Retro Computer Memories: Sinclair ZX81


ZX81 is home computer produced by Sinclair Research.

It was launched in 1981 and was designed as ultra low cost entry level home computer.

While the Apple ][ cost about $1300 at that time, ZX81 was just $100 and could be bought also in form of kit so you can solder it yourself.

Here is picture of ZX81 kit content:


ZX81 was designed to be small, simple and cheap. It uses TV tuner so you connect it to your TV and in 8KB of ROM it holds BASIC interpreter which have 1KB of RAM memory!

Its distinctive design brought its designer, Rick Dickinson, a Design Council award.

The unexpanded ZX81’s tiny memory presented a major challenge to programmers.

Simply displaying a full screen takes up to 768 bytes, the system variables take up another 125 bytes and the program, input buffer and stacks need more memory on top of that.

Nonetheless, ingenious programmers were able to achieve a surprising amount with just 1 kB. One notable example was 1K ZX Chess by David Home, which actually managed to squeeze most of the rules of chess into only 672 bytes!

The ZX81 conserved its memory to a certain extent by representing entire BASIC commands as one-byte tokens, stored as individual “characters” in the upper reaches of the machine’s unique (non-ASCII) character set.

Let’s see what is inside:


nice red soldermask laquer 🙂



surprise: the top side have no protection mask – remember the PCB have to be cheap and soldermask cost money, as the components are soldered only on the bottom side there is just bottom solder mask.

As you can see from the picture above the ZX81 have just few ICs and all glue logic is in ULA Ferranti chip in the left of Z80. The two small ICs in the right side are the RAM memory and the big chip with the window is 8KB of UV-light Erasable ROM where the BASIC interpreter is.

I like ZX81 as it’s master piece for it’s time – showing how one home computer can be produced on budged and to be profitable.

RETRO COMPUTER MEMORIES: Acorn BBC Micro model B and Acorn BBC electron



BBC Micro and Acorn Electron – I like these two computers from my retro vintage computers collection as there is interesting story around them.

Everyone has heard for ARM processors, the fastest growing architecture, they are everywhere: in phones, tablets, handheld devices, GPS navigators, media players, set top boxes and in most of Android devices. Nova days we have even Cortex-M0 ARM processors for $0.30 which penetrates the holy territory of the 8-bit processors.

If you now ask yourself What ARM processors do have with the above retro computers, the answer is simple, because of these computers ARM processors were designed in first place.

How this happend:

Back in 1970s Chris Curry is Cambridge University professor with interest in the microprocessors, he launched together with his friend Herman Hauser two companies: first with research in the microcprocessors named Cambridge Processor Unit Ltd (CPU) and another named Acorn Microcomputer with aim to make mini computer based on the CPU processors.

According to Curry the Acorn name was choosen so it appear before Apple in the telephone directory, which show which company they were influenced at that time.

Some engineers joined to work in Acorn Computers and one of them Sophie Wilson designed few computer systems there. These were primary laboratory oriented kits with LCDs, keyboards, processors which allow laboratory exercises to be completed with students – looking same as Olimex’s development boards now 🙂

In 1979 Curry influenced by ZX81 computer launch decided also to move to home computer design and this is how Acorn Atom was build. Based on 6502 as any other home computer at that time.
After the Atom was released Curry and the team started thinking for more powerful processor, Hauser suggested compromise 6502 with enhanced peripheral co-processor which to unload 6502 from the part of the tasks so they designed the next version Proton.

BBC in 1981 selected Proton for their educational program and the computer was renamed to BBC micro. Acorn sold more than 1.5 million BBC microcomputers and this helped the company to make enough profit to start working on their own processor. With the launch of IBM PC in 1981 and the Apple Lisa Acorn saw that they need more powerful processor, first attempt was to contact Intel to licensee 268 and improve it, but Intel refused so they as University professors decided that its not so hard to make their own 🙂

At that time everyone has put efforts to make processors which machine language is close to higher level programming languages i.e. so named CISC (complex instruction set computing) processors. The idea was that programming language would be easy to translate and execute faster if the machine language instructions are close to it. This lead to very complex implementation of the processor, long signal paths, which didn’t allow high frequencies (most processors at that time run at 1-2Mhz) and it was taking about 100 man years to develop something like 6502. Acorn had no such resources so they went to different route – to make RISC (reduced instruction set computing) processor which was easier to develop with smaller budged and team. Sophie Wilson set about developing the instruction set, writing a simulation of the processor in BBC Basic that ran on a BBC Micro with a 6502 second processor. Another benefit from the RISC architecture came the smaller paths to implement and the higher frequencies and lower power supply the RISC processor required compared to CISC processor. First RISC processors were running at 4Mhz and the power consumption was less than CISC.  Smaller size also means less cost to manufacture.

