Joust (Atari, 1983)

[toinet]

Ride gallant birds into knightly combat!

Joust is a trademark and (c) Williams 1982, manufactured under license from Williams Electronics Inc.


PROTECTION TYPE
A non standard diskette:
- One big sector of $1000 4*4 nibbles per track

That protection has also been used for Moon Patrol, please refer to that topic for info (in French only): http://www.hackzapple.com/phpBB2/viewtopic.php?t=13


BOOT TRACE
- 9600<C600.C6FFM
- 96FB: 4C 59 FF
- 9600G

Let's examine the code at $0800:
- the arm is moved to track 1 (routine in $804, in A the phase you want to go to)
- data is loaded from $B000 to $B7FF
- $08FB is a JMP $B000

Let's break that:
- 96FB: A9 98 8D FD 08 4C 01 08
- 9800: 4C 59 FF
- 9600G

Let's examine the code at $B000:
- clear the screen,
- set $CC = 0
- load data at $B100 (1)
- execute code loaded at $7000
- set $CC = 1
- load data at $B100 (2)
- execute code loaded at $0800

(1) The first tables that load data per $800 bytes are located at:
- phase to move the arm to (equals to track *2): $B0CD
- address to load data into memory: $B0DB
- checksum of read data: $B0E9
=> data is read from track 3 to E from $2000 to $7FFF because $CC = 0

(2) The second tables that load data per $800 bytes are located at:
- phase to move the arm to (equals to track *2): $B0F7
- address to load data into memory: $B0FA
- checksum of read data: $B0FD
=> data is read from track 2 from $0800 to $0FFF because $CC = 1

Code at $0800 loads data to the $2000.$BFFF space from phase 1E to phase 44. Then final code execution is passed to $3E00. For those of you who would like to make a ProDOS version of the game, the program is from $2000 to $BFFF.

As you may see, the remaining free space is $1000..$1FFF or the $0200.$07FF space, that may be useful for your own routines...



READ ROUTINE
The read routine located at $B100 zeroes its index at $C8 and depending on the value of $CC sets parameters from table 1 or table 2. The routine uses the $C0Ex softswitch and not the $C08x,X softswitches!

Entry parameters at $B13D
A: phase you want to you (one phase = track number * 2)
phase 2 means track 1, phase E means track 7
$C9..$CA: address of data to load

Other parameters:
$50: checksum value
$C2: target phase
$C7: current phase

The routine loads data for 8 pages of $100 bytes (or $800 bytes, or $1000 nibbles), then compares the checksum between data read and $50, then increments the index at $C8.


DISK COPY
Let's use what Atari offers us: its own routine in $B100 but we need to modify it slightly:
- read one track
- copy to the IIgs memory
- loop until the end of the disk

Here is the code: