> On Feb 17, 2025, at 10:36 AM, Peter Corlett via cctalk
> <cctalk@classiccmp.org> wrote:
>
> On Mon, Feb 17, 2025 at 09:11:17AM -0500, Paul Koning via cctalk wrote:
> [...]
>> int f1(int i) {
>> int j;
>> int f2(int x) { int y; y = j*2; ... f1(x+1); }
>> f2(...);
>> }
>> f1(42);
>>
>> The local variables go into stack frames, one for each call of each
>> function. So when recursive calls are made as in this example, there are
>> multiple stack frames belonging to calls of f1 and of f2. Each f1 frame
>> has a j in it. So how does the code for f2 find "the right f1 frame" to
>> resolve the reference to j that I showed?
>>
>> The answer is to have a display, which is a vector of pointers, indexed by
>> "static scope", in other words by textual nesting level. In this case that
>> vector would have three entries: one for the program, f1, and f2. When a
>> call to f1 is made, the stack frame is created. In the stack frame the
>> previous value of the display entry for f1 is saved, and that entry is
>> then set to the stack frame address. On function return, the previous
>> display entry is restored.
>>
>> To resolve the reference to j in f2, the code would load the display entry
>> for f1 (the second entry in the display vector), and add the offset to j
>> in the stack frame.
>
> That's the very old-school approach for weird CISC architectures which were
> mainly programmed in assembly language or with naivee compilers. It's most
> definitely not a *good* way to do it, performance-wise.
Could be. Optimizing compilers weren't much of a thing in the mid-1960s.
I just found a good descrption in EWD 101
https://www.cs.utexas.edu/~EWD/ewd01xx/EWD101.PDF starting with "simple
variable addressing" (page 2). Fortunately that is in English.
The D register mentions points to the display; the addressing modes M<num>[i]
are references to frame offset i via display entry "num", and MA[i] is indexed
addressing with offset i from the A register. (That's odd, I would have
expected the B register which is the default stack pointer.)
paul
