20 Sep
2015
20 Sep
'15
3:52 a.m.
On 19 September 2015 at 19:53, tony duell <ard at p850ug1.demon.co.uk> wrote:
A lot of disk-based BASICs had a statement that would
merge a program from
disk in this way. Sometimes the program had to be saved in ASCII, not tokenised,
the BASIC interpretter then essentially read the file as if you were typing it on the
keyboard. So program lines would indeed replace those with the same line number.
OK, that I've seen, yes. Even, very carefully, used it.
But the examples I've seen do not behave as you describe. Note, I am
not saying it's impossible or never happens, merely that it does not
match the behaviour I've seen.
In ever BASIC interpreter I've ever used, the LOAD command loads a new
program in from $MEDIUM. This implicitly gets rid of the program
previously in memory. It is not the same as a NEW command, which
usually also resets all variables etc.
If you want to keep the existing program in memory *and* load
additional lines from storage, there was a different command: MERGE.
But in everything from ZX BASIC to BBC BASIC to GWBASIC, loading a
program erases all lines of code in interpreter RAM and replaces the
whole program with the one loaded from disk, but leaves variables etc.
intact.
This means that, in effect, the program modules being loaded are
overlays: you can pass state (a whole set of initialised variables and
their values) from one module of code to another. I maintained a large
MS-DOS payroll program written this way: it consisted of 18-30+
sub-64kB modules of code. (How many depended on the client's
requirements.) The first module initialised all the variables in
memory and drew a menu. The menu LOADed the other modules, many of
which LOADed each other to handle the task. All the variables
remaining in place in RAM, all effectively globals, as GW-BASIC didn't
offer local variables, named procedures etc.
The reason was that you couldn't have a program of >64kB on GW-BASIC.
The original developer taught me to think of each chunk as a
procedure, and all the variables as globals. The code was extensively
commented to explain what variables it expected to find in place, what
they held, which ones it would modify etc.
Once I understood it, I found it really elegant.
I don't think I've *ever* seen a program that MERGEd in code during
execution, though. That sounds terrifying!
One of the extension ROMs for HPL on the HP9825 (a
BASIC-like language) had
a command to store a string as a program line. It could be used within a program,
thus leading to an official way to have self-modifying code.
Nifty!
--
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