Tech Pages

These pages are dedicated to the Texas Instruments TI-99/4A home computer. They contain mainly technical information on the various processors and memory chips found in the console, on peripheral cards and cartridges and on additional hardware. Although the emphasis is on hardware, I included numerous examples of assembly language programs to demonstrate how to access hardware or illustrate undocumented features.

These pages are not for beginners! To follow the discussion you'll need some knowledge of digital electronics and notions of assembly language programming. I tried to adopt a fairly didactic style, but this is by no means a tutorial. Nevertherless, you'll find an assembly language primer, an introduction to GPL language, an explanatory page for digital TTL chips and tutorials on how to solder and how to open your console and PE-box.

These pages are meant to load fast over a phone line: no fancy graphics, no pictures inserted in the text and (God preserves us) no frames! In addition, the whole site is available for download as .zip files.

This site is currently hosted by the University of Geneva, Switzerland.  It used to be hosted by Stanford, then the University of Sheffeild, but this changed when I moved back to my hometown. So if you've bookmarked the old site, please replace it with

Now folks, I hate to brag,but I must admit that it feels good to be recognized by one's peers. So let it be known that this site was granted the Edgar Mauk Award in the catagorie `TI Websites`, at the European TI-Treffen in Gent (Belgium), on October 7th 2000. Click here for details.

In 2003, it was granted the Jim Peterson Award in the category "Community Service".

RTP award
And in 2005, it was selected for the Rob Tempelmans Plat award, a joint initiative from the Chicago TI users group and the Dutch TI Gebruikersgroep to honor the best internet sites for the TI-99/4A community.

Below is the site map. Note that some pages may be listed under more than one header.

The TI-99/4A console

General architecture
Pinout of ports and chips

TMS9901 Programmable system interface
TIM9904 Four-phase clock generator and driver
TMS9918A  (or  TMS9928A or TMS9929A) Videoprocessor
TMS9919 Sound generator
TMS5220 Speech synthesizer

Scratch-pad RAM

CRU (communication register unit)
Cassette tape interface
TTL chips featured in the schematics on this site

Peripheral cards

The Peripheral Expansion Box
The TI disk controller. Part I: Theory, FDC , Part II: Card, drives
The TI RS232C interface card
The german 128K GramKarte
The P-Gram card
The 32K memory expansion card
The SupeRAMs memory card
The p-code card , Part II: the p-system
The Horizon ramdisk
My IDE interface card, PartII: Software , (Old version)
The MBP clock + ADC card
The ForTI sound card
My USB/SmartMedia card , Part II: Software , Part III: Build it (see below for related software).


Mini memory

Extra hardware

TI joysticks
PC joysticks adapter
WiPo mouse 99
Navarone's Widget
GROM reader for the PC (Courtesy M. Comstedt)

Upgrading your TI-99/4A

Do-it-yourself primer (Soldering, making boards, tools, where to buy, etc).
Open-console surgery guide
PE-box surgery guide
Index of modifications

Adding a second RS232 card
Replacing card ROMs with EEPROMs
Multi-bases Widget
Installing the speech synthesizer inside the PE-box

Console modifications
Powering the joystick port
PC joysticks adapter
Overclocking the console
Supressing wait states
Jeff Brown's interrupt mod
Replacing the console ROMs with EEPROMs
Disabling the console GROMs
Console CRU interface

Programming the TI-99/4A

Assembly language primer
GPL: the GROM language (I) , Part II
The TI-Basic interpreter

Tips & tricks
Standard headers: writing DSRs and Calls
Calling GPL from assembly
Using interrupts
Floating point numbers calculations
Using the internal timer *
Sound generation *
Speech synthesis *
Direct access to the disk drive controller *
Scanning the keyboard *
Keyboard challenge (+ tutorial)
* Part of another page

Blowfish encryption
AVL trees
Writing DSRs for the USB-SM card , Part II: EEPROM loader 

Known TI-99/4A bugs

Annotated pictures

The Floppy disk controller
The RS232c / PIO card
The Horizon Ramdisk
My IDE card
The Mini-memory cartridge
The Speech synthesizer


Free software
Disassembled listings
The whole site
How to download


Comments, corrections
Information wanted
An interview with yours truly
E-mail me

The TI-99/4A world

ti99-4a online user group
ti99-4a programming group
comp.sys.ti newsgroup

The TI-99/4A Web Ring
This TI-99/4A Ring site is owned by Thierry Nouspikel
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