TMS34010
C
Run-Time
Environment
Notes:
1. The status register
is
not
saved.
2
..
There
is
not
an
explicit argument
pointer
in this scheme. The frame
pointer
(A13)
is
located at the end
of
the argument list and at the
beginning
of
the local frame, so
both
arguments and local variables
can be indexed from the frame pointer.
3.
Register
AS
is
used
as
the return value register; and
thus
is never saved
in a normal function. Therefore, this register can be modified
without
having
to
save it on the system stack.
5.5.4
Function
Termination
5-10
At
the termination
of
a function, the function:
1)
Handles any return values
to
be passed
to
the caller.
2)
Restores the caller's registers.
3) Restores the local frame
of
the caller.
There
are
three
Cases
of
return value types and each has a method
of
handling:
Pointers
or
integer
return
values:
Returned in register
AB.
Because
of
this use
of
register A8,
it
is
never saved at function entry. Thus,
A8
may be
used
as
a scratch register between function calls. Note
that
since integer
values
are
not
returned on the stack,
it
is
not
necessary
for
the caller
to
al-
locate
space for them prior
to
passing parameters.
Floating
point
return
values:
Returned on the stack. Either a single
or
a
double
precision value can be returned. Upon return, the value appears
to
have been "pushed" on the stack. The space
for
the return value must be
allocated by the caller before the call,
followed
by a save
of
the program
stack pointer to the system stack.
Structure
return
values:
Returned on the program stack,
as
floating
point
values
are.
It
is
assumed that the space
is
allocated, and
that
the program .
stack pointer (STK)
is
saved on the system stack.
Note:
The size
of
a structure returned
is
rounded
up
to
the next
word
boundary.
This makes handling easier. Upon return, the structure appears
to
have
been
"pushed"
on the stack.
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