Performance and Evaluation of Lisp Systems - Dreamsongs

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3. Activating the frame by making the open frame the active
frame, by stashing the next PC in the current function in
the PC slot of the call frame, and by making the function
that it contains the current function.
In the third step, the microcode checks to see that the function is being
called with the proper number of arguments, and if the function can take different
numbers of arguments, it selects an entry point. The called function then defaults
any unsupplied optional arguments and bumps the stack pointer to allocate space
for local variables. A call frame looks like this:
Frame Header
Function
Previous Active Frame
Previous Open Frame
Previous Binding Stack Pointer
Saved PC of Caller
Argument 0
Argument 1
. . .
Local 0
Local 1
. . .
If a function tries to return multiple values, the microcode looks at the stack
frame’s header word, which indicates whether or not the caller is expecting mul-
tiple values. If the caller is not expecting multiple values, only the first value is
returned.
CATCH and THROW are implemented in much the same way as call and re-
turn (indeed, the microcoded instructions share a good deal of code). The CATCH
instruction creates a catch frame, which looks much like a call frame except that
the function that created the catch frame takes the place of the ‘function to be
called,’ and the PC of the code to be executed when the catch frame is thrown to
takes the place of the ‘return PC.’ A catch frame looks like this: