TRS-80
Radio Shack's surprise success
The TRS-80 brought personal computing to Radio Shack stores across America, becoming an unexpected bestseller and establishing the home computer market. Tandy's own technical handbook shows a machine built to a price in every detail — including a character code that is only six bits stored, with the seventh worked out by a logic gate.
Radio Shack expected to sell a few thousand TRS-80s to hobbyists. They sold 10,000 in the first month and 200,000 in the first year. The “Trash-80” (as critics called it) proved that personal computers could be mass-market products, sold in shopping malls rather than electronics stores.
Fast facts
- Manufacturer: Tandy Corporation (Radio Shack).
- Released: August 1977.
- CPU: Zilog Z80 at 1.77 MHz.
- RAM: 4KB expandable to 16KB (Model I).
- Graphics: 128×48 “graphics” via block characters.
- Sound: None (Model I).
- Storage: Cassette, later floppy disk.
- Price: $599.95 (including monitor).
- Nickname: “Trash-80” (affectionately or not).
Everything is memory
Tandy’s technical handbook states the design principle plainly: “the TRS-80 microcomputer system is Memory Mapped”. The Level I memory map shows what that means in practice.
| Address | Contents |
|---|---|
0000–0FFF |
Level I ROMs |
3800–38FF |
Keyboard |
3C00–3FFF |
Video Display |
4000–41FF |
RAM used by BASIC |
4200– |
“Useable RAM starts here” |
The keyboard is not a port. It is 256 bytes of address space you read, and the screen is 1,024 bytes you write. The handbook draws the distinction itself: “In memory mapping, the CPU knows where the data is. In a port, the CPU does not know and couldn’t care where the data is located.”
A keyboard that doesn’t know its own letters
The 53 keys are “single-pole, single-throw normally open”, wired as a matrix, and the handbook is clear about what they do not do:
this Keyboard does not output ASCII. It is scanned, like calculator-type keyboards. Each key represents a switch across a matrix node. When closed, the switch will short out a horizontal line to a vertical line. ROM software will detect the node short and generate ASCII equivalents for that particular key.
There is no keyboard encoder chip. The machine reads a grid of shorted wires and works out which letter that was in software, which is a few pence of hardware replaced by a few bytes of ROM — the trade this whole machine is made of.
Six RAMs, seven bits
The best example is in the video section. A character code needs seven bits. The TRS-80 stores six.
There are seven RAMs. Six are used for ASCII storage, and the seventh is used as a graphic/alphanumeric definition bit. There are eight data lines. Notice the line labeled Bit 6. It is sourced by NOR gate Z30 at pin 13. If Bit 5 and Bit 7 are low, then Bit 6 will be high. Z30 is a sneaky way of squeezing a seventh ASCII bit out of six RAMs.
Bit 6 of every character on screen is not stored anywhere. It is computed, at display time, from the state of two other bits. The seventh RAM is spent not on the code but on a flag saying whether this cell is a letter or a graphic. Saving one memory chip per machine was worth designing the character set around.
The graphics that flag selects are equally frugal: the character rectangle is divided “into six smaller rectangles” — two across, three down — so a 64 × 16 text screen doubles and triples into the 128 × 48 the machine is remembered for. The “graphics generator” chip, as the handbook says, “does not generate anything”; it selects which of six blocks are lit.
The 1977 trinity
Three machines defined 1977:
- TRS-80: Cheapest, available everywhere.
- Apple II: More capable, more expensive.
- Commodore PET: Business-oriented all-in-one.
Radio Shack’s retail presence gave TRS-80 unmatched accessibility.
Distribution advantage
Radio Shack stores were everywhere:
- Over 3,000 US locations in 1977.
- Customers could see, touch, try computers.
- Staff trained to demonstrate and support.
- No mail-order uncertainty.
This accessibility drove early adoption.
Business applications
The TRS-80 found unexpected professional use:
- Word processing: Electric Pencil, Scripsit.
- Databases: Simple filing systems.
- Small business: Affordable automation.
- Before IBM PC: TRS-80 was the business standard.
Model evolution
The line expanded significantly:
- Model I (1977): Original, massive success.
- Model II (1979): Business-focused, 8-inch floppies.
- Model III (1980): Integrated design, FCC compliant.
- Model 4 (1983): Final Z80 model.
- Color Computer (1980): Different architecture, consumer focus.
Software library
Despite limitations, software thrived:
- Games: Scott Adams adventures, arcade ports.
- Productivity: Word processors, spreadsheets.
- Languages: BASIC, Pascal, C.
- CP/M compatibility: Access to business software library.
Limitations
The Model I had notable weaknesses:
- Graphics: Block characters only, no bitmap mode.
- Sound: None without add-on hardware.
- RF interference: Failed FCC certification, discontinued.
- Keyboard: Mushy feel, questionable reliability.
The Trash-80 legacy
Critics mocked, but sales spoke:
- First computer for many users.
- Proved mass-market viability.
- Established Z80 as standard processor.
- Paved way for later home computers.
Legacy
The TRS-80 demonstrated that personal computers could be retail products. Radio Shack’s distribution network made computing accessible to people who’d never enter a computer store. The machine’s success surprised everyone—including Tandy—and proved the market existed.