Re: How about in INC opcode

["Sims, Ken" <KSIMS@xxxxxxxxxxxxxxxx>]

Hi Don -

>Date: Wed, 8 Mar 2000 09:55:25 -0500 (EST)
>From: Don <dr2@cssas400.com>
>Subject: Re: How about in INC opcode
 ...
>OK, it's been a while since I've studied compiler construction theory
>here, but, since the interpretation process in effect parses and
>translates the the command into some machine runnable instrucion(s), I
>don't see the strength in the interpreted vs compiled line of thought
>here...What am I not seeing? (caveat: I'm still only on cup 2 of coffee
>this morning...:)

The problem is not in a lack of understanding about compilers but a lack of
understanding about interpreters.  An interpreter is not a mini, on-the-fly
compiler.  It does *not* change the language instructions to machine
instructions and then run them.  It parses the command and then does what
the commands says to do, no machine instruction generation is involved.
(That's why porting an interpreter to another platform is so much easier
than porting a compiler.)

I'm not familiar with Perl, so I'll use interpreted BASIC as an example (I
wrote a BASIC interpreter in RPG back on the S/34 or S/36 many years ago).
Take a FOR/NEXT loop:

FOR A=1 TO 20
 do something
NEXT A

A BASIC interpreter does not build machine code for the loop.  Instead, it
goes to its variable table and looks for "A".  If it doesn't find it, it
creates an entry.  Then it sets the value for of A to 1.  Then it adds an
entry to the loop control table for a loop based on "A" that points to the
start of the A and has the ending limit and the increment value.  Then it
goes to the statement following the FOR command.

When it hits the NEXT command, it takes that variable name and gets the
value from the variable table.  It finds the loop control record, adds the
increment value to the variable and compares it to the ending limit.  If the
loop is done, it removes the loop control record and reads the statement
following the NEXT statement.  If the loop is not done, it uses the
statement pointer from the loop control table to go up and read the
statement following the FOR command.

Ken
Southern Wine and Spirits of Nevada, Inc.
Opinions expressed are my own and do not necessarily represent the views of
my employer or anyone in their right mind.

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