MEGA65 Style Cartridge (Work in Progress)

Owned by Oliver Graf
Last updated: Aug 05, 2024
3 min read

There are defined ways how a cartridge is recognized in a C64 or C128 system . But what defines a MEGA65 cartridge and how is it started? The Slot 0 core needs to know and also the ROM has probably to do something.

We decided to go both the C64 and C128 way: a MEGA65 cartridge has the bytes $4D $36, $34 at $8007. This spells “m65” (the C128 has “cbm” at that location, while the C64 can have “CBM80” at $8004).

With M65 carts, EXROM/GAME lines are both high. The system does not map any cart memory to bank 0. The M65 ROM handles auto-start by reading cart memory directly from $400xxxx.

Like the C128 $8000 (in cartridge space) has the coldstart and warmstart code:

* = $8000 cartVectors: jmp coldStart // cold-start vector jmp warmStart // warm-start vector cartFlags: .byte %00000001 // flags, bit 0 = 1 to auto-start cartMagic: .byte $4D,$36,$35 // "m65" string

Cartridge Startup

If the ROM sees the identifier at $4008007, and bit 0 of $4008006 is 1, the ROM uses DMA to copy 8 KB from $4008000 to $8000 and executes a JMP $8000. This executes the coldStart routine of the cartridge. Everything else is up to the cartridge.

  • A simple cart storing a program less than 8 KB in size can just run from 0.8000.

  • A cart larger than 8 KB in size will need a bootstrap routine to set up memory as desired. All cartridge hardware lines are available at 400.xxxx, so the actual bootstrap code will depend on the cartridge hardware platform.

  • EasyFlash 1 can store up to 1 megabyte, making 16 KB visible at a time in two 8 KB chunks at 400.8000 (ROML) and 400.E000 (ROMH). Only EF bank 0 ROML is DMA’d to 0.8000 during cart start-up. The cart’s bootstrap code can write to the 400.DE00 EasyFlash bank register to select one of 64 banks to replace ROML and ROMH. This does not change 0.8000 automatically: the cart’s bootstrap needs to decide how to access and install cart data read from 400.xxxx.

  • Gmod2 can store up to 512 KB, making 16 KB visible at a time in one 16 KB chunk at 400.8000. Only the first 8 KB is DMA’d to 0.8000 during cart start-up.

  • Custom hardware must serve a DMA’able bootstrap program at $8000-9FFF.

CRT files [DRAFT PROPOSAL]

In the C64/128 ecosystem, CRT files are used to capture data from common cartridge types, for use with recreation hardware based on EEPROMs or flash memory (EasyFlash, Gmod2), and for use by emulators like VICE. We will define a CRT extension for similar purposes with MEGA65 carts. Minimal changes to the CRT format are needed.

MEGA65 CRT files begin with this 16-byte signature: MEGA65 CARTRIDGE

Cartridge types:

  • 0: generic

    • 16KiB ROML at $8000-BFFF, 16 KiB ROMH at $C000-$FFFF, non-banking

    • One or two CHIP packets, one for each of ROML and ROMH

    • See “C128 generic” for reference

  • 1: EasyFlash

    • 8KiB ROML at $8000, 8KiB ROMH at $E000, with banking

    • See “EasyFlash” for reference

  • 2: Gmod2

    • 16KiB banks, always mapped to ROML at $8000-BFFF

    • See “Gmod2-128” for reference

Other format changes might be justified if this is added to the CRT standard by the VICE team, e.g. incrementing the format version bytes.

CRT files and EasyProg

Someone writing software intended to be installed on an EasyFlash 1CR cart is likely to distribute that software as a CRT file that an end user can install on their own EF1CR hardware. With C64/C128 carts, the end user uses EasySplit and EasyProg to write the CRT to the EF1CR using their C64.

We don’t yet have comparable tools for MEGA65 carts, but it’s worth discussing tool needs.

  • A fork of EasyProg would be required to recognize the CRT signature and install the M65 cart header described above.

  • A fork of bin2efcrt would be useful for developers. The change to the CRT signature would be straightforward.

  • I have not yet tested EasyProg from GO64 mode. It works well with the C64 core.

  • Ideally we’d have a new version of EasyProg that runs in MEGA65 mode and can read full CRT files off the SD card, but that’ll be a big project. In the meantime, it should be possible to use a forked EasyProg with the C64 core to make MEGA65 carts.

External References