Advances in Language Design

1. Advances in Language Design Lecture 18: Dolores Zage

2. Topics • Variations on Subprogram Control • Parallel Programming • Formal Properties of Languages • Language Semantics • Hardware Developments • Software Architecture

3. Where do advances come from? • Formal models of the programming process • experience->theory-> evaluation-> • experience - programs have bugs, programming is hard • theory -> structured programming, BNF grammars, data abstractions • evaluation -> build languages using these concepts • These have lead to further improvements • object-oriented classes, SLR(1) and LALR(1) parsing techniques

4. Von Neumann Architecture • Computer consists • relatively large memory • a much faster but smaller control memory • CPU • All present languages implement these features

5. Changes in hardware and software architecture • Machines increase by a factor of 2 every 3 years • same basic problem remains -> control memory and CPU are always about an order of magnitude faster than the larger main memory • a computer spends much time simply waiting

6. Waiting Problem Solutions • Software that allows for more efficient use of the hardware by concurrency • This permits the hardware to execute more efficiently by executing another program when one program is blocked waiting for data. • However, programmers are poor in rewriting applications to handle concurrency

7. Waiting Problem Solutions • Another solution is develop more effective hardware • More cache • however the nature of computing of changing -> client/server

8. Variations on Subprogram Control • Four assumptions on our usual activation record • explicit call statements are required • subprograms must execute completely at each call • immediate transfer of control at the point of call • single execution sequence

9. Exception and Exception Handlers • Error conditions • conditions detected by the virtual computer • conditions generated by the semantics of the programming language • unpredictable conditions • tracing and monitoring

10. Coroutines • Drop the assumption to allow subprograms to return to their calling program before completion of execution. • Two subprograms swapping control back and forth as each executes • now used in simulation languages

11. Scheduled Subprograms • Relax the concept that execution of a subprogram should always be initiated immediately upon its call • one way to think of a call is it is to be scheduled for execution immediately • can be thought of exception • again used for discrete time simulation • note no main, just a scheduler

12. Nonsequential Execution • Take away the assumption of single execution sequence • Parallel programming • Concurrent programming

13. Parallel Programming • Multiprocessor system -> several CPUs • Distributed or Parallel computer system has several computers each with its own memory and CPU connected with communication links into a network • multiprogramming and time sharing • major block - lack of support in PLs • for the most part, languages use additional OS calls

14. Parallel Programming Languages • Add complexity since several processors may be accessing the same data simultaneously • need 4 concepts • Variable definitions - mutable or definitional • Parallel composition - a statement to define threads • program structure - transformational or reactive • communication - shared memory or messages

15. Commands of Concurrency • Construct to allow for parallel - the and • statement 1 and statement 2 and … • the statement beyond the and does not execute until all the parallel statements finish • guarded statements - nondeterministic execution • tasks - dependent • semaphores ...

16. Formal Properties of Programming Languages • Chomsky Hierarchy • Undecidability • Is there a limit to what we can compute with a computer? • Instead of writing a C program, can you write another program that reads a description of the C program (its source file) and determines whether it would halt

17. Programming Verification • About the correctness and reliability • languages are being designed with features that enhance these attributes • assert in C++

18. Hardware Developments • Improve the performance of a single CPU • develop alternative architectures

19. Processor Design • CISC - making instructions more powerful, fewer instructions, less movement of data • RISC

20. System Design • Single bus • crossbar switch • Omega network

21. Software Architecture • Persistent or transient data • read in persistent data into transient data within the program and then rewrite the transient data back into the persistent storage • this type of model is inefficient for some systems • reservation system

22. Transaction System • Program executing continually • reservations occur at any time • just keep the data in transient • problem of system going down and losing the transient updates • solve this by a persistent programming language - no distinction between transient and persistent data

23. Networks and Client/Server Computing • Major impact on programming language design is the lack of a global data store for the program. • Both client and server only have limited access to the total information content of the program