CIS 400 LECTURE 22

Continuing from last time: dynamic activation stack

• Dynamic scoping is used for dynamic chain activation
• Static scooping follows static chain of activation

Static chain activation stack:

C has a link back to B

Often denoted C[static link[I]]

Or

C®®X, ( a pointer to a pointer)

For non-local references, there is an alternative to traversal of static chain:

In order to speed things up…

1) Central referencing table, (similar to what is found in Lisp)

2) Display, (copy of static activation chain)

Exceptions:

Issues:

1) Raising exceptions

2) Propagating the exception

3) After exception is handled, where does execution go?

Fundamental issues:

1) Who raises exception, (programmer or machine)?

2) Propagation issue, where do you look to find a handler? (static or dynamic scope)

3) Execution: continue, terminate and clean-up, or continue with next statement?

Semantics for dealing with exceptions:

Exceptions:

PL/1, (first with exception handlers)

• Conditions to be enabled or disabled, (most under programmer control)
• Lexically scoped, (inheritance by block)
• On unit, (overriding system default behavior)
• Executions resumes next statement after condition is raised, (some exceptions to this)

CLU

• Exceptions may only be propaged to calling procedure
• Signaling procedure is terminated
• Procedure header: normal return or signal return

e.g.

for_fun = proc(a:int, b:real)

return(int)

signals(zero(int), overflow, bad_format(string))

MESA

Terminology: signals and catches

• Association made by dynamic activation
• Exceptions can be raised by either:

1) Signal

2) An error

3) Return with error

ADA

• If a condition not handled inside procedure, termination occurs and signal is propagated, (dynamic calling chain)
• Select, (similar to a switch)
• If a raised exception is not handled, termination occurs

Flaws in Ada handling:

1) No parameter transmission

2) Dynamic chain sometimes takes you t unexpected places

3) No compiler on type checking for exception