22 Feb
2009
22 Feb
'09
8:19 p.m.
Small correction: The string "EXEC" takes does not seem to need to only
hexadecimal digits, the only requirement seems to be that it have a
non-zero, even length.
So I'm unsure of the encoding -- strings on the Tek are limited to 7-bit
ASCII, so using CHR() to build a string directly from 6800 machine codes
isn't going to work. Given that the call requires an even-length
string, clearly two characters are combined somehow to form one machine
code, but how isn't immediately obvious. I've played around with a few
simple combinations with no luck so far.
Josh
Josh Dersch wrote:
Looks like your memory is not too bad... the
documented method for
calling ROM code (in expansion packs) is via the "CALL" command with a
string that names the function to be called. (i.e. I have the EDITOR
rom, and that's invoked by doing 'CALL "EDITOR"').
Well, I just tried this:
A$="ABCDEF"
CALL "EXEC",A$
And the machine immediately reset and gave me a system error :). So
it seems evident that this was executing the string as machine code
(and crashing horribly).
Further experimentation reveals that the string can only contain
hexadecimal digits (strings containing other characters returns an I/O
error), so I'm wagering that the "EXEC" call does a simple translation
from the hex string to bytecodes, then executes it.
Now to work out various sundry details like calling conventions, where
the code is in memory when it's executed (so I can do jumps) and then
onto more fun things like doing something useful!
Thanks!
Josh
Rick Bensene wrote:
I recall that there was a way to make the 4051
execute machine code. I
don't remember the exact method, but I remember that the bytes of 6800
machine code were encoded into a character string using the CHR$()
function. For example, hex 43 would turn into CHR$(67), or a "C". The
bytes were put into a string with the limit of program size being the
max. size of a character string. The next part is where my memory fails
me -- there was a way to cause the BASIC interpreter to branch to the
first character in the string, executing the code there. It involved
doing the transfer in such a way that it was if the code was branched to
with a "jump to subroutine" call, and when the code was complete, a
"return from subroutine" instruction would be in the code, and that
would cause a branch back to the BASIC interpreter. As I'm writing this,
a statement called EXEC$ or something along those lines pops into my
head.
I don't know if this is any help at all, but I really clearly remember
seeing 4051 programs (I worked at Tektronix from 1977 to 1990, and did a
lot of tinkering with 4051/2/4) that filled character strings with
hand-assembled machine code, and had a means to execute it. Hopefully
someone can find some docs that might substantiate these memories and
solve the mystery.
Rick Bensene
The Old Calculator Museum
http://oldcalculatormuseum.com