Snake Byte (Sirius Software, 1981-1982)

[toinet]

What has 48K Bytes and is addictive? Snake Byte! It's the game that works like a charm. A tail of Perilous Purple Plums that's ahead of its time. A game you can sink your teeth into. An antidote for boredom. Snake Byte. Fangs alot, Sirius Software!

It is the first game I ever played on an Apple II in 1983


DISK STRUCTURE
It is a non-standard disk, only T0/S0 can be copied! Each track contains the equivalent of thirteen sectors with markers DD AD DA followed by thirteen 512 4*4 coded nibbles.

Last but not least, the first track is track 1.5


BOOT TRACE
Sirius games are actively protected against boot tracing. That is quite mandatory as, once loaded, there are no more calls to the disk, meaning all the program is loaded into memory!

- 9600<C600.C6FFM
- 96FB: AD E8 C0 60
- 9600G
Bing! At $0801, the program loads 1024 bytes from $0400 to $07FF, clears memory, EORs our boot page, resets the stack pointer, EORs ZP and then jumps to the next boot stage at $0400

Let's get the code:
- 96FB: A9 14 8D 12 08 A9 18 8D B1 08 A9 00 8D B7 08 4C 01 08
- 9600G
Bing! We now have the $0400..$07FF code at $1400..$17FF.
We have another ZP eoring, the setting of the reset vector, another EOR, another stack pointer change.
Beginning at $0472, the real load process, which ends in $0500 where memory is once more EORed. It finally jumps to $7680!

Let's try to get the program (unless you have a IIgs and enter the control panel :
- 96FB: A9 80 8D 7E 08 A9 02 8D 7F 08 4C 01 08
- 0280: 48 A9 59 8D 1C 05 A9 FF 8D 1D 05 68 4C 00 04
- 9600G
Bing! The program should now be completely loaded into memory.


MEMORY ORGANIZATION
- T0/S0 - $0800..$08FF
- T0/S1..4 - $0400..$07FF
- T1.5 - $1400..$1FFF
- T2.5 - $2000..$2BFF
- T3.5 - $2800..$33FF (buffer crosses what has been previously loaded)
- T4.5 - $3400..$3FFF
- T5.5 - $4000..$4BFF
- T6.5 - $4C00..$57FF
- T7.5 - $5800..$63FF
- T8.5 - $6400..$6FFF
- T9.5 - $7000..$7BFF
- TA.5 - $7C00..$87FF


DISK COPY
You now have all the information to get the complete game and create a DOS binary file. As always, I believe it is better to rewrite the read routines

The following messages will contain the original boot codes and my own versions.

What you have to take care of is the stack pointer which is really important in a Sirius software. If you do not set it correctly, the game will hang. Take care also of the false opcodes at $7680, ah! I like such games.


GETTING THE STACK POINTER VALUES
At $0867, you have the first EOR routine that sets the stack pointer. The second one at $045F. We cannot modify the original, that would change the stack pointer value. So what?
Make a copy of the EOR code somewhere else in memory and executes it, just print out the result, let me show you what to do for the first routine:
- 0367<0867.0871M
- 0371: 20 DA FD 60
- 0365: A2 00
- 365G
=> result is #$20


That's all folks. You have all the useful information to make a backup of it

Toinet

[toinet]

The original boot 0 code at $0800:

[toinet]

The original boot 1 code at $0400:

[toinet]

My modified first boot process at $0800. Note that the ROM routine could have been reused, it is only for fun

[toinet]

My modified program loading code at $0400. Shorter, faster