Doubt about developing a 128kb MegaROM (ASCII 16K Mapper)

Yep, I had the same doubt when coding Freedom Fighter. Once you get it right (and trust me, try it and you get it in no time) it is a joke to manage

The Z80 signals a read request (/RD pin) or a write request (/WR pin) together with the address on the address bus A0-A15 and the value on the data bus (D0-D7). In a plain 32K ROM the ROM chip is directly connected to these via the cartridge slot (and /WR is left disconnected). However in MegaROM there is a mapper IC on the cartridge in-between the Z80 and the ROM.

When a /WR write signal is received from the Z80 the ASCII16 mapper IC checks if the address is in the range 6000H-6FFFH or 7000H-7FFFH and if so will set the value specified on the data bus into its corresponding bank select register, otherwise the write is ignored. It only checks the address bits A12-A15 so that’s why it spans a range rather than a single address (this reduces transistors needed).

When a /RD read signal is received from the Z80 the ASCII16 mapper IC takes address bits A0-A13 and concatenates them with the current value of the corresponding bank select register to form a 22-bit physical ROM address A0-A21 (max 2^22 bytes = 4 MB). This address is passed to the ROM chip, which will then return the value to the CPU on the data bus.

The differences between different mapper types lie in what addresses they respond to for their bank select registers, and which address bits they use for their banks. E.g. the ASCII8 mapper with 8K banks has four bank registers instead of two, and uses A0-A12 together with the bank select register to form a 21-bit address (2MB).

As for emulators, in absence of a physical cartridge you provide them with the ROM contents + a mapper type to emulate. If no mapper type is specified, the emulator first tries to look up a hash of the ROM in a database. If it is not recognised, it tries to guess the mapper type by looking if the ROM has code that writes to bank select addresses that belong to a particular mapper type. This guessing is not 100% so it can fail under circumstances, so best to specify the mapper type explicitly.

Thanks, @Grauw and @mcolom, it's indeed an interesting way to solve this problem, cheers to the designers.
Also, it's cool how is required some (at least) basic hardware knowledge to develop in ASM.

It depends on the mapper.
For the one I use (I think it's the "Generic8": https://www.msx.org/wiki/MegaROM_Mappers#Generic8), I do this:

set_two_segments_P8000:
; A: segment. It'll also choose segment A+1
ld (Seg_P8000_SW), a ; 0x8000 - 0x9FFF
inc a
ld (Seg_PA000_SW), a ; 0xA000 - 0xBFFF
ret

So, it switches 0x2000 bytes every write. So, a total of 0x2000*256 = 2 Mb.
But actually I'd say that in a real cartridge you're not really much limited. The hardware can could do crazy things to page memory (like ready sequentially bytes from 0x8000 to get a large address, for example). I've never found anything like that, but I don't see why you wouldn't be able to build such a thing. Of course, I mean only with real hardware.

Unfortunately I have to tell you that 99% of the time it is not black magic, but just sleight of hand... Practically these emulators just cheat.

In openMSX share-folder there is a file called softwaredb.xml that contain description of each MSX cartridge. If you open it, add your own software entry there, add inside correct mapper type and your ROM-file's valid SHA-1 checksum, openMSX will then "magically" select the correct type.

When this check fails I've heard rumors that it then tries to make some "educated guess" based on pattern search, but at least those few times I've worked with unknown ROMs, that recognition has just failed miserably... I've seen some ROM source codes where people have copy/pasted some dummy, unused mapper select routines here and there around the code, so unless this is pure superstition I guess it may have helped at least someone on some case... but this then indeed belongs to the black magic category where you just try to exorcise emulator to use it's supernatural hardware imagination powers. I don't have any good spells to share for emulators that are currently in use.