TRS-80 "The Trash 80"

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The development of Level II BASIC gave the TRS-80 its first real chance to make good on all the promises Radio Shack had been making to the public. This vastly improved language was able to be used for business and home applications. Most of the computers were Level II, but those who had bought the original Level I machines could have their machines upgraded for $120. Video games started to appear using the graphics characters, and the stock of available cassettes in all categories increased.

By 1978, semiconductor technology had advanced to the point where 16K memory chips had come down in price, and it was possible to offer an upgrade to the TRS-80. This removed the original memory chips and replaced them with new 16K RAM. Many users had this upgrade done when they had Level II BASIC installed. It was quite expensive to have Radio Shack make the upgrade and several small companies came out with do-it-yourself upgrade kits. These sold for about $140, about half the price of the Radio Shack installation, and many users bought the kits, opened their case for the first time, and installed their own RAM.

By 1976, Radio Shack was selling the Level II, 16K TRS-80 for $849. This included a numeric keypad, a 6K RAM, and the 12K ROM containing the Level II BASIC. The enhanced BASIC included such features as string handling, multi-dimensional arrays, and multi-letter variable names (Level I only had two letter variable names.) You could name cassette files, and full editing was supported. Since BASIC came in a 12K ROM, the entire 16K was available for user programs. In 1979, this was a huge amount of available memory. The TRS-80 was equal to a 28K system, and it was hard to imagine that anyone would ever need more than THAT!

The Expansion Unit and Disk Drives

The tape cassette had been considered a great improvement over paper tape for storing programs and data for personal computers, but soon it became apparent that it was not a satisfactory solution as a data storage medium. The serial transfer rate was too slow, and the method of converting digital data into audio frequencies for recording was subject to error. The inexpensive tape cassettes, designed for audio, were prone to failure and tape stretch that rendered them unreadable.

The S-100 computers were adopting floppy disk storage. Eight inch diskettes and the recently-invented 5 1/2-inch mini-floppies were coming into use. Apple Computer was selling their 5 1/2-inch disk drives for the Apple II as fast as they could make them.

For Radio Shack to develop a disk drive system, a major system unit had to be designed and manufactured. This was the Expansion Interface, a unit that could be mounted under the Video Display. It contained the RAM expansion_up to 32K could be installed. It also provided a real-time clock, serial interface, printer port, and, most importantly, the disk controller. The controller could support up to four floppy disk drives. The TRS-80 Mini Disk System used a diskette with a capacity of 83K formatted. The disk with the operating system on it had 53K remaining. There were 35 tracks per disk with 2500 bytes per track, divided into ten sectors. Data could be moved in or out of the disk at a rate of 12.5 Kbytes per second, a great improvement over tape cassettes. The operating system, TRSDOS, had to be on the first drive for the system to operate.

TRSDOS - the Radio Shack Operating System

The dominant disk operating system for 8080 and Z-80 CPU's was CP/M. It was used by almost all of the manufacturers of this type of computer, and therefore a considerable library of software had developed for CP/M systems. It was considered the standard for this type of computer.

Radio Shack chose to ignore the standard and develop their own operating system. It was more important for them to completely control their system than make a considerable stock of programs available to Radio Shack owners. In fact, Radio Shack made it impossible to run the standard CP/M on a TRS-80 computer by preempting certain entry points on the memory map. Since these points were in the system ROM, it was impossible to change them.

As a result, TRSDOS developed on its own. While it was possible to port many CP/M programs to TRSDOS, it was a task for an expert and required a rework of the program. In time, TRSDOS came to have as a big a library of software as CP/M and as much support among software publishers. There were several versions of TRSDOS, including some produced by third-party software developers, which were compatible, but included features not found in Radio Shack's versions. Eventually there were versions of CP/M that could be run on the TRS-80. These were not compatible with standard CP/M, and programs had to be specially configured to run on them.

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