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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.arch
Subject: Re: Stacks, was Segments
Date: Sun, 19 Jan 2025 17:33:53 +0100
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On 18/01/2025 09:59, Niklas Holsti wrote:
> On 2025-01-18 5:08, John Levine wrote:
>> According to MitchAlsup1 <mitchalsup@aol.com>:
>>>> Stacks are small because OS people make them small, not because of
>>>> a valid technical reason that has ever been explained to me.
>>>> "To avoid infinite recursion" is not a valid reason, IMHO.
>>>
>>> Algol 60 only had stack allocation for dynamically sized arrays,
>>> so stacks had to be as big as the data are.
>>
>> Huh?  Algol 60 routines could be mutually recursive so unless it was
>> a leaf procedure or the outer block, everything not declared "own"
>> went on the stack.
> 
> 
> Mitch's point AIUI was that Algol 60 had no heap allocation (and no 
> explicit pointer types), so indeed all data were either on the stack or 
> statically allocated.
> 
> I'm not an English native speaker, but it seems to me that Mitch should 
> have written "Algol 60 had only stack allocation" instead of "Algol 60 
> only had stack allocation".
> 
> The most-used Ada compiler, GNAT, uses a "secondary stack" to reduce the 
> need for heap. Dynamically sized local data are placed on the secondary 
> stack, and dynamically sized return values of functions are returned on 
> the secondary stack. So a function can return "by value" an array sized 
> 1..N, with N a function parameter, without needing the heap.
> 
> Of course the programmer then has the problem of setting sufficient 
> sizes for /two/ stacks, the primary and the secondary. For 
> embedded-systems programs one usually avoids constructs that would need 
> a secondary stack.
> 

A two-stack setup can be used in C too.  (The C standards don't require 
a stack at all.)  On the AVR microcontroller, it is not uncommon for C 
implementations to work with a dual stack, since it does not have any 
kind of "[SP + n]" or "[SP + r]" addressing modes, but it /does/ have an 
"[Y + n]" addressing mode using an index register.

Two stacks are also pretty much required for FORTH.

The use of a dual stack could also significantly improve the security of 
systems by separating call/return addresses from data.