5 - Forth Interest Group

SEPARATED HEADS
Klaus Schleisiek
Memory in RAM-based systems can be
used more efficiently by means of a
"Symbol Dictionary Area ," which allows
words and/or name and link fields
which are needed only at compile time
to be thrown away after compilation.
Incremental use of these techniques
will result in more efficient memory
usage and will. also encourage the use
of more and shorter definitions
because there is no longer the need
to pay the penalty of taking up
memory space with numerous name and
link fields.
In the course of a two-year
project I developed some tools which
allow a significant compression of
code in RAM-based systems. I also
feel these methods will have a sig-
nificant impact on programming style,
particularly because they will encour-
age the use of more and shorter
definitions. The followicg is an
explanation of these various functions
in a somewhat historical order.
My programming task was to develop
a lightpen-operated sound system,
which would allow control of a number
of small sound synthesizers by point-
ing a lightpen to various dots, light
potentiometers, and the like on a
video display. There was to be no
keyboard intervention. A "dot" was
put together by compiling a word
which associated the following
information:
A) The shape of the dot itself as
an address of some programmable
character.
B) The dot's location on the
screen as an address relative
to the upper left hand corner
of the screen.
C) As an option, either a text
string or a string of program-
mable graphics characters to
be displayed above, below, or
to either side of the dot.
Thus, every "dot" served a double
purpose. On the one hand, it des-
cribed a portion of the display
itself which had to flash on the
screen. Secondly, it supplied the
key to a large keyed CASE statement
which associated the dot with the
function to be performed when the
lightpen was pointed to it. In other
words, the definitions of the dots
themselves were only needed at
compile time .
The dot definitions were used to
create a densely packed "image" definition
to flash the picture on the
screen, while the addresses of the
dot locations were used as keys in
the CASE statement. So, to be memory
efficient, I wanted to set up some
mechanism which would allow the presence
of "symbols" at compile time
that could then be thrown away after
compilation to free memory. By
"symbol" I mean any legal FORTH
definition that is only needed at
compile time. This led to the idea
of dividing the dictionary into "main
dictionary" and " symbol dictionary . "
Figure 1 shows the arrangement of
this scheme based on the 6502's unique
memory mapping.
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Figure 1
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