🚀 go-pugleaf

>                                 The basis of the architecture is
> a hardware stack. The stack frames a[re] referenced by a set of registers
> called D registers. There are no user accessible registers in the
> machine, the top of stack being the equivalent of registers.
> The other interesting part of the architecture is that it is tag based.
> Each word in memory has an associated tag which gives the type of
> the word. Odd tags are read only words. These apply to code words, which
> cannot be modified by a user program. Tag 0 words are data, and tag
> 2 words (even), are double precision.

The tagged words mean that the A Series archetecture is object oriented,
with the hardware aware of the basic types of objects.  Thus, the hardware
prevents improper operations on objects.  The mechanisms used for this are
quite general.

Operations are done on the stack, which is both an expression stack and a
history stack.  Procedures or blocks are addressing environments, which
exist as regions on the stack (most recently activated or entered on top).


The hardware can tell from the tag if an object is executable code or
program data or a software structure.  Operators specify stack offsets
for data items (objects) as an activation and an offset within that
activation.  The D-registeres are an optimization to find the base of
an activation quickly.

Historically, stack machines such as the A Series have been respected by
computer science types because of their elegance and ease of implementing
recursion, rentrant code, dynamic arrays, etc., but suffered from slower
performance because of having to access items through stack indexing.  This
performance issue has traditionally been alleviated by making the operators
powerful enough that fewer are needed, thus programs as a whole run quickly.
Recently, additional performance improvements have been gained by dividing
the processor into multiple concurrant sections, some of which fetch data
ahead of the logic unit.

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  = sua cuique voluptas              (everyone has his own pleasures) =
  = Randy Gellens            randy%mpa15ab@trenga.tredydev.unisys.com =
  = >>>>>>> If mail bounces, forward to rgellens@mcimail.com <<<<<<<< =
  = Opinions are personal; facts are suspect; I speak only for myself =

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