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TEACH POPSYS                                        Steve Hardy Dec 1982


                       HOW THE POP11 SYSTEM WORKS


Compiler, interpreter, LISP,  machine  code,  syntax  analysis,  lexical
analysis, list cell, garbage collection.

This handout points out those areas of computer science necessary for an
understanding (even slight) of how the POP11 system works. More detailed
discussion of some of the topics presented here can be found in TEACH VM

We can divide an understanding of how the POP11 system  works  into  two
parts.  Firstly,  how  is  the  translation from POP11 to a language the
machine can  understand  done,  and  secondly,  how  are  the  important
features of POP11. (In particular list structures,) implemented?

An important phase in the translation process is  the  formation  of  an
intermediate  representation  of  the  program  that  is called a 'parse
tree'. This parse tree is in many ways analagous to a LISP  program  and
so  an  understanding  of  LISP itself is useful. There is then a second
phase of translation from LISP to machine code. To completely understand
the second phase of translation, the reader must understand something of
machine code itself.

The two principal data structures of POP11 (and LISP) are "the word" and
"the  list". We need to know how these can be represented on the VAX's
memory - this, of course,  requires  an  understanding  of  the  VAX's

A third important aspect of understanding POP11 is the operating system,
for it is via the operating system that POP11 programs communicate with the

                         Translation Into LISP

Translation of a  high  level  language  like  POP11  into  a  LISP-like
language  is  usually called syntax analysis. This topic is described in
most books on compiling  techniques  and  is  usually  dependent  on  an
understanding of "transformational grammars".

A related topic  is  'lexical  analysis',  that  is  the  conversion  of
characters (e.g. 'C','A','T') into symbols, (e.g. "CAT").


No actual compiler would produce LISP as an intermediate  representation
of  a  program.  However,  the parse trees produced will usually be very
like LISP and as LISP is well documented I will use it as a analogy  for
a parse tree. There are many manuals on LISP itself.

Here is an example of a POP11 program:

       define sum(list);
             if      list = []
             then    0
             else    hd(list) + sum(tl(list))

Here is the same program in LISP

       (define (sum list)
             (cond ((null list) 0)
                     (t (plus (hd list)
                             (sum (tl list))))))

Notice that in LISP, expressions are represented by lists.   The  HD  of
such  a  list  is  the  name of a procedure to be applied and the TL its
arguments.  Notice, also, that the arguments of COND  are  'condition  -
consequent' pairs.

An important feature of LISP is that it can be "interpreted" by  a  LISP
interpreter.   Therefore, it is useful if a reader understands something
of the way LISP is interpreted in a LISP system.

                            Code Generation

One of the disadvantages  of  interpreting  code  is  that  one  usually
requires  a  complicated  program, written in a simple language (usually
machine code) to  do  the  interpreting.  Furthermore,  interpreting  is
inevitably  slower  than  direct  execution  by  the  machine  itself. A
tremendous increase in efficiency can be achieved  by  translating  LISP
into  machine  code.  When  carried  out  by a compiler, this process is
called "code generation".

Here is an example of a LISP expression;

       (plus (hd list)
             (sum (tl list)))

and here is its translation into a simple machine code:

       push list
       call hd
       push list
       call tl
       call sum
       call plus

                              Machine Code

Machine code is a language, specific  to  each  machine,  which  can  be
understood  by  the  hardware  of  the  machine  itself.  That  is,  the
interpreter for machine code IS the machine. Perhaps surprisingly,  most
modern  computers  have a very similar sort of language as their machine
code. Each instruction in a machine code program  will  usually  perform
some very simple operation, for example:-

    (1)  Putting the value of a variable on to a stack.
    (2)  Putting a literal (e.g. a number) on to a stack.
    (3)  Taking the top item from the stack and putting it into a variable.
    (4)  Calling a sub-routine.
    (5)  Returning from a sub-routine.
    (6)  A jump instruction which  breaks  the  normal  sequential  flow  of
    (7)  All programming languages  require  the  ability  to  conditionally
     execute  code. One way of doing this is to have a "conditional jump
     instruction". Unlike the jump instruction, this label will only  be
     "gone  to"  if some 'condition' is met (e.g. the top element of the
     stack being TRUE).

                             Machine Memory

Most computers have a very simple memory structure. A  large  number  of
numbered "words" - each word of the computer's memory can hold a number.
This contained number can be interpreted in any way we wish, for example
as  an  instruction  to  the hardware, as a character (e.g. A = 1, B = 2
etc.), as the address of  another  word  in  the  machine's  memory,  "a
pointer" or, of course, as a number.

For a more detailed explanation of how we can  represent  POP11  objects
such as words and lists in this sort of memory, see the * WAL demo.

                           Garbage Collection

The computer only has a finite memory (in our case about 1 million words)
As  the  POP11 program is running it is creating various data structures
(e.g. lists,  words,  procedures,  etc.).   These  data  structures  are
represented  using some of the machine's memory. A problem arises - what
happens when the POP11 system has used all of the memory allocated to it
by  the operating system?  Now, many data structures are only temporary,
they are created, used for a while  and  then  no  longer  wanted.  (For
example, a procedure is re-defined, the data structure used to represent
the old definition  is  no  longer  wanted.  POP11  contains  a  set  of
procedures collectively called the "garbage collector" which attempts to
guess which of the data structures  currently  existing  are  no  longer
wanted.  These  unwanted data structures are then made available for re-

                          The Operating System

There is a set of programs collectively called  'the  operating  system'
which exist to help programs make the best use possible of the computer.
They have two important rules:-

     (1)  The computer would be under-used if only one person could  use
     it  at  a  time. When that one person was sat thinking, the machine
     would be idle. To prevent  this  happening,  the  operating  system
     allows  many  people  to use the machine on a "time sharing" basis.
     First one user has her program running  and  then,  whilst  she  is
     thinking,  another user's program can run. If more than one program
     is runnable at any given moment, the operating  system  gives  them
     equal  shares  of  the  computer's  time. The operating system also
     protects users from one another (and indeed, protects  itself  from
     them)  if  one  user has an area that should not affect others. For
     this reason, the operating system  clearly  defines  what  sort  of
     access  each user can have to the machine (for example, how much of
     the computer's memory they are allowed to use).
     (2)  Almost all programs want to communicate with the world in some
     way,  for  example  by  reading  input from the teletype or sending
     output to the teletype or accessing the discs. These tasks are very
     fiddly  and  moreover,  it  is essentially the same task, no matter
     what program the user wants to run. For this  reason,  instructions
     on  how  to  work  the  individual  devices  are  made  part of the
     operating system (thus preventing unnecessary duplication of code).

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