Trying to define a smaller MMC3 ROM results in a wrong sized output

[wendelscardua]

I'm making a MMC3 ROM with mostly C++, and if I compile it along a config.s with these definitions:

        .global __chr_rom_size
        .global __prg_rom_size
        .global __prg_ram_size
        .global __four_screen
        .global __mirroring

                                ; Kilobytes
        __prg_rom_size = 64
        __chr_rom_size = 8
        __prg_ram_size = 8

                                ; Flags
        __four_screen = 0
        __mirroring = 1 ; horizontal mirroring

I get a ROM with 81936 bytes instead of 73744 (16 header + 65536 PRG + 8129 CHR)

(for now I'm omitting the _prg_rom_size part and living with a mostly empty 500K+ ROM)

[mysterymath]

This was an off by one in the MMC3 linker script; depending on the bank layout, the fixed region may use take either 2 or 3 banks. The default setup uses 3 but neglects to subtract out a swappable bank for any size other than the max. I've added regression tests for this behavior for all of the supported size boundaries.