Texas Instruments TI-99/4A

From Academic Kids

The Texas Instruments TI-99/4A was an early home computer, released in June 1981, originally at a price of $525. It was an enhanced version of the earlier TI-99/4, which had been released in late 1979 at a price of $1150. The TI-99/4A added an additional graphics mode and a full-travel keyboard (the 99/4 had a calculator-style chiclet keyboard).



In the TI-99/4A, the CPU, motherboard, and cartridge drive were built into a single unit with the keyboard. Available peripherals included a 5¼" floppy disk drive, an RS-232 interface, an in-line speech synthesizer module, and a 32 KB memory expansion card. Many of these peripherals came in two forms, a card which plugged into the bulky "Peripheral Expansion Box", an eight slot chassis containing its own power supply and 5¼" floppy bay; or a 'sidecar' version which plugged into the side of the console. These 'sidecar' expansion units could be connected together in a continuing chain. Early models (the 99/4) included a built-in equation calculator, but in the 99/4A this feature was discontinued. All consoles included TI BASIC, a strict ANSI-compliant BASIC programming language interpreter which was largely incompatible with the more popular Microsoft BASIC. Later consoles, identified by a "2.2" on the title page, also removed the ability for the system to execute ROM-based cartridges, locking out third-party manufacturers such as Atarisoft. In the early 1980s, TI was known as a pioneer in speech synthesis, and a plug-in speech module was available for the TI-99/4A. The system also supported saving and loading to two cassette drives through a dedicated port, and had a joystick port that supported two digital joysticks, which TI referred to as "wired remote controllers". Composite video and audio were output through another port, and combined by an external RF Modulator for use with a television.

The TI-99/4 series holds the distinction of being the first 16-bit personal computer. The TI-99/4A had a 16-bit TMS9900 CPU running at 3.3 MHz. However, several design decisions substantially impacted the performance of the machine. First, there were only 256 bytes of CPU-addressable RAM, all remaining memory (16k) was attached to the VDP (see below). Secondly, except for the system ROMs and this 256-byte 'scratchpad', all memory and peripherals were connected to the CPU through a 16-to-8 bit multiplexer, requiring twice the cycles for any access and introducing an additional wait state. The machine also accessed most hardware through a single bit serial I/O bus called the Communications Register Unit (CRU - 9901), although in some devices this hardware was only used to map memory and hardware into the CPU memory space for direct access.

Because the machine only had 256 bytes of RAM attached to the CPU, the TI99/4A's display coprocessor, a TMS9918A, had its own RAM referred to as VDP RAM (visual display processor RAM).

This memory was also used for storing the programs users wrote in the built-in BASIC programming language. BASIC was implemented on the TI-99 series using a second interpreted language called 'Graphics Programming Language', or GPL. The GPL interpreter resided in the ROMs and took control of the machine at powerup, and was very close to the native 9900 machine code, adding instructions to transparently access the different types of memory in the machine and perform higher level functions such as memory copy and formatted display.

Adding CPU memory additional to the 16 KB of the videochip was considered too expensive. (The video chip only supported a 4k mode, and the 16k mode. The 4k mode was insufficient for all display modes, and so the 16k mode was used.) All video memory had to be accessed through the video processor a single byte at a time, as this memory was not directly connected to the CPU.

The computer also had unusual features such as GROMs (Graphics Read-Only Memory) and an accompanying programming language called GPL (Graphics Programming Language). GROMs were another set of memory accessed a single byte at a time through a dedicated memory port, and were auto-incrementing read-only devices. (Although there is support in the console for 'GRAM', simulators for which were created by third parties later.) As there was no realistic amount of RAM addressable by the CPU, machine code programs would not run unless more RAM was added in the form of either the 32 KB expansion card or the 4 KB "Mini Memory" module. Because of these, the TI-99 series gained a reputation for being quirky and eccentric, which endeared it to some and maddened others. Many people who had only experienced TI BASIC also considered it very slow, although assembly programs actually managed fairly good speed despite the hardware issues to overcome.


Initially, the TI-99/4A was reasonably successful, and it has been estimated that it had about 35% of the home computer market at its peak. However, TI quickly found itself engaged in a price war, particularly with Commodore International, and was forced to lower the computer's price in order to compete. By August 1982, the computer was losing shelf space and TI offered a $100 rebate, which caused spokesman Bill Cosby to quip about how easy it was to sell a computer if you paid people $100 to buy one.

In February 1983, TI lowered the price to $150 and was selling the computers at a loss. And in June 1983, TI released a redesigned, cost-reduced version that it sold, also at a loss, for $99. TI lost $100 million in the second quarter of 1983 and $330 million in the third quarter. In October 1983, TI announced it was exiting the home computer business.

A total of 2.8 million units were shipped before the TI-99/4A was discontinued in March 1984.

The TI-99/4A was technologically a competitive computer, offering more memory and more advanced graphics capabilities than the Commodore VIC-20 and in some regards rivaling the Commodore 64, which was aimed at a higher point in the market. However, a number of elements of its design attracted criticism: All peripherals plugged directly into the right-hand side of the unit (unless the user purchased the expensive and heavy Peripheral Expansion Box), which caused the computer to not fit well on top of a desk if a user added many peripherals besides a tape drive and a printer. In addition, the 48-key keyboard layout didn't match that of a typewriter very closely, which made it unpopular for word processing.

However, the 99/4A's biggest drawback was its limited software library. TI closely controlled software production for the machine, which resulted in a software library of around 300 titles and few of the big-name hits available for other computers of its day. By comparison, the VIC-20, whose history paralleled the TI-99/4 series except its software development was completely open, had a library of more than 700 titles.

As a result, the TI-99/4A found itself selling for around the same price as the VIC-20, even though it was much more expensive to manufacture.

The TI-99/4A maintained a cult following for years after its death in the marketplace, in part because of its eccentricities, and a number of PC-based emulators for it exist. There is still some life, in 2004 a Universal Serial Bus (USB) card and Advanced Technology Attachment controller for IDE hard drives for the PEB were released, and there is still a yearly Chicago TI Fair where people get together for the machine. Third party devices such as expanded memory cards, improved floppy controllers, and hardware RAMdisks are very stable and popular additions to the machine, although there are no current known sources for these devices.

Technical specifications

  • CPU: TI TMS9900, 3.3 MHz, 16-bit
  • Memory: 16 KB video RAM (not expandable), 256 bytes CPU RAM (expandable to 32k + 256 bytes)
  • Video: TI TMS9918A VDP
    • Text resolution: 32×24 with a 40×24 resolution available (not directly in Basic)
    • Graphics resolution: 256×192
    • 15 colors (one color reserved for 'transparent')
    • 32 single-color sprites
  • Sound: TI TMS9919
    • 3 voices, 1 noise (white or periodic)

External links

fr:TI-99/4A it:Ti-99/4a sv:TI-99/4


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