It convinced the Acorn engineers that they were on the right track. Before they could go any further, however, they would need more resources. It was time for Wilson to approach Hauser and explain what was afoot. Once the go-ahead had been given, a small team was put together to implement Wilson’s model in hardware.
The official Acorn RISC Machine (ARM) project started in October 1983, with Acorn spending GBP 5 million on it by 1987. VLSI Technology, Inc were chosen as silicon partner, since they already supplied Acorn with ROMs and some custom chips. VLSI produced the first ARM silicon on 26 April 1985 — it worked first time and came to be known as ARM1. Its first practical application was as a second processor to the BBC Micro, where it was used to develop the simulation software to finish work on the support chips (VIDC, IOC, MEMC) and to speed up the operation of the CAD software used in developing ARM2. The ARM evaluation system was promoted as a means for developers to try the system for themselves. This system was used with a BBC Micro and a PC compatible version was also planned. Advertising was aimed at those with technical expertise, rather than consumers and the education market, with a number of technical specifications listed in the main text of the adverts. Wilson subsequently coded BBC Basic in ARM assembly language, and the in-depth knowledge obtained from designing the instruction set allowed the code to be very dense, making ARM BBC Basic an extremely good test for any ARM emulator.

In 1982 Acorn released budged version with lower cost of BBC micro called BBC Electron, it was with co-processor IC ULA which had inside all logic and peripherals of BBC implemented.

Unfortunately Electron come to the market late as meantime Commodore 64 wiped all competitors in 8-bit scene, Atari and many others went in bancrupcy in 1985, Apple was about to sink also, same happen with Acorn. Olivetti take it over, but the new processor activity was kept in such secret that Olivetti even didn’t know that they buy it.

Good move from Acorn management was to present the new ARM processors to Apple and they decided to use it in their handheld Newton device as were impressed by the price, power and performance of this new chip, so Apple and Acorn began to collaborate of developing the ARM so they formed new company ARM Ltd in November 1990 where Acorn Group and Apple had each x 43% and the VLSI was both investor and first ARM licensee.

Now back to our retro computers:


Here is how BBC Micro looks on bot:


and back:


Note the many peripheral chips, this was one of most advanced computers at that time, it had, LAN, Graphics co-processor, Audio co-processor, needles to say some of most interesting games were on BBC micro 😉


The BBC Electron






as you can see all glue logic and peripheral were fit inside custom chip :


which allow BBC Electron to be sold under 200 GBP, sure this didn’t help to complete the mighty Commodore 64 which wiped all 8-bit market, but the research to make newer faster better processor for the Acorn computers lead to the design of one of most successful architecture at our time ARM

Retro Computer Memories: TRS-80 Model 100


TRS-80 Model 100 is one of my favorite computers from my vintage computers collection.


Although build in 1983 it’s in so cute and compact form that you immediately fall in love when you look at it.

This is the mother of all Notebooks and Laptops 🙂 with of only 1.4 KG with the batteries (4xAA 1.5V type) and size of only 300x215x50mm.


It was designed by Kyocera under the name Kyotronic 85, this model was so well designed for it’s time that Tandy, Olivetti and NEC decided to license the design instead to make their own.

More than 6 Million units were sold just by Tandy in US and Canada.

The processor inside TRS-80 Model 100 is 80C85 running at 2.4 Mhz. The RAM memory is humble 8K expandable up to 24K by slots inside and at the back of the computer.

The LCD display is 240×64 pixels 8 lines x 40 characters, there is build-in 300 bps modem, parallel printer port, RS232, bar-code reader, cassete input, RTC, external CRT connector.




The firmware is inside of 32KB ROM and contain Microsoft BASIC interpreter with good support for all hardware.


here is our Hello World program written in Basic:


What is remarkable with this firmware is that the Model 100 firmware was the last Microsoft product that Bill Gates developed personally, along with the Japanese hardware developers.
According to Gates, “part of my nostalgia about this machine is this was the last machine where I wrote a very high percentage of the code in the product”.

When introduced, the portability and simplicity of the Model 100 made it attractive to journalists,who could type about 11 pages of text and then transmit it using the built-in modem for electronic editing and production. The batteries allow up to 20 hours of work without connection to external power supply.

The Model 100 was also used in industrial applications as a programming terminal for configuration of control systems and instruments.

This is how main board looks inside:


and this is the LCD and keyboard board which is connected to the main board with ribbon cable:


This 30 years old machine is so well build, that works fine even today, all it need is 4 x AA batteries!

