Amiga Format
This article was originally written by Simon N Goodwin for Amiga Format magazine, and is used by permission of the joint copyright holders. It should not be copied any further without written permission. The free emulators mentioned were included on the accompanying CD with the magazine, and are generally available - often in updated form - from this site. The text is as originally submitted to the magazine - which means there are some differences, mostly extra text which was cut to fit into the pages available - with the addition of hyperlinks to new or updated information which has since become available. This approach has been taken to preserve the structure of the original series, while making new information readily available. Simon welcomes comments from readers, care of Tomas Amsrud, who has generously prepared the material for publication on the Internet.
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Texas Instruments


Texas 99/4A

The TI 99/4A home computer was developed by Texas Instruments, the enormous chip firm that invented TTL - the lego bricks of 1970s electronic design. It's rare for a chip maker to develop and market a complete system, with software and peripherals as well as raw silicon. Most chip makers were content to leave the cutthroat, fashion-conscious marketting battles to third party firms.

TI made almost aall the parts in the TI/99/4A, including the 16 bit processor and the graphics chip. The graphics chip was a major hit on the micro scene, favourite choice of manufacturers who lacked the resources to follow Commodore, Atari or Sinclair and roll their own silicon.

The TI 9900 processor is bizarre - a genuine 16 bit minicomputer, easy to program and elegant by any standards, but slower than most eight bit chips of its era. The bottleneck in the 9900 processor is its lack of on-chip registers. All temporary results must be read and written to memory. This squanders the potential advantage of the 16 bit processor, as the time to process each instruction was much longer than for eight bit micros that kept temporary results on-chip. It also limited the scope for speed improvements, at a time when processor internals were out-stripping memory performance.

Texas VDP

The Texas VDP (Video Display Processor) offers 16 colours at a resolution of 256 by 192 pixels, similar to the ZX Spectrum. Sprites can move across the display without affectibg the background graphics, which . Unfortunately the display memory is not directly accessed by the processor - usually a Z80, although Texas used their own bizarre 16 bit chip - so screen changes much be poked, byte by byte, through an IO port, mming whole -screen updates relatively slow. This bottleneck turns out to be quite convenient for emulators.

Some tricks are possible by redefining the character set dynamically. As on eight-bit Ataris and Commodore machines, this instantanty changes every instance of the corresponding character on the screen. It's great for simple effects like waves on the sea, but complicates emulation because one change in VDP memory might require hundreds or thousands on the Amiga screen.

The TI micro was well made, in a neat black and silver case with a good keyboard and considerable plug-in expansion potential, but UK marketing was poor. In a cock-up reminiscent of the Amiga Video Toaster saga, Texas dithered for years before producing a model compatible with European TV standards. The first systems were imported from the states with a dual standard NTSC/PAL television, more than doubling the price.

CPU emulators

MFA-Simulator is an Amiga emulator for an educational computer system based on Intel's 8085 microprocessor. It runs on any computer with at least Workbench 2, but a fast 68030 is recommended for acceptable speed.

MFA training remains a mandatory part of some German engineering courses, so the simulator makes sense there, but it's rather an obscure choice elsewhere, especially as most of the documentation is still only available in German. MFA-Simulator is useable and includes a reasonable BASIC interpreter.

There's only one TI emulator for the Amiga:
TI 99-4A - http://www.geocities.com/Athens/7374/ti.html

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