Texas Instruments TI-99/4A

From Free net encyclopedia

(Redirected from TI99/4A)

Image:TI-994A.jpg Image:994.jpg The Texas Instruments TI-99/4A was an early home computer, released in June 1981, originally at a price of $525 USD. The TI-99/4A was an enhanced version of the less-successful - and quite rare - TI-99/4 model released in late 1979 at a price of $1,150. The TI-99/4A added an additional graphics mode, "lowercase" comprised of small capitals, and a full travel keyboard. Its predecessor, the TI-99/4, featured a calculator-style chiclet keyboard and lacked any provision for lowercase text.

Contents

[edit]

Features

The TI-99/4A's CPU, motherboard, and cartridge ("Solid State Software Module") slot were built into a single unit with the keyboard. The power supply regulator board (linear in early systems, switching in later systems) was housed below and in front of the cartridge slot under the sloped area to the right of the keyboard.

Available peripherals included a 5¼" floppy disk drive, an "RS-232" card comprising two serial ports and one parallel port, a "P-Code" card for PASCAL support, a thermal printer, an acoustic coupler, and a 32KB memory expansion card.

In the early 1980s, TI was known as a pioneer in speech synthesis, and a plug-in speech module was also available for the TI-99/4 and 4A (and was very popular). Speech synthesizers were offered free with the purchase of a number of cartridges and were used by many TI-written video games (notable titles offered with speech during this promotion were Alpiner and Parsec). This synthesizer used a variant of linear predictive coding and had a vocabulary built-in. The original intent was to release small cartridges that increased the device's built in vocabulary, but the success of the text to speech system in the Terminal Emulator II cartridge cancelled that plan. (Most speech synthesizers still shipped with a door that opens on top, although very few have the connector inside. There are no known speech modules in existence for those few units with the connector.) In many games, although mostly those produced by TI, the speech synthesizer had relatively realistic voices. As an example, Alpiner's speech included male and female voices and could be quite sarcastic when the player made a bad move.

Many of the peripherals came in two forms, a card (encased in cast aluminum) which plugged into the bulky "Peripheral Expansion System" (usually known among TI owners as the Peripheral Expansion Box or "PEB"), an eight slot chassis made of two plies of thick-gauge steel and containing its own linear power supply and 5¼" floppy bay. Each card also had its own "access light", an LED which would blink or flicker when the card was being used by software. Another unique feature of the Peripheral Expansion System was that the section of the power supply which powered the card slots was unregulated - each card had on-board regulators for its own requirements, thus reducing power consumption on a partially-loaded PEB and allowing for future expansion cards which might have unusual voltage requirements. The alternate expansion method was a 'sidecar' version which plugged into the side of the console, originating before the conception of the PEB. These 'sidecar' expansion units could be connected together in a continuing chain, but could rapidly occupy an entire desk and cause crashes and lockups due to the large numbers of connectors on the system's buses.

Early models (the TI-99/4, identified by its keyboard and "(C)1979 TEXAS INSTRUMENTS" on the title page) included a built-in equation calculator, but in the 99/4A ("(C)1981 TEXAS INSTRUMENTS") 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 "(C)1983 TEXAS INSTRUMENTS V2.2" on the title page, also removed the ability for the system to execute ROM-based cartridges, locking out third-party manufacturers such as Atarisoft.

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". Aftermarket adapters were available which allowed the use of two Atari-compatible joysticks. Composite video and audio were output through another port on NTSC-based machines, and combined by an external RF Modulator for use with a television. PAL-based machines output a more complex component-like signal which is also modulated externally.

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, built into the console, 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 - a TMS9901), although in some devices this hardware was only used to map memory and hardware into the CPU memory space for direct access. A popular user modification in later years involved "piggybacking" static RAM chips onto the console's 16 bit ROM chips, allowing a 32K RAM expansion without the wait state and approximately a 30% speed increase for many applications. Most hardware was based on the system clock, not the program execution speed, and the hardware access still ran through the 8-bit bus with the wait states intact, so this particular modification was not known to impact any peripherals.

The unusual architecture of the TI-99 machines is documented to be due to the failure of the 9985, an 8-bit processor which was being created especially for the machine. When it was abandoned, the 16-bit 9900 was selected to replace it, and a great deal of 'glue logic' had to be added to fit the processor into the existing design, while no changes were made to take advantage of the 9900's strengths.

