Target

Mapped ROM program

Note: MegaROM refers to a ROM of 128KB or more. Even though they are not widely supported, there is nothing to prevent you from creating a 64KB ROM using a mapper. We therefore prefer to use the term MappedROM here rather than MegaROM.

Setup

You can create program using one of this supported ROM mappers:

• ASCII-8: 8KB segments for a total to 64KB to 2MB,
• ASCII-16: 16KB segments for a total to 64KB to 4MB,
• Konami: 8KB segments for a total to 64KB to 2MB,
• Konami SCC: 8KB segments for a total to 64KB to 2MB.

In your build.bat, chose as target on of the following type: ROM_ASCII8, ROM_ASCII16, ROM_KONAMI, ROM_KONAMI_SCC. You can specify the ROM size (in KB) in the ROMSize variable. Default value is 128 (KB). Any multiple value of the mapper's segments size is valid, but I recommend using powers of 2 values (64, 128, 256, etc.) to help emulators to autodetect the right mapper.

Some Build Tool configuration examples:

  REM -- 128KB ASCII-8 mapped-ROM
  set target=ROM_ASCII8

  REM -- 4MB ASCII-16 mapped-ROM
  set target=ROM_ASCII16
  set ROMSize=4096

  REM -- 512KB Konami SCC mapped-ROM
  set target=ROM_KONAMI_SCC
  set ROMSize=512

See s_vgm sample for a concrete use case.

Data

Mapped-ROM use a system of sub-page (called "bank" here) the user can point to any segment of the ROM. Those bank are mapped to page #1 and #2 (4000h~BFFFh) of the memory space. The number of banks depends on the size of the mapper:

• 8KB mappers have 4 banks:
  • bank 0: 4000h~5FFFh
  • bank 1: 6000h~7FFFh
  • bank 2: 8000h~9FFFh
  • bank 3: A000h~BFFFh
• 16KB mappers have 2 banks:
  • bank 0: 4000h~7FFFh
  • bank 1: 8000h~BFFFh

The Build Tool build the first 32KB of the mapped-ROM like a 32KB plain ROM. This represents the first 4 segments for a 8KB mapper and the first 2 for a 16KB mapper.

The following segments are added to the ROM by the Build Tool by searching for files with a particular nomenclature: <project_name>_s<segment_number>_b<target_bank>.c.

For example with a ASCII-8 ROM of 128KB:

   program.c          | Main program including segment #0~#3 (4*8KB)
   program_s4_b2.c    | Segment #4 to be used in bank #2 (8000h~9FFFh)
   program_s5_b3.c    | Segment #5 to be used in bank #3 (A000h~BFFFh)
                      | No files found for segment #6 to #14: padding data added to the ROM
   program_s15_b2.c   | Segment #15, the last one, to be used in bank #2 (8000h~9FFFh)

Select segment

In your program, you can then select what segment is visible from what bank using the SET_BANK_SEGMENT(bank, seg) macro. For example, to put segment 15 of the above example in bank 2, just use SET_BANK_SEGMENT(2, 15);

For information, at startup, the following segments are selected:

• 8KB mappers:
  • bank 0: segment 0
  • bank 1: segment 1
  • bank 2: segment 2
  • bank 3: segment 3
• 16KB mappers:
  • bank 0: segment 0
  • bank 1: segment 1