When plug Cartridge, does display memory change from $9c40 to $7c40?

[emarti]

Hi,

I am using Altirra emulator. I am coding ROM program to works on cartridge. When I debug for xex program (not rom), display memory ( $58: 40, $59: 9c ) is $9c40. But If I attach rom on emulator, It displays $7c40.

 

Is this normal?

[flashjazzcat]

It is normal, because the display is 8K lower in RAM when ROM occupies $A000-$BFFF. Otherwise, the cartridge area is RAM and the display sits immediately below the start of the OS.

[emarti]

Thank you, @flashjazzcat.

[ClausB]

True but it appears you have 8K cart ROM in the first case and 16K in the second, assuming you're using the GR.0 text screen.

Edited July 25, 2018 by ClausB

[Rybags]

If you're doing a Rom program and it's not a language which would have variable requirements then you're best off using a fixed screen location and doing the DList setup yourself.

Cartridge games usually have minimalist requirements so to work on systems with not much Ram.

Some early games like Star Raiders can actually run on machines with only 8K of memory. Most games work in 16K. A lot of later ones need 32 or 40K but that's because many are just ports of games that were on disk originally like Ballblazer, and just use the Rom as storage and copy the game to run from Ram.