Review of “Register Binding for FPGAs with Embedded Memory”

by Hassan Al Atat and Iyad Ouaiss

Lisa Steffen

CprE 583

Outline

Introduction Motivation Proposed Memory Binding Technique Results Strengths and Shortcomings Conclusion

Introduction

Early FPGAs– Trend was to use as many logic cells as possible– External memory banks were typically attached to

the FPGA to hold data structures Communication with external memory banks

very slow– Motivating need for embedded memory in FPGAs– Many register binding techniques were developed

to minimize storage requirements of designs

Introduction (cont.)

Register Binding– Aim is to minimize the number of registers

necessary to hold all of the design’s variables– Registers are shared by multiple variables who’s

lifetimes do not overlap Variable lifetime is time between when the variable is

first written to and the last time it is read Result is decreased number of required registers

Motivation

Current trend in FPGAs is to include large amounts of embedded memory banks– Summary of Altera and Xilinx device families

shows this trend– Trend shows increase in logic cells and memory

bits– Ratio of embedded memory bits to logic cells

sharply increasing Altera devices have increased from ~8% to ~88% Xilinx devices have increased from ~5% to ~82%

Motivation (cont.)

Research in the area of using memory blocks for variable storage in ASICs

Techniques that implement register binding and the registers are then grouped into memory pads

Techniques which map all variables to one memory and then the memory is partitioned

Techniques that group variables into memory models and then binds them into registers

Motivation (cont.)

When targeting an ASIC, one goal when mapping variables to memories is minimizing the number of memories used

FGPAs have a fixed number of embedded memories, minimizing the number used is not necessarily a goal

Interconnection overhead may be improved by using more memories

Motivates need for an FPGA memory binding technique

Memory Binding

Registers are traditionally implemented using logic resources

Results in decreased utilization from placement and routing congestion

Logic utilization can be improved by making use of the available memory blocks through the use of memory binding

Memory Binding

Register binding maps variables to registers based on their lifetimes

Memory binding maps variables to memories based on their read access times and write access times

Variables may be mapped to the same memory if they do not conflict– Variables conflict if they are read in the same time

step or if they are written to in the same time step

Memory Binding

The Algorithm– A list is obtained of every variable in the design– Two lists maintained for every memory bank used

for memory binding beginList contains all of the writing times for all of the

variables mapped to the memory bank endList contains all of the read times for all of the

variables mapped to the memory bank

Memory Binding

The Algorithm (cont.)– Any unassigned memory who’s write time(s) are

not found in the beginList and who’s read time(s) are not found in the endList are added to the memory, read/write times are added to the appropriate list

– Continue until all variables are mapped

Results

Performance of Memory Binding technique was shown by applying the technique to three benchmark synthesis results

Number of logic cells used with no variable binding techniques were compared with the number of logic cells used with the memory binding technique as well as with two register binging techniques; Clique Partitioning and Left-Edge Algorithm

Results

All three techniques reduce the total number of logic cells occupied in the device

Improvement by the Clique Partitioning was marginally better than the Left-Edge Algorithm

Improvement by the Memory Binding technique was clearly greatest of the three techniques

Resluts

Memory Binding improvement over Clique Partitioning was ~6% with a small resource bag and ~40% with a large resource bag

Memory Binding improvement over the Left-Edge Algorithm was ~6% with a small resource bag and ~50% with a large resource bag

Strengths

Memory Binding makes use of the memory blocks which are on the FPGA

Memory Binding results in clear area utilization improvements over other available variable mapping techniques

Shortcomings

Memory Binding introduces a lot of complexity into the design

Memory Binding introduces three new sources of delay

– Variables bound to memories rather than registers are at a greater distance from their operators

– Memories have a larger access delay than registers– Time is needed to calculate the addresses of the variables

in memory

Shortcomings

The algorithm must have memory blocks available which can be utilized for memory binding– Many FPGA designs are data-oriented, many

designs may use the majority of the memory for data storage

Performance was only studied in the area of logic cell utilization, no timing results shown from applying the three techniques

Conclusion

For specific designs, the proposed technique could provide many benefits– Designs which utilize a large number of logic

cells, but little embedded memory– Designs which are able to utilize the memory

binding without effecting the timing requirements of the design

Conclusion

Number of logic elements in an FPGA are also fixed, as the number of memory blocks are fixed

If the design fits into the number of logic elements available, the cost incurred by applying the memory binding would be greater than any benefit received

Conclusion

Much more analysis needs to be performed with regards to the timing of the design after performing memory binding

Most designs would not be able to incur the extra delay of having to access all variables from memory