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What's needed to get taipan onto a cart:
joey_z is willing to manufacture carts like this:
+---------------------------------------------------------------------------+
| Type 13: XEGS 64 KB cartridge (banks 0-7) |
+---------------------------------------------------------------------------+
One of the two variants of the 64 KB XEGS cartridge, that's built on either
a C100649 board with the W1 solder point not connected, or a C026449 board
with pin 9 of the 74LS374 chip unconnected.
This bank-switched cartridge occupies 16 KB of address space between $8000
and $BFFF. The cartridge memory is divided into 8 banks, 8 KB each.
Bank 7 (the last one) is always mapped to $A000-$BFFF. Three lowest bits of
a byte written to $D500-$D5FF select the bank mapped to $8000-$9FFF.
The initially selected bank is random, although it seems that 0 gets chosen
the most often. Atari800 always selects bank 0 initially.
Reference:
http://www.atarimax.com/jindroush.atari.org/acarts.html#xegs
"For bank-switched cartridges banks are numbered in the order they appear
in the image file, starting with 0."
So far, he's built a prototype (old EPROM, dead-bug mounted logic ICs,
hand-wired) and is working on a cleaner version, using a 99 cent flash
chip, for production. The only cart shells we've been able to find are
actually for the C=64, and will supposedly work on any Atari 8-bit except
the 1200XL.
...so:
Bank 7 has the startup code, uncompressed title screen, title code,
and code to copy from the other banks to RAM.
That gives me banks 0-6 to store code in. The code is 27174 bytes, so I
have plenty of space: it'll occupy 4 banks, leaving 2 empty ones. The
last code bank will only be 1/3 full of code... but 1K of it will be
used for the font, so 4.5K free there.
Code in bank 7 will copy all the chunks to correct place in RAM... and
I don't need to leave room for DOS or anything else, so the code can be
ORGed at $0400 (romable_taimain.raw target in the Makefile does this).
$0400 + 27174 means the code ends at $6e26. The BSS is right after that,
and is less than a page. The OS will place the GR.0 display list at $7c20,
and the stack will grow down from there to $7a20 (except it never grows
that much).
Copying the game code to RAM causes a short delay at boot (0.43 seconds).
It's barely noticeable, and lots of cartridge games have similar delays.
The copying code could maybe be optimized to speed it up a little,
but it's probably not even worth the effort.
Amusingly, the Taipan cart will work on a 32K 800. Not a 16K Atari though.
bank 7: fixed bank
$a000-$bxxx - title screen data, dl, menu code,
memory size checker, code to
copy romable_taimain to RAM
$bffa-$bfff - cart trailer
banks 0, 1, 2: full banks of romable_taimain code
$8000-$9dff - 31 pages (7936 bytes, 7.75K) of code
$9f00-$9fff - unused
bank 3: last (partial) bank of romable_taimain, plus the font.
This bank stays enabled after copying is done.
$8000-$9bff - up to 7K of code (28 pages)
$9c00-$9fff - font (1K)
Unused areas are filled with $ff (this is the default state for both
flash and EPROM). For the banks that map at $8000, this includes the
cart trailer area. A non-zero byte (our $ff) at $9ffc tells the OS that a
cart isn't inserted in the right slot, so it won't try to initialize/run
our cart as a right cart. Only bank 7 (that maps as a left cart) has a
valid cart trailer... according to cart.txt, every once in a while, bank
7 might come up selected in the $8000 bank at power on. This shouldn't
matter: it'll be in both bank areas, and if the OS tries to init it as
a right cart, the init/run addresses will point to the left cart area.
banks 4, 5, 6 are unused (24K total). Possibly the manual goes here,
if I write one.
--
Changes the game needed for a cart version: Not many.
.org $400 for taimain.xex. It could be a little higher, but making it
as low as possible allows room for bugfixes or new features (if I ever
add any).
checkmem.s isn't needed any longer... though there is a new memory checker
(in bank 7) that says "32K required" if someone tries it on a 16K machine.
Exiting the game (N at "Play again?" prompt) returns to the title screen,
since there's no DOS to exit to. This might be slightly useful: you
might decide to change the colors or disable sound for your next game.
Pressing the Reset key does a coldstart (reboot) in the cart version.
You end up at the title screen again.
The title decompression is gone: it just displays the title screen DL
and data straight from ROM. The menu help text and the tail end of the
display list are in RAM though, so the menu can change them. Moving the
end of the DL to RAM means one extra black (border-colored) scanline
appears due to the DL jump instruction. I may make the border the same
color as the text bg, to make this less noticeable.
The font is located in ROM, on a 1K boundary, so CHBAS can point
to it, rather that $2000 like the xex version.
num_buf and firm are located in the BSS rather than page 6 as they
are in the .xex version.
Since I have 3 empty banks... Why not include a manual on the cart,
with pseudo-hypertext UI?
--
What would be *really* slick: figure out a way to split the code up
across banks and include bankswitching in the logic, so it runs from
the cart and switches banks as needed. This would allow Taipan to run
on a 16K or even an 8K 400/800!
Can it be done? Surely. Can it be done without rewriting everything
in asm? Probably. Do I want to? Not really.
--
The cartridge label should look like a classic brown 1st-gen 400/800 cart,
even if the cart shell itself doesn't.
Yellow text:
TAIPAN!
Computer Game
and a bogus serial number.. CXU000001 or such (U for Urchlay).
The top should say LEFT CARTRIDGE.
If there's going to be a printed manual, it should be based on the Apple II
version's manual. If there was one. If I can find a copy.
|
