Disclosure Number: IPCOM000100924D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 62K

Publishing Venue

IBM

Related People

Cocanougher, D: AUTHOR [+4]

Abstract

In IEEE standard 754 Floating-Point Units (FPUs), results which underflow or overflow require additional loops through the data flow for adjustments or denormalization. A Guess Underflow and Guess Overflow scheme is implemented to predict, very early in the instruction execution cycle, when an underflow or overflow will occur. This improves performance by not having to wait until the final result is calculated before allowing following instructions to enter the data flow pipeline (see the figure). This scheme also circumvents the problem of backing instructions out of the pipeline, when it is needed to denormalize a result.

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IEEE Floating-Point Guess Underflow/Overflow

       In IEEE standard 754 Floating-Point Units (FPUs), results
which underflow or overflow require additional loops through the data
flow for adjustments or denormalization.  A Guess Underflow and Guess
Overflow scheme is implemented to predict, very early in the
instruction execution cycle, when an underflow or overflow will
occur.  This improves performance by not having to wait until the
final result is calculated before allowing following instructions to
enter the data flow pipeline (see the figure).  This scheme also
circumvents the problem of backing instructions out of the pipeline,
when it is needed to denormalize a result.

      If the result of every arithmetic instruction was a normalized
number, then a new instruction could be received and decoded every
cycle.  Once the pipeline (which consists of a multiply, add, and
write cycle) fills, a result could be generated every cycle.  But if
a result overflows or underflows, additional loops through the data
flow pipeline are required to add adjustments or denormalize the
result.

      Using the Guess Underflow and Guess Overflow scheme, it can be
determined in the multiply cycle whether additional passes through
the pipeline will be required.  If no Guess Underflow or Guess
Overflow is generated in the multiply cycle, then the instruction in
the decode cycle can move down to the multiply cycle and a new
instruction brought down to the decode cycle.

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