MiSTer: Cycle-Accurate Atari ST Core FX CAST

ijor has been developing an Atari ST core for some time and just released a preview build for MiSTer. He boasts that it is so precise as to be cycle-accurate. Atari ST uses a Motorola 68000 processor that numerous consoles and computers also use. If the core is cycle-accurate, then the components that ijor has ported will be upgrades for cores that share the same ICs. His pre-compiled MiSTer core can be downloaded from the release post.

Audio Sculpture original copy protected SCP image

X CAST ATARI ST core for MiSTer platform
Copyright (c) 2018 by Jorge Cwik

Public beta release

Cycle accurate Atari ST core.

The core features a cycle accurate 68000 CPU and cycle accurate chipset, including cycle accurate FDC that can load and run low level copy protected disk images.

This is a preliminary beta release. Several features are not fully implemented yet. They will be available in future releases. See the limitations sections below.

Special issue

This release requires a special version of the main linux MiSTer application to be able to use all the available features. The distribution package includes two variants of the core. There is a limited build that can be used with the stock MiSTer application.

Currently the limited build can low standard ST disk images only. It doesn’t support copy protected images.

The package includes the custom build of MiSTer. If you decide to install it, please keep the original MiSTer as a backup just in case the custom version has some kind of incompatibilities with other cores.

Type of disk images supported

– ST standard images
– MSA compressed images (custom MiSTer only)
– SCP copy protected raw flux level images (custom MiSTer only)

Loading SCP images might take considerable time. Those are huge images that sometimes might fill most of the available RAM.


– ST/STFM only. No STE functionality. No Blitter. ***
– No hard disk support.
– No user selection of the TOS version. Has TOS 1.0 UK.
– No user selection of amount of RAM. Hardwired for 1MB.
– Single floppy disk drive. Read only.
– No option for faster than real floppy disk.
– No turbo mode (CPU always runs at nominal frequency)
– No support for ROM cartridges
– A few copy protected images might not load correctly.

Reporting bugs

Note that many ST programs run only on a special configuration. If you find something that doesn’t work, make sure that runs on real hardware, or at least on a modern emulator, using exactly the same configuration (most important is TOS version and RAM size).

If you find any software that doesn’t run correctly, please provide as much details as possible. Also, there are multiple version for most games and titles. So you might need to provide a download link, or at least some way to identify the version you are using. If you know how to do it, a checksum hash (like MD5) of the disk file image might be helpful.

Known issues

– There might be some cosmetic issues, especially on monochrome mode.
– After loading you have to hit F12 to bring the menu manually and load an image.
– VGA output on the I/O board is not well tested and not fully implemented. Color output is scan doubled, there is no 15KHz mode.
– The core seems to be sensitive to powering issues. If the joystick controller doesn’t work correctly, you might need to try using a powered usb hub.

FX CAST 0.1 core. Version for standard MiSTer

This build that doesn’t require a custom version of the MiSTer app. It doesn’t support copy protected images. Some ST images with exotic geometry might not load.


#mister, #fpga, #atarist, #fxcast