At 13:45 3/22/98 -0800, Sam Ismail wrote:
>On Sun, 22 Mar 1998, Hotze wrote:
>
>> OK. What exactly does a language card do? (Sorry, I'm new at this)..
>
>A "language card" basically had an additional 16K of memory that mapped
>into the upper 16K of the Apple's memory and was accessed by bank
>switching.
>
>To switch to the RAM bank, you would access memory location $C083. To
>switch back to the ROM, you would access memory location $C081.
>
>The Apple ROM (including the monitor and BASIC interpreter) was also
>located in this region. If you switched to the RAM, your computer would
>lock up because all of a sudden its monitor code disappeared (you just
>turned on the RAM which overrode the ROM space) and it would execute an
>invalid instruction and halt. A reset is required.
>
>A neat thing you could do if you wanted to play around with the memory up
>there is to copy the monitor ROM into the language card RAM and then
>switch to the RAM. Since the same ROM instructions were now in the RAM,
>switching to the RAM was no big deal because the system just continued
>executing ROM code in RAM.
>
>So to do this, first get into the monitor if you're not already there by
>doing:
>
>]CALL -151
>*
>
>Now do:
>
>*D000<D000.FFFFM
>
>This moves the ROM code starting from $D000 through the end of memory at
>$FFFF (which includes BASIC and the monitor) to the RAM. Any writes to
>locations $D000-$FFFF will still be stored in the RAM (but since the ROM
>is active, any reads will return the bytes in the ROM). By executing this
>move instruction, you're copying ROM to RAM.
>
>Now turn on the language card memory:
>
>*C083
>
>This simple memory access gates the RAM. You now have before you a fully
>modifiable Apple ROM! You can start haphazardly poking bytes between
>$D000-$FFFF which will eventually have the effect of royally screwing the
>system up, but just temporarily...a simple reset will restore the ROM.
>However, the changes you made to the RAM are still in effect! So if you
>switch back over to your corrupted ROM images you'll likely still run into
>problems.
>
>A more amusing thing to do besides having fun killing your system is to
>change the BASIC keywords. These are stored starting at location $D0D0.
>All but the last character of each command has their high bit off. The
>last character of the command has the high (8th) bit set. This is how
>Applesoft BASIC knows when it has reached the end of a command when its
>parsing a statement (more clever than using an end-of-command byte, such
>as $00, which would be a waste of a byte).
>
>So the END command (which starts at $D0D0) is stored as:
>
>*D0D0- 45 4E C4
> E N D <-- 'D' = 44 + 80 = C4
>
>You could change the END command to FIN by putting in:
>
>D0D0: 46 49 CE
> F I N <-- 'N' = 4E + 80 = CE
>
>Of course, renaming all the BASIC keywords is fun but rather pointless. A
>better thing to do would be to re-write BASIC altogether if you were so
>inclined, and perhaps add features to the monitor ROM (which occupies
>$F800-$FFF). But in order to do this, you had to strip out unneeded
>features of the ROM since the code was so compact to begin with there is
>absolutely no space left (a testament to the coding brilliance of the
>Woz!) Some people would remove the montior cassette routines (which were
>used to write/read blocks of raw memory to/from cassette) which were
>generally of limited usefulness after disk drives and replace them with
>routines to help them crack games.
>
>If you made your own ROM, you could simply save it as binary to disk, then
>every time you booted your system you could have your "HELLO" program
>BLOAD it back into the RAM and then switch to RAM. A customizable home
>computer! Of course, you could also burn your code into an EPROM and and
>replace Apple's ROMs, but the beauty of the Apple was that you didn't have
>to go through all this trouble...it was so damn easy to hack!!!
>
>Sam Alternate e-mail:
dastar_at_siconic.com
>---------------------------------------------------------------------------
----
>Computer Historian, Programmer, Musician, Philosopher, Athlete, Writer,
Jackass
>
> Coming Soon...Vintage Computer Festival 2.0
> See http://www.siconic.com/vcf for details!
>
>
>
__________________________________________
Kip Crosby engine_at_chac.org
http://www.chac.org/index.html
Computer History Association of California
Received on Sun Mar 22 1998 - 17:48:37 GMT