Summary of 1/28/97 Operating Systems Class

Differences between Monitors/Serializers and CSP/Ada
CSP
Commands in CSP
CSP Example
Ada

Differences between Monitors/Serializers and CSP/ADA

Monitor/Serializer	CSP/Ada
The called procedure begins executing right away. It waits if the object is not in a proper state.	The call is only accepted by the called procedure if certain conditions are satisfied.
Passive synchronization.	Synchronization between active threads.
Inherently non-distributed.	Possibility for distributed programming using synchronous message passing.

CSP: Communicating Sequential Processes

Sequential process aspects: single thread of control, autonomous, encapsulated, named, static.
Communication aspects: synchronous, reliable, unidirectional, point-to-point, fixed topology.
Syntax:
Rendezvous semantics: Senders remain blocked at the send until a matching receive operation is made and vice versa.
A limitation: Each process must be named and programmed for.

CSP Commands

Guarded command: <guard> --> <command list>
Alternative command: [ G1 --> S1 [] G2 --> S2 [] ... [] Gn --> Sn ]
1. evaluate all guards.
2. if more than one guard is true, nondeterministically select one.
3. if no guard is true, terminate.
Repetitive command: * [ G1 --> S1 [] G2 --> S2 [] ... [] Gn --> Sn ]
Repeatedly execute the alternative command until none of the guards are true.

CSP Example: Bounded Buffer Problem

buffer: (0..9) portion;
in, out: integer; in := 0; out := 0;

* [ in < out + 10; producer?buffer(in mod 10)
    --> in := in + 1;
    []
    out < in; consumer?more()
    --> consumer!buffer(out mod 10);
        out := out + 1;
]

Ada

select is like the alternative command.
when specifies the conditions (guard).
To get the semantics of a repetitive command, put the select inside a loop.

Cara Cocking