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# Fast and Modular 2048-Bit to 11-Bit Priority Encoder

IP.com Disclosure Number: IPCOM000110828D
Original Publication Date: 1994-Jan-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 6 page(s) / 109K

IBM

Patel, M: AUTHOR

## Abstract

Disclosed is a 2048-bit to 11-bit priority encoder that is fast and does not require high speed clocks required by sequential solutions.

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Fast and Modular 2048-Bit to 11-Bit Priority Encoder

Disclosed is a 2048-bit to 11-bit priority encoder that is fast
and does not require high speed clocks required by sequential
solutions.

A priority encoder generates the location of the bit in the
input that has a one in the lowest address.  For example, DIN
(2047:0) is a vector of 2048 bits (bit 0 is least significant and bit
2047 is most significant).  The 11-bit priority encoded value, PE
(10:0) (bit 0 is least significant and bit 10 is most significant),
for DIN (2047:0) = XXX...XXX10000000 is PE (10:0) = 00000000111.

There are sequential methods which require very fast clocks to
iteratively compute one bit at a time for computing the 11-bit
priority encoded number from 2048-bit value.  The problem is solved
for groups of 16-bits first and then the results of the first stage
are used to compute which group is to be selected for the second
stage and so on.  Fig. 1 shows a block diagram of a 2048-bit to
11-bit priority encoder.  It is made up of eight 256-bit to 8-bit
priority encoders (PE8), Fig. 2, that compute individual values
simultaneously and then one of the eight 8-bit result is selected via
the PSEL3 to select the lowest address PE8 that had a bit on.  An
8-bit to 3-bit priority encoder (PE3), see Fig. 3, is used to compute
the value of the lowest address PE8 that had a bit on.  The PE3
outputs form the high order 3 bits and the PSEL3 outputs form the
lower order 8 bits of the 11-bit prioritized value of the original
2048-bit value.

PE3 and PE4 are well known priority encode functions so
truth-table description is provided for PE3 in Fig. 3.  PE4 is
similar to PE3 but for 4 bits.  PCNTL3, Fig. 5, describes the
truth-table for the priori...