All TI-99 models, from the earliest TI-99/4 to the unreleased TI-99/2 and TI-99/8, included "plug and play" support for all peripherals. Device drivers (called "Device Service Routines", or DSRs) were built into ROMs in the hardware; when a new card was inserted, it was immediately available for any software which needed or wanted to use it. All device access utilized a generic file-based I/O mechanism, allowing new devices to be added without updating software to use it. However, each card ran at a hard-wired address on the CRU bus, and so multiple cards could not be supported without modification. The only official card known to be modifiable was the RS232 card, which supported two different base addresses. Most hobbyist created cards released after TI's exit from the hardware business included switches to set the base address.

[edit]

VDP RAM and GPL

TI slipped up with the marketing of the TI-99 series because they were only advertised as being 16K machines. However, Texas Instruments engineers afforded the TI99/4A's graphics coprocessor, a TMS9918A, a separate store of 16K of proprietary VDP (Visual Display Processor) RAM.

VDP RAM was also used for temporarily storing users' BASIC programs. 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 power-up, 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.

The advanced Stackless RISC architecture of the TMS9900 series meant that the "Workspace" of registers currently in use were stored in main memory. Because very fast RAM was also very expensive in the early 80's, TI only gave the machines 256 bytes of fast "scratch pad" RAM where register workspaces could be stored.

The video processor as its name suggests is a processor in its own right. It alleviates the main CPU of worrying about graphics processing. The CPU just passes instructions to the VDP via an 8 bit link. The advantage of this means that like all TI-99 hardware, it is very easy to upgrade the graphics capabilities of the machine. The same VDP was used in the MSX machines and an upgrade chip was produced called a V9938 which allows the graphics capabilities of the TI-99/4A to be greatly upgraded to allow 512 * 424 pixels and 16 colours, or 256 * 424 and 256 colours. Using an external palette chip means up to 19200 colours can be used.

[edit]

Graphics Read-Only Memory

Graphics Read-Only Memory was another set of memory accessed a single byte at a time through a dedicated memory port, and were auto-incrementing read-only devices. (There is also 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 could not be loaded 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.

[edit]

History

Image:TI994A-CosbyAd.jpgInitially, 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 still losing shelf space. 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 beige 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, and there was (at the time) no option for an 80-column display. The keyboard and display limitations made it unpopular for word processing.

However, the 99/4A's biggest drawback was its limited software library. TI closely controlled both hardware and 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. No official technical documentation was released until the "Editor/Assembler" assembly language development suite was released in 1981; no system schematics were ever released to the public until after TI had discontinued the computer. By comparison, the VIC-20, whose history paralleled the TI-99/4 series except its hardware and software development was completely open (Commodore even included schematics in the owner's manual, allowing anyone to build hardware for the machine), 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. It is worth noting that Texas Instruments and Commodore each owned their own IC fabrication facilities, allowing creation of custom ICs to combine functions of smaller ICs. Commodore used this aggressively to reduce the cost of their consoles, while TI continued to use off the shelf components and making only relatively small revisions to their motherboards. Commodore also made other cost-cutting changes including using aluminized cardboard to build RF shields for some of their systems. Texas Instruments never followed suit, electing instead to continue to use the highest quality components and materials with the unfulfilled hope that the marketplace would recognize it.

Image:Ti start screen.pngThe TI-99/4A maintained a cult following for years after its death in the marketplace, in part because of its eccentricities, and in part because TI had actively supported a network of user groups during the production of the machine. There is still some life: several of these user groups still exist with histories of first supporting a state-of-the-art machine, then die-hards discussing their obsolete machine, and now enjoying today's "retro computing" resurgence. In 2004 a Universal Serial Bus (USB) card and Advanced Technology Attachment controller for IDE hard disks for the PEB were released, and there is still an annual Chicago TI Fair where people congregate to celebrate the historic TI-99 family of computers. 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. A number of PC-based emulators for the TI exist, though they cannot emulate the experience of the blinking lights on the PEB, the warmth and buzz of the console's power supply, or the satisfying clunk of the Shugart-built 5.25" full-height floppy drive.

There was also a portable sibling to the TI-99/4A. Dubbed the CC40 (Compact Computer 40), it was a battery-powered compact with an LCD display and a version of TI BASIC. It also pioneered TI's HexBUS interface, a high speed serial expansion port similar in concept to USB. The HexBUS peripherals were compatible with all members of the TI-99 family; CC40 cartridges were not.

In 1987, the "Turbo XT" was introduced by Triton. Though rare, it allowed a TI-99/4A and an XT to share the same desktop space, though without sharing such things as memory or disk drives. Pictures of this unusual peripheral are available here.

