Browse Prior Art Database

Look-Ahead Single-State Checker

IP.com Disclosure Number: IPCOM000102380D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 3 page(s) / 92K

Publishing Venue

IBM

Related People

Gervais, G: AUTHOR [+2]

Abstract

Parity checking has been used in the past to detect multiple active bits in a given bit string. This checking approach is 50% effective, as an error is generated only if there is an even number of active bits in the bit pattern. All odd combinations of bits in the pattern are left unchecked. The look-ahead single-state (LASS) checker described below provides a multiple active bit detection mechanism that checks all possible bit combinations and even reduces the logic required for single bit checking.

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

Look-Ahead Single-State Checker

       Parity checking has been used in the past to detect
multiple active bits in a given bit string.  This checking approach
is 50% effective, as an error is generated only if there is an even
number of active bits in the bit pattern.  All odd combinations of
bits in the pattern are left unchecked.  The look-ahead single-state
(LASS) checker described below provides a multiple active bit
detection mechanism that checks all possible bit combinations and
even reduces the logic required for single bit checking.

      The basic principle of the LASS checker is to detect the first
active bit in any given bit string, starting, for example, with the
most significant bit, and to generate a look-ahead mask (LAM) that is
used to check if any of the subsequent bits are active.  All bit
positions in the LAM corresponding to the bit string positions
following the active bit detected first are set to a binary "1".  A
logical AND operation is performed on the LAM and the bit pattern.
If any bits are set in the result, then multiple bits were active in
the bit pattern.  If the last bit in the LAM (carry bit) is a "0",
then none of the bits in the pattern were set to "1".  Fig. 1 shows
a few examples.

      A circuit implementation of the look-ahead single-state LASS
checking principle is shown in Fig. 3.  The bit pattern bits are
identified as "S01" to "S07" and the LAM bits are identified as "M02"
to "M07".  The Boolean expressions for the LAM gen...