17 Feb
2025
17 Feb
'25
6:11 a.m.
On Feb 16, 2025, at 8:00 PM, ben via cctalk
<cctalk(a)classiccmp.org> wrote:
...
I have trouble understanding the fine points of accessing a local variable in Algol with
a display. Books tend to spend more time
on the evils of a dangling else, and gloss over the run time action of
a display.
Displays are a solution to the problem of finding variables in an ancestor function block.
It doesn't appear in C since (standard) C doesn't have nested functions.
I'll use C notation here for simplicity, though. Suppose you have:
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.
On the X8 a reference to j through the display is a single instruction using a specialized
addressing mode that does the whole job. It assumes no more than 63 statically nested
blocks, but that's quite a reasonable limitation.
Have a good example or reference book I can find free
on line.
Also is there a ENGLISH description of the EL-X8?
There's the article in Wikipedia. Apart from that, find the "EWD archive"
at UT Austin. Many of the early EWD papers, up to number 150 or so, deal with the
development of the THE operating system. Some are in English, some in Dutch, for no
obvious reason. A number of the Dutch ones have been translated by volunteers working on
that archive. I think there are some useful summaries of the machine in there.
One thing you should not miss is the paper "The structure of the THE operating
system". It describes the design principles and why they were used; it lays the
foundation for techniques that were used by many others afterwards.
paul