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ao486 port for MiSTer by Sorgelig.

Core was greatly reworked with many new features and performance added.


  • 486DX33 performance (no-FPU).
  • 256MB RAM
  • SVGA with up to 1280x1024@256, 1024x768@64K, 640x480@16M resolutions
  • Sound Blaster 16 (DSP v4.05) and Sound Blaster Pro (DSP v3.02) with OPL3 and C/MS
  • High speed UART (3Mbps) internet connection
  • MIDI port (dumb and fake-smart modes)
  • Extrnal MIDI device support (MT32-pi and generic MIDI)
  • 4 HDDs with up to 137GB each
  • 2 CD-ROMs
  • Shared folder support

How to install

  • Copy ao486.rbf to the _Computer folder on your SD card. This will be automatically copied there if you are using the update script.
  • Create games/ao486/ directory on SD card and copy boot0.rom, boot1.rom there.
  • For CD images, the typical location is games/ao486/cd/ and you can either have the game folders, or CHD, IMG, ISO, BIN/CUE files in the root for mounting. The imgset command can be used in DOS to mount/unmount images placed here by passing the exact path.
  • For floppy: copy desired floppy raw image with extension img to games/ao486/floppy/ directory. The imgset command can be used in DOS to mount/unmount images placed here by passing the exact path.
  • For HDD: create an empty file of desired size with extension vhd in ao486 directory (or prepare .vhd file separately with any tool supporting .vhd or hard disk .img files and copy to games/ao486/ folder)
  • Boot ao486 and in OSD choose desired floppy, hdd, cd, and boot order. A valid DOS or supported OS boot is required for the core to boot an OS. Typical use cases: DOS Install disks on floppy with a clean VHD mounted for HDD. Pre-configured VHDs with OS and programs, Boot CDs, etc.
  • Save settings and press "Reset and apply HDD"
  • Optional shared folder can be used in conjuction with MisterFS.exe running as a TSR in your DOS image to exchange files between games/ao486/shared from inside DOS.

HDD image is a raw image with MBR. It can be opened in Windows/Linux by many applications, so it's possible to prepare a HDD in windows/linux (in most cases you need to work with .img files, then just rename it to .vhd extension)

Core Speed and Options and Drivers

The default core speed is set to 90Mhz with both L1 and L2 caches enabled. This will give you the maximum speed for the release version of ao486. Some games, especially older games, are sensitive to speed and cache so you can change the speed options and cache to fit the game. Optionally you can use the sysctl.exe program to automatically change these settings before launching your game. This is especially useful for batch scripts where you want to change those options and then set it back after the game exits.

Core options under Hardware:

  • CPU Clock: 15MHz, 30Mhz, 56MHz, 90Mhz (90MHz)
  • L1 Cache: On, Off (On)
  • L2 Cache: On, Off (On)
  • RAM Size: 256MB, 16MB (256MB)

Optional programs and drivers:

  • MISTERFB.DRV/INF: Windows Video Driver
  • imgset.exe: Used for mounting fdd, cd, and ide drives. Run 'imgset' for options.
  • misterfs.exe: You will need to copy this file to a VHD and run it using, "misterfs LETTER" where letter is the drive letter you want to mount the shared folder to. Create a /games/ao486/shared folder and copy files named to the 8.3 filename standard (SFN - short filename). This allows for file copy from the shared folder and accessable from within dos on a mounted drive. The tool is basic so it should be mainly used for file copy and not for running programs.
  • modem9x.inf: Used for Windows modem setup.
  • mpuctl.exe: tool for sending commands to the optional mt32-pi baremetal synth. See reference below.
  • sbctl.exe: set Sound Blaster configs. Supported: I5,I7,I10,H5,H1,T4,T6
  • sysctl.exe: Used for setting core options and cache referenced above from command line. Usase: SYSCTL SYS/MENU 90Mhz/56Mhz/15Mhz L1+/L1- L2+/L2-

Sound Blaster

Default config: A220 I5 D1 H5 T6 Supported alternative configs with IRQ 7 or 10 and/or no-HDMA (16bit DMA through 8bit DMA). Standard SB16 config (diagnose) can be used to set alternative settings, or included sbctl util. Windows driver manages alternative settings by itself when manually configured.

Current implementation supports SoundBlaster Pro specific commands (not available in SB16 originally) as well. Compatible config for SBPro is "A220 I5 D1 T4" in case if some game will require it.

ASP/CSP is not implemented, but some specific commands with dummy replies are added to let Windows driver work.

Simple volume (only master volume) is implemented in mixer so windows volume can be used in additional to standard MiSTer volume control.

C/MS Audio

C/MS (dual SAA1099) can be enabled in OSD. When enabled it prevents OPL2/3 access on ports 220-223. OPL2/3 still can be accessed on ports 388-38B (AdLib).

OPL2/3 (Adlib)

OPL2/3 can be accessed as a part of Sound Blaster board at ports 220-223 (if C/MS is not enabled) and 228-229(OPL2 only). However it's recommended to use AdLib ports 388-38B instead.

MIDI settings:

  • address: 330h
  • IRQ: 9

MT32-pi support:

MT32-pi connection is supported through USER I/O port. MT32-pi is automatically detected. Supported straight and crossed RX<->TX cables.

MT32-pi config and settings are here.

MT32-pi interface board is on Hardware repository

LCD and buttons are for convenience but not required. Basically only USER I/O connector is required, so it can be assembled without interface board.


  • Press WIN+F12 to access OSD on ao486 core. F12 alone acts as generic F12 PC key.


Currently only data portion of CD is supported. Best image format is ISO, but BIN/CUE and CHD format discs still can be mounted. You just need to mount bin file or if it's multi-file image, then usually first track is data which you need to mount.

IDE 1-0 has special function - it's a placeholder for CD. So you can hot-swap CD images in this drive. Regardless special function this drive also supports HDD images. Once HDD image is loaded upon reboot this drive loses CD placeholder feature.

Hard Disk and CD-ROM in Windows

Core supports up to 4 HDD images up to 137GB each. Up to 2 CD-ROMs are supported.

Currently due to unknown source of issue Windows works with IDE devices through BIOS. Most likely some BIOS function doesn't supply correct info for Windows drivers. It doesn't prevent windows from working but you need to keep in mind some specifics:

  • Hard disks mounted on Primary IDE (0-0/0-1) need no attentions. They simply work.
  • Driver for Secondary IDE (1-0/1-1) have yellow mark. It's recommended to delete this device from device list - it's not used anyway but will be installed every time you start automatic HW detection procedure.
  • CD-ROM needs oakcdrom.sys and mscdex.exe added to config.sys and autoexec.bat respectively (you may find them on bootable Window98 CD). Windows will use it to access CD-ROM.
  • CD-ROM autostart won't be automatically triggered (because work through BIOS). After replacing image file in CD-ROM you need to refresh the folder with drives (F5) and icon of CD will be changed if provided by image. Autostart will be triggered when you click on CD icon.

Original core repository