Mega99

The Mega99 is a TI-99/4A core for the MEGA65 computer. Currently r3 and r6 boards are supported.

The core is written from scratch specifically with the MEGA65 as its target. It tries to match the timing of the original TI-99/4A as closely as possible, but also features a 36x turbo mode, where the TMS9900 runs at 108 MHz.

ROMs

In order for the Mega99 to work, the ROM chips inside must be loaded with content from a real TI-99/4A. These are not included with Mega99, but can be found for example in distributions of MAME. The ROM images must be placed on a SD card inserted into the MEGA65 when the Mega99 core is started. The ROM images can be loaded either from separate files, or from the same zip archives that MAME uses. Please consult the following table for the correct file names of the ROM images and the respective zip files.

ROMs are searched for first on the external SD card, and then on the internal. If there is a directory MEGA99 in the root of the SD card, it is searched before the root directory for the file.

Peripherals

Keyboard

Alphanumeric keys and the space bar map directly from the MEGA65 keyboard to their corresponding TI-99/4A key. Other keys are mapped as indicated by the following table:

The OSD menus can be navigated with the arrow keys and RETURN.

Pressing TAB enables pseudokeys mode, in which keys are remapped according to the symbols on the keycaps of the MEGA65. Thus for example SHIFT+2 gets remapped to FCTN+P in order to generate the " character. FCTN in combination with an alphanumeric key or + can still be used as normal in this mode. FCTN (MEGA) in combination with non-alphanumeric keys is remapped to generate the ASCII symbol on the front of the keycap if any, e.g. MEGA together with the comma key gives ~.

Pseudokeys mode can not handle multiple keys pressed at once, and is not recommended for games. To leave pseudokeys mode, press TAB again.

The following mapping of non-printing keys is done in pseudokeys mode:

The cursor keys and HELP key are remapped according to the table above both when pseudokeys are enabled and when they are disabled.

Joysticks

Atari style joysticks can be connected to the joystick ports of the MEGA65. There is currently no support for TI style double joystick connectors.

Cartridges

Cartridges are loaded from RPK files as used by MAME. To insert a cartridge, open the main menu and select "Load RPK", then choose the RPK file to load. Supported cartridge layouts are "standard", "paged", "mbx" and "minimem".

Disk drive

The Mega99 includes a TI FDC card with three emulated drives. To attach a disk image on SD card, open the main menu and select "Mount DSK1 disk image" for DSK1, etc. Then select the image file to use. The image file must either contain a valid Volume Information Block (VIB), or have one of the following sizes:

Enabling the CPU turbo function also speeds up the disk drives by the same amount.

Cassette Tape

The Mega99 supports loading and saving to an emulated tape deck. To load data from tape, use the following procedure:

1. Start the tape load function in the relevant software (e.g. "OLD CS1" in TI BASIC).
2. When the prompt "PRESS CASETTE PLAY THEN PRESS ENTER" appears, don't press ENTER yet but instead open the main menu.
3. Select the "Open CS1 input file" option, and pick a file in the file selector.
4. Exit the menu, and then press ENTER to proceed with the tape dialogue.
5. The file should now be heard loading.
6. When the prompt to "PRESS CASETTE STOP" appears, there is no need to do anything special, just press ENTER.

Either WAV files or raw tape data files can be used. The latter should contain the actual bytes written to tape starting with the >FF data mark (the initial file sync of 768 >00 bytes should not be included, this is added automatically).

It is also possible to save to tape. Just proceed through the complete tape save process and the resulting data will be available in the service processor's memory afterwards, from whence it can be saved to SD card using the main menu. Tape saves are always created as raw tape data files.

Mini memory

The 4K battery back-upped RAM of the Mini memory cartridge can be loaded from, or stored to, SD card using the main memory. Load the RPK first, then the RAM memory file.

Speech synthesizer

The speech synthesizer is included and functional.

32K RAM expansion

The 32K RAM expansion is included and always enabled.

Running on Nexys A7

The core can also be used on a Nexys A7 or Nexys 4 DDR board. Use the mega99_nexys_a7_50t.bit bitstream for Nexys A7 50T and mega99_nexys_a7_100t.bit for Nexys A7 100T or Nexys 4 DDR.

SD card

In addition to the ROM images, the file mega99sp.bin should also be placed on the SD card when using the Nexys board. This is the main program for the service processor, which handles ROM loading and the OSD menu. It is possible to run Mega99 without mega99sp.bin, but then many features become unavailable, including all OSD functionality and the ability to load ROMs from zip files.

Keyboard

The following mappings are specific to the USB keyboard:

Pseudokeys mode is not available on the Nexys board, but the cursor keys can still be used as pseudokeys.

Joysticks

Atari style joysticks can be connected to PMOD connectors JA (joystick 1) and JB (joystick 2). Like on the MEGA65, TI style dual joystick connectors are currently not supported.

Pin mappings for the joystick connections:

Ideas for future enhancements

• Load and save to real tapes might be possible using a 1531 datasette connected to the Expansion Board

• Reading and writing 3.5" floppies using the MEGA65 builtin floppy drive should be possible, but maybe not so useful considering TI-99/4A floppies are 5.25"?

• There could be a DSR to access the contents of the SD card directly (using TIFILES)

Acknowledgments

• The VGA to HDMI IP was written by Adam Barnes

• Thierry Nouspikel's TI-99/4A Tech Pages has served as an invaluable resource in making this core

• Also instrumental was the Internet Archive for providing the full schematics of the TI-99/4A motherboard