v0.21 26 August 2024

This is a minimalist Apple Macintosh 128K/512K emulator. It uses the Musashi 68K interpreter and lashes the absolute minimum in hardware emulation around it so as to boot and run basic apps. It's been tested with System 2.0 up to System 7.5.5, and runs MacWrite, MacDraw, and Missile Command. (For a Mac 128K, System 3.2 is the last OS that will run, but by configuring 2MB+, even System 7 will work.)

You can write, draw, and unwind with an apocalyptic game. Live well.

This was written as one of the old "hey I wonder how hard it'd be to" exercises; bit of fun, but not intended as a replacement for existing emulators. But, it might inspire others working on similar (better) projects, and as a basis to explore these early but innovative machines.

This grew without a plan, just playing around with the Mac128K ROM: it turns out that there's very little HW emulation required to get to an Unhappy Mac screen, or even an attempt to boot via FDD (disc-question-mark screen). Then, you discover IWM is way painful to emulate, and spend 80% of the time working around that – almost all Mac emulators immediately patch the ROM to insert a paravirt block driver over the top of the IWM driver, circumventing the problem. That's what MicroMac does, too.

This emulates the following hardware:

• VIA A/B GPIO ports, and IRQs (1Hz and Vsync)
• VIA shift register for keyboard
• SCC DCD pin change interrupts, for mouse
• Paravirtualised disc storage
• Defaults to 128K of RAM, but will run as a Mac 512K by building with MEMSIZE=512. Or, you could use 1024, 2048, or 4096 to make a 1/2/4MB Mac Plus-like machine. Straying off-piste a little, you can also use unsupported values like 256, 208, 192KB; when this is done, the ROM is patched to "probe" the correct memory size.

There's no emulation for:

• IWM/realistic floppy drives
• SCSI; the machine is sort of like a Mac Plus without SCSI.
• More than one disc, or runtime image-switching
• Sound (a lot of work for a beep)
• VIA timers (Space Invaders runs too fast, probably because of this)
• Serial/printer/Appletalk
• Framebuffer switching: the Mac supports double-buffering by moving the base of screen memory via the VIA (ha), but I haven't seen anything using it. Easy to add.

The emulator is structured so as to be easily embeddable in other projects. You initialise it, and pass in UI events (such as keyboard/mouse), and read video from the framebuffer:

  umac_init(pointer_to_ram, pointer_to_patched_rom,
            pointer_to_struct_describing_mmaped_disc_images);

  while (happy) {
    if (one_second_passed)
      umac_1hz_event();

    if (vsync_happened_60Hz_kthx) {
      umac_vsync_event();

      update_UI_video_from(pointer_to_ram + umac_get_fb_offset());
    }

    if (keyboard_event_happened)
      umac_kbd_event(mac_scancode);

    if (mouse_movement_happened)
      umac_mouse(delta_x, delta_y, button_state);

    umac_loop();
  }

A simple SDL2-based frontend builds on Linux.

To build on Linux, you'll need SDL2 installed (packaged as libsdl2-dev on Ubuntu).

Musashi is included as a submodule, currently a custom branch with some optimisations for static/tiny/fast builds. git submodule update --init

You'll then need some Mac artefacts. Because we need to patch the ROM a little, we require specific versions. Currently the only supported ROM is Mac Plus V3 ROM, checksum 4d1f8172.

(Note this makes this a Mac 128Ke, fancy!)

Then, get a boot disk. Any System up to 3.2 should work fine on a 128K (though I don't think I've tried 1.0). I just tested a random German System 3.2 disc from WinWorld and it works fine. You might want to use Mini vMac or Basilisk (i.e. a proper emulator) to prepare a disc image with some apps to run... MacDraw!

It doesn't have to be a specific size. A 400K or 800K floppy image works. Make sure it's a raw image; the first two bytes should be the chars 'LK'. Some emulators append a header (which can be dd'd off).

make

No surprises here. No autoconf either. :D You can add a DEBUG=1 to make to compile in debug spew, and add MEMSIZE=<size_in_KB> to control the amount of memory.

This will configure and build Musashi, umac, and unix_main.c as the SDL2 frontend. The Musashi build generates a few files internally:

• m68kops.c is generated from templates in m68k_in.c: this "multiplies out" N instructions by the 200,000 addressing modes of 68K, generating specialised code for each individual opcode.
• Using a custom/new Musashi build option, a large (64K pointers) opcode lookup table is generated, 16-bit 68K opcode is generated at build time, as m68ki_static_instruction_jump_table. This was previously generated at runtime, i.e. used up RAM.

After the Musashi prepare step, tools/decorate_ops.py does an in-place update of m68kops.c to decorate some of the opcode functions with M68K_FAST_FUNC. This macro does nothing by default, but can be defined to apply a function attribute. If this project is being built in the RP2040 Pico environment, the functions gain an attribute to place them in RAM instead of flash – making them much faster.

