This eight-word area is allocated by any non-leaf routine prior to a call. The
exact size of this area is defined because the caller uses it to locate the
formal arguments from the previous frame. (Any standard procedure can identify
the bottom of its own frame, and can therefore identify the formal arguments in
the previous frame, because they will always reside in the region beginning
with the ninth word below the top of the previous frame.)
Previous SP: Contains the old (procedure entry) value of the
Stack Pointer. It is only required that this word be set if the current frame
is noncontiguous with the previous frame, has a variable size, or is used with
the static-link.
Relocation Stub RP (RP"): Reserved for use by a relocation stub
that must store a Return Pointer (RP) value, so the stub can be executed after
the exit from the callee, but before return to the caller. See Chapter 3 for
detailed discussion of Parameter Relocation stubs.
Clean Up: Area reserved for use by language processors; possibly
for a pointer to any extra information (i.e. on the heap) that may otherwise be
lost in the event of an abnormal interrupt.
Static Link: Used to communicate static scoping information to
the callee that is necessary for data access. It may also be used in
conjunction with the SL register, or to pass a display pointer rather than a
static link, or it may remain unused.
Current RP: Reserved for use by the called procedure; this is
where the current return address must be stored if the procedure uses RP (gr2)
for any other purpose.
External/Stub RP (RP'), External sr4/LTP', and
External DP/LTP: All three of these words are reserved for use by
the inter-modular (external) calling mechanism. See Chapter 5 for more details.
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