Owing a piece of computer history: PRAVETZ 8D – ORIC ATMOS retro vintage computers


Oric was the name used by the Tangerine Computer Systems in UK for a series of home computer.

As you can see from the company name they were inspired by the very successful Apple Computer 🙂

Tangerine first released Oric-1 in 1983 and after they sold 160 000 pcs in UK and 50 000 in France they made Oric-Atmos in 1984 which was much improved version with better keyboard and more memory (48KB of RAM).

Oric-Atmos was a bit more sophisticated than Apple ][ as it had GI8912 programmable sound generator.

Oric-Atmos inspired lot of clones, one of them was Pravetz-8D, produced in Bulgaria from 1985 to 1991.

Pravetz computers were Bulgarian personal computers brand. They were designed by Ivan Marangozov in ITKR (Institute for Technical Cybernetics and Robotics) which was part of Bulgarian Academy of Science.

First computer designed by ITKR was IMKO2 in 1980 an Apple ][ clone with Mostek 6502 at 1Mhz processor, the processor was also cloned and produced in Bulgaria under the name CM630.

These computers were great success and good opportunity to export them to all around Ex-communist economy zone. Statistics says that in the period 1980-1990 around 40% of all computers sold in ex-communist countries in Eastern Europe were made in Bulgaria.

Pravetz computers had several modifications:

  • Pravetz-8M – dual processor computer with both 6502 and Z80A which could be selected by motherboard jumper so you could run Apple DOS or CP-M on same computer
  • Pravetz-8E – upgrade of original Pravetz-82 with more RAM memory (E=extended memory)
  • Pravetz-8A – based on CM630 with up to 1MB RAM divided in 64KB memory banks
  • Pravetz 8С – cut down Pravetz-8А version, 128Kb RAM integrated, but not expandable, less number of slots, integrated Centronics, FDD controllers, Joystick and sometimes RS-232.
  • Pravetz 8S – 128Kb RAM integrated, exapandable to 1080KB, less number of slots, integrated Centronics, FDD controllers, Joystick and sometimes RS-232. Could control 3.5″ floppy disk drive and a 5MB HDD.

as you can see ITKR did not blindly copy the Apple computers, but also improved the hardware, there were also team of software developers which had released lot of system software including DOS, compilers etc which support Cyrillic alphabet, also talking program which add voice to Pravetz and many other originally developed software.

The major problem with Pravetz 82 and etc to become real home computer was it price, the computer with the special monitor cost about 1600-1800 Bulgarian Leva which was about 6 average salaries at that time and not many people could afford to have at home.

This was changed with Pravetz-8D which was copy of Oric-Atmos and could connect to any normal TV and was with sale price only 420 Leva. This is why many students at that time had Pravetz-8D as their first computer.

The production of Pravetz-8D started in 1985 (!) i.e. it was cloned one year after Tangerine released it to the market.

Initially Pravetz-8D had no floppy interface but only cassette interface, but later Borislav Zahariev wrote DOS-8D operating system and there were lot of amateur hacks how to attach floppy disk from Apple ][ / Pravetz-82 to Pravetz-8D. Borislav also was famous at that time with his text to speech program for Pravetz which was performing pretty well for that time.  Checking where Borislav is now found it in Linkedin  seems he is now in Canada and working for local company and enjoying playing guitar in his free time .

I’m still keeping my Pravetz-8D and recently I bought on e-bay the original Oric-Atmos, so there they are:


at left side is Oric-Atmos at right side is Pravetz-8D

As you can see Pravetz-8D is bit bigger but have integrated power supply inside, while Oric need external power adapter.

Let’s see what is under the box in Pravetz-8D:


In the upper left corner is the RF TV modulator, then the composite video output, cassete interface, parallel port/printer interface and phone modem interface. The power supply is inside the box with large aluminum heatsink.


here is side view of the connectors


on the back we can see the year of manufacture 1986 and serial number 03163.


the keyboard is on separate board with ribbon cable connected to main board.


Oric-Atmos is with very compact size as power supply is with external adapter. This way UK company makes it easy to export to European countries which use different style of power supply plugs.


to open it I had to remove the warranty label 🙂


this is the main board assembly


the keyboard is on separate board again connected with ribbon cable:



side view show TV connector, then composite video, cassete interface modem connector (hiden with red tape for some reason) and the parallel printer port and power supply adapter.

Both computers although at 27 years age are still in working condition and in good order 🙂 I have still some cassetes with retro games which run on both

With the years I collected a lot of vintage computers from 1980s and next week will show you something else from my collection.