[edit]

Successors and clones

At the time they left the home computer market, TI had been actively developing two successors to the TI-99/4A. Neither actually entered production, though several prototypes of each are in the hands of TI-99/4A collectors. Both machines were to feature the TMS9995 CPU and would therefore have been substantially faster than the original TI-99/4A, and both were to use TI's "HexBUS" serial interface (which was available as an option on the TI-99/4A and could be viewed as a prototype for today's ubiquitous USB - the link for the TI-99/8 includes some images of HexBUS peripherals).

  • TI-99/2, a low-cost black-and-white only machine with no sound, absurdly reminiscent of the ZX81, TS1000 and TRS-80 MC-10 given its 16 bit processor.
  • TI-99/8, a premium successor for the TI-99/4A, with a large keyboard, 64k of RAM expandable to 15 megabytes, built-in speech synthesis, and the full 16 bit data bus available on the expansion port.

The Myarc Geneve, an enhanced TI-99/4A clone which was built as a card to fit into the TI Peripheral Expansion System and used an IBM PC/XT detached keyboard. Introduced after TI left the home computer market. In many ways, similar to the TI-99/8. Included a faster processor, 80 column text mode, 16 bit wide RAM, MDOS, and was fully compatible with all TI software and slot-mounted hardware (an adapter was available to allow the sidecar-only Speech Synthesizer to be installed inside the PEB). A toggle switch was mounted to the side of the PEB to allow insertion of wait states to bring the computer down to the same speed as the original console, allowing compatibility for games and other timing-critical software.

[edit]

Technical specifications

  • CPU: TI TMS9900, 3.3 MHz, 16-bit
  • Memory: 16 KB video RAM (expandable to 192K with the use of YAMAHA V9938), 16K bytes CPU RAM (expandable to 80k using a 32K memory expansion card and also a 32K Superspace cartridge in the cartridge slot). Additionally the machine has 256 bytes of fast "scratchpad RAM" intended for the the TMS9900 to maintain register workspaces.
  • Video: TI TMS9918A VDP (TMS9918 in the earlier 99/4, TMS9919/9919A in PAL versions. Distinct in being the only chip on the TI motherboard which had a heat sink.)
    • 32 single-color sprites in defined layers allowing higher-numbered sprites to transparently flow over lower-numbered sprites. Sprites were available at 8x8 pixels or 16x16 pixels, with a 'magnify' bit that doubled all sprites' size but not their resolution. A single bit was available in hardware for coincidence (collision detection), and the console supported automatic movement via an interrupt routine in the ROM.
    • 16 fixed colors (15 visible, one color reserved for 'transparent' which merely showed the background color. Transparent was intended for the 9918's genlock functionality, not available in the TI-99 for a number of technical reasons. Some users have reportedly succeeded in making this work via hardware hacks, but there is no documentation to prove it.)
    • Text mode: 40×24 characters (256 6×8 user-defineable characters, no sprites, foreground and background color only, not accessible in BASIC)
    • Graphics mode: 32×24 characters (256 8×8 user-definable characters, full 15 color palette + transparent (available in groups of 8 through the character table) and 32 sprites (The only mode available in BASIC. Extended BASIC is required for sprites, and can only access 28 of them.)
    • Bitmap mode: 256×192 pixels (no more than two colors in an eight pixel row, full 15 color palette + transparent, all 32 sprites available but interrupt-based motion through the ROM routine is not due to the memory layout, not available to BASIC or the original 9918). Bitmap mode could be arranged in such a way as to use less memory but still provide improved color or improved pattern layout, leading to the popularity of so-called "half-bitmap" modes. In fact these modes were not undocumented modes of the VDP (which fully documented this masking) but simply clever layout of Bitmap mode.
    • Multicolor mode: 64×48 pixels (each pixel may be any color, all 32 sprites are available)
    • All of the above comprise 36 "layers" starting with the video overlay input, then the background color, then two graphics mode layers, then a layer for each of the 32 sprites. A higher layer would obscure a lower layer in hardware, unless that higher layer was transparent.
  • Sound: TI TMS9919
    • 3 voices, 1 noise (white or periodic)
    • Voices generate square waves from 110Hz to approximately 115kHz
    • Console ROM includes interrupt-driven music list playback
[edit]

External links

es:Texas Instruments TI-99/4A fr:TI-99/4A it:Texas Instruments TI-99/4A nl:Texas Instruments TI-99/4A ja:TI-99/4A sv:TI-99/4

Retrieved from "http://www.netipedia.com/index.php/Texas_Instruments_TI-99/4A"