Some low-quality (so uncommitted) and undocumented profiling code was used to generate the tools/fn_hot200.txt list of the 200 most frequently-used 68K opcodes. This was generated by profiling a System 3.2 boot, using MacWrite and Missile Command for a bit. :D Out of 1967 opcodes, these hottest 200 opcodes represent 98% of the dynamic execution. (See RISC.)

./main -r <path_to_MacPlusV3_rom> -d <path_to_disc_image>

The RAM is actually a memory-mapped file, which can be useful for (basic) debugging: as the emulator runs, you can access the file and see the current state. For example, you can capture screenshots from screen memory (see tools/mem2scr.c).

For a DEBUG build, add -i to get a disassembly trace of execution.

Finally, the -W <file> parameter writes out the ROM image after patches are applied. This can be useful to prepare a ROM image for embedded builds, so as to avoid having to patch the ROM at runtime. That then means the ROM can be in immutable storage (e.g. flash), saving precious RAM space.

If you're writing an emulator for an olden Mac, some pitfalls/observations:

• The ROM overlay at reset changes the memory map, and the address decoding for read/write functions has to consider this. Overlay is on for only a handful of instructions setting up RAM exception vector tables so, for performance to avoid checking on every access, there should be two versions of memory read/write functions that are selected when the overlay changes. This has been implemented only for instruction/opcode fetch.

• IWM is a total pig to emulate, it turns out. There's some kind of servo loop controling the variable rotation speed via PWM (a DAC!), and the driver/IWM vary it for a particular track until the syncs look about right... too little fun for a Sunday.

• The disc device emulation is a cut-down version of Basilisk II's disc emulation code: A custom 68K driver (in macsrc/sonydrv.S, based on B2's driver code) is patched into the ROM, and makes accesses to a magic PV_SONY_ADDR address. These are then trapped so that when the Mac OS makes a driver call (e.g. Open(), Prime(), Control(), Status()) the call is routed to host-side C code in disc.c. The emulation code doesn't support any of the advanced things a driver can be asked to do, such as formatting – just read/write of a block. The read/write can be performed internally, by passing a pointer to an in-memory mapping of the disc data (e.g. unix_main.c just mmap()s the disc); or, op_read/op_write callbacks can be used for when a host disc op needs to be performed. When the disc is asked to be ejected, a umac callback is called; currently this just exits the emulator. The beginnings of multi-disc support are there, but not enabled – again, bare minimum to get the thing to boot.

• The high-precision VIA timers aren't generally used by the OS, only by sound (not supported) and the IWM driver (not used). They're not emulated.

• The OS's keyboard ISR is easy to confuse by sending bytes too fast, because a fast response's IRQ will race with the ISR exit path and get lost. The main.c keyboard emulation paces replies (kbd_check_work(), kbd_rx()) so as to happen a short time after the Mac sends an inquiry request.

• Mouse: The 8530 SCC is super-complicated. It's easy to think of the 1980s as a time of simple hardware, but that really applies only to CPUs: to compensate, the peripheral hardware was often complex, and SCC has a lot of offloads for packetisation/framing of serial streams. It is this chip that enables AppleTalk, relatively high-speed packet networking over RS422 serial cables. Two spare pins (for port A/B DCD detect) are used by the mouse; the Mac 128K is a nose-to-tail design, no part of the animal is wasted. Uh, anyway, only enough of the SCC is supported to make the mouse work: IRQ system for DCD-change, driven from one half of a quadrature pair on X/Y. The ISR for those lines then samples the VIA PORTB pins for the corresponding other half of the pair. Finally, the emulator interface takes a movement delta dx:dy for convenience; a quadrature step is performed for each unit over a period of time.

• I didn't use the original Mac128 ROM. First, Steve Chamberlin has done a very useful disassembly of the MacPlus ROM (handy to debug!) and secondly I misguidedly thought that more stuff in ROM meant more RAM free. No, the 128K MacPlus ROM uses more RAM (for extra goodies?) than the original 64K Mac 128K ROM. It does, however, have some bug fixes. Anyway: the MacPlus ROM runs on 512K and 128K Macs, and was used as the 'e' in the Mac 512Ke.

• https://github.com/kstenerud/Musashi
• https://www.bigmessowires.com/rom-adapter/plus-rom-listing.asm

The basis for the 68K sonydrv.S and host-side disc driver code in disc.c/b2_macos_util.h (as detailed in those files) is from Basilisk II, Copyright 1997-2008 Christian Bauer, and released under GPLv2.

The keymap.h and keymap_sdl.h headers are based on Mini vMac OSGLUSDL.c Copyright (C) 2012 Paul C. Pratt, Manuel Alfayate, and OSGLUAAA.h Copyright (C) 2006 Philip Cummins, Richard F. Bannister, Paul C. Pratt, released under GPLv2.

Some small portions of main.c (debug, interrupts) are from the Musashi project's example/sim.c. Musashi is Copyright 1998-2002 Karl Stenerud, and released under the MIT licence.

The remainder of the code is released under the MIT licence:

Copyright (c) 2024 Matt Evans

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.