ZX Computings - OpenLibra

Machine code
saving machine code
Summary Of
Possible Methods
I first considered all the
possibilities:
1. Establishing a dummy array
with DIM A(x) and then
POKE into VARS.
2. Moving RAM TOP down (as
Mr. Sinclair has provided
two nice little POKEable
bytes 16388/16389 for
just this purpose) and then
POKEing the code into the
available space NEW and
CLS can't touch it, which is a
major advantage.
3. POKEing into a REM statement.
Here at least, the code
would be saved and safe
from attack, but I was worried
about displaying the
REM and the dreaded
system crash that would
follow when the basic interpreter
found naughty, nondisplayable
codes for it to
put on the screen.
4. POKEing above program and
display file, but far enough
below the stacks, to prevent
collision with the "calculator
stack". At least the
code is immobile here, but I
never liked this one with the
ZX80, as it's a bit "by
guess" as to whether the
code gets overwritten or
not.
5. Setting up a string array to
contain the code and stripping
it out byte by byte. This
gobbles up memory at a
phenomenal rate, plus the
fact that you are tied to
relative code as the strings
shift.
The Sinclair Manual
The Sinclair Manual is as good
as useless on using machine
As a self-confessed machine code
freak, who gets withdrawal symptons
without a daily shot of bytes, Mike
Bidden decided to push some code up
the ZX81, only to discover that it was
a totally different machine from the
ZX80. and that he was unable to use
his tried and tested favourite
methods. This was due to lack of RAM
or because the overseeing 8K ROM did
naughty things to the code he POKEd
in. Let Mike tell you the story...
code (read Chapter 26), but to
be fair, it's directed at those
new to programming and in this
it succeeds. The Sinclair
Manual mentions methods 2, 3
and 5 with scant detail, but I
began to wonder why there
was no mention of a dummy
array and VARS.
Experimentation
So I set out to try this method
first. I have always favoured
MICROMON (the ZX80 magic
book) for this purpose. Micromon
pokes hex code into VARS
(system variable area). I converted
Micromon into something
the ZX81 could recognise
(no TL$ on the 81 you have to
use (2 to ) instead). Having
got the program in and running,
I discovered that there were only
about 50 bytes left for the
code and that DIM A(25) was
about all that was possible. Not
only that, I could not POKE into
the VARS area, all my carefully
considered bytes were overwritten
to zeros. Perhaps I was
doing something wrong, but at
this stage I stopped trying to
POKE into VARS
Method two worked as
described in the Sinclair
Manual, but what's the use of
code you cannot save, without
a special routine that consumes
precious memory. So I discard-
1 P E N 1 0 0 C H A R A C T E R S O F
Y O U R C H O I C E
LO L E T A = 155:L4-
1 5 CLS
P R I N T A ; " " ; P E E K R
i FT A=A-H
3 INPUT
- *'" THEN GOTO IS
1,0 IP >,$= M" THEN GOTO 90
C0 IF = THEN GOTO 120
7-2- POKE R 1 j. UAL A*
£ O T O 15
INPUT A
100 GOTO 15
3 2 0 C L S
125 FAST
1 3 0 L E T U - U 3 R