Precision Timed Machine (PRET)

http://chess.eecs.berkeley.edu/

February 21, 2008
Center for Hybrid and Embedded Software Systems
Precision Timed Machine (PRET)
Isaac LiuBen Lickly
MissionThe goal of the PRET project is to develop new abstractions for computer systems that emphasize correctness of timing on the same level as performance and traditional functional correctness.We feel that all abstraction layers, from the high level programming specifications to the low level architecture, must be augmented in order to support these aims.Hardware ArchitectureIn our research, we have pursued a bottom-up approach. Starting with the underlying architecture (in our case an open source SPARC core), we have modified and added constructs to improve the timing predictability at the architectural and instruction-set-architecture level
PipeliningStalls, branch predictions, caches introduce unpredictability
Interleaved multithreadingBy using interleaving threads through our piprline, we are able to remove the data hazard and dependencies in the pipeline, creating a timing predictable pipeline.
Processor modelBefore making extensive architectural modifications to our SPARC core, we built a SystemC model of our processor:

SystemC transaction-level model more abstract than than register transfer level (such as VHDL)Faster and easier modeling and explorationBut, could not abstract away details of timing of the system!
ISA AugmentationFinding the best way in which an instruction set can be extended to support timing is an ongoing area of research. One approach that has been implemented involves adding deadline instructions to the ISA, which allow a program to specify how long certain blocks of code should take.
SPARC ModificationsFor our research, we chose to modify an open source VHDL implementation of the SPARC version 8 architecture processor created by Gaisler Research Labs called the LEON3. It contains a simple 7 stage pipeline with no branch prediction and adjustable peripherals. It also included compiler tool chains and support for convenient debug and testing.

Center for Hybrid and Embedded Software Systems

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