how to make an autostart cartridge binary image?

[llcameron]

I'm surprisingly not finding any specifics on how to format the first bytes of a cartridge binary image to have it autostart at reset! I have a game I wrote that I would like to burn into an EPROM for use with the cart PCB from TFW8B, but I'm stumped as to how to frame the start of the bytes at $A000!

I know there is a magic byte sequence that needs to appear at the start (?) of the binary (at $A000?), and the VIC will look for that sequence at reset. If it finds it, it runs the code. So where does the code start - immediately after the magic sequence? Is there anything special that the code needs to do, for example clear the interrupt flag, setup any interrupt vectors, etc?

If anyone could share a recipe with me I would most appreciate it!

[mrr19121970]

Look into a .bin file of an existing cart ?

It's probably like the c64
http://blog.worldofjani.com/?p=879

[llcameron]

Thank you for your reply!

The procedure for creating the VIC autostart cart I'm sure is very simpliar to the C64 but must differ in details.

Thank for the link - I need a version of that, just for the VIC. I'm surprised how much difficulty I am having trying to locate the details.

[Mike]

I'm surprisingly not finding any specifics on how to format the first bytes of a cartridge binary image to have it autostart at reset! [...]

A thorough look into the reset sequence beginning at $FD22 in the KERNAL ROM (using the ROM listing of your choice) is entirely sufficient to sort this out.

Bytes $A000..$A001 contain the cartridge Reset vector, i.e. the address of code which is immediately called by the KERNAL, just as the signature has been checked, before anything else (RAM, I/O) is initialised. If you want to use KERNAL I/O, you should at least copy part of the continued reset routine (what would normally run at $FD2F following) before entering your game, so zeropage and I/O are set up properly. If your game is written in BASIC, you'll need to copy a bit more code, but instead of showing the start-up prompt, transfer the program into RAM (including the values at $2B..$2E!) and then use a certain sequence of calls into the BASIC interpreter (JSR $C533, JSR $C659, JMP $C7AE) to start the payload.

Bytes $A002..$A003 contain the cartridge NMI vector, i.e. the address of code which is called by the KERNAL NMI routine in place of its own vector and I/O reinit routines - your game should either just acknowledge the NMI source and do nothing, or copy the relevant part of the NMI reinit beginning at $FEC7 and change it to your liking to 'restart' your game (without it losing highscore lists, or the like). Note this needs a proper initialised NMI vector at $0318 pointing to $FEAD.

Bytes $A004..$A008 contain the A0CBM signature (nothing 'magic' about it!): $41, $30, $C3, $C2, $CD.

...

For an example, take a look at my TRON lightcycles game, which I made into cartridge form quite some time ago. It has been a BASIC program for unexpanded VIC-20. I took some extra preparations to make the game proof against extra RAM expansions (it disables them as part of the extended reset sequence), also STOP and STOP/RESTORE are switched off.

...

It's no bad idea to test the cartridge image in VICE first, before burning it into an EPROM.

[llcameron]

Thanks for your reply! It was very helpful!

What ultimately worked for me (with the least amount of effort and thought!) is to disassemble with x65dasm a gorf cartridge and see how it did its initialization.