, meaning Galaxy
) was originally a build-it-yourself computer
designed by Voja Antonić
. It was featured in the special edition Računari u vašoj kući
(Computers in your home
, written by Dejan Ristanović
) of a popular eponymous science magazine, published late December 1983 in Belgrade
. Kits were available but not required as it could be built entirely out of standard off-the-shelf parts. It was later also available in complete form.
In the early eighties, various laws in Yugoslavia
prevented importing computers into the country. At the same time, even the cheapest computers available in the west
were nearing average monthly salaries. This meant that regardless of demand for home computers, only a relative minority of people owned one - mostly a ZX Spectrum
or a Commodore 64
According to his own words, some time in 1983, Voja Antonić, while vacationing in Hotel "Teuta" in Risan, was reading the application handbook for the RCA CDP1802 CPU and stumbled upon CPU-assisted video generation. Since the CDP1802 was very primitive, he decided that a Zilog Z80 processor could perform the task as well.
Before he returned home to Belgrade, he already had the conceptual diagrams of a computer that used software to generate a video picture. Although using software as opposed to hardware would significantly reduce his design's performance, it also simplified the hardware and reduced its cost.
His next step was to find a magazine to publish the diagrams in. The obvious choice was SAM Magazine published in Zagreb, but due to prior bad experiences he decided to publish elsewhere.
The popular science magazine Galaksija appeared incompatible but he heard that they were working on a special issue dedicated to computers. He proposed publishing entire do-it-yourself diagrams, instructions, etc. to the author of the issue, Dejan Ristanović. Everything made its way into the special issue called Računari u vašoj kući It was released late December 1983, although it was dated January 1984.
They tried to guess the number of Galaksijas that would be built by readers. Their estimates ranged from a hundred to a thousand (a number that sounded so optimistic it provoked laughter). The actual number built by known "do-it-yourselfers" - was around 8000! This number may in reality be greater if people who did not purchase any kits (including PCB and ROMs) are accounted for.
Components were provided by various manufacturers and suppliers:
Later, Institute for school books and teaching aids together with Elektronika Inženjering started mass commercial production of Galaksija computers, mainly to be delivered to schools.
Galaksija was almost not comparable by any measure of features to the commercially available computers at the time. However, that was not important. The computer sparked the minds of many people. Many enthusiasts
have learned how computers actually work by looking at Galaksija's schematic diagrams and Voja's
great descriptions. It was a great learning tool. Making a computer yourself boosts confidence and brings on the challenges of making the best out of it.
The Galaksija computer's popularity was significant enough that it became commercially available. Many educational institutions were given some. Although many of them were not ready for the experience, many others used it as a great tool to teach computer science (computer architecture and programming) even in elementary schools (in 1984!).
- CPU: ZiLOG Z80A 3.072 MHz
- ROM "A" or "1" - 4 KB (2732 EPROM) contains bootstrap, core control and Galaksija BASIC interpreter code
- ROM "B" or "2" - 4 KB (optional, also 2732 EPROM) - additional Galaksija BASIC commands, assembler, Machine code monitor, etc.
- Character ROM - 2 KB (2716 EPROM) contains character definitions
- RAM: 2 to 6 KB of 6116 static RAM in base model, expandable to 54 KB
- Text mode only: 32 x 16 characters, monochrome
- Pseudographics: 2x3 dot matrix combinations in graphic character subset - 64x48 dots total. More was done using tricks - see video generation.
- Sound: None according to specifications, but tape interface was occasionally used as audio output port - like the "EAR" port on ZX Spectrum can be used both as audio and cassette port. See cassette port for details.
- Storage media: cassette tape, recording at 280 bit/s rate
- I/O ports: 44-pin Edge connector with Z80 Bus, tape (DIN connector), monochrome video out (PAL timings, DIN connector), and UHF TV out (RCA connector)
Galaksija BASIC is a BASIC interpreter originally paritally based on code taken from TRS 80 Level 1 BASIC, which creator believed to have been a Microsoft BASIC. However, after extensive modifications to include video generation code (as the CPU was a major participant to reduce the cost of hardware) and improve the language, what remained from the original is said to be mainly flow-control and floating point code. It was fully contained in 4 KB ROM "A" or "1". Additional ROM "B" or "2" provided more Galaksija BASIC commands, assembler, monitor, etc.
The chip labeled as "A" by the creator of Galaksija, Voja Antonić was commonly referred to as "ROM 1" or just "ROM". ROM "A" contained bootstrap code of Galaksija, its control code (rudimentary Operating System), video generation code (as Galaksija did not have advanced video subsystem its Z80 CPU was responsible even for generating video signal) and Galaksija BASIC.
Fitting all this functionality in 4 KB of 2732 EPROM required a lot of effort and some sacrifices. For example, some message text areas were also used actual code (e.g. "READY" message) and the number of error messages was reduced to only two ("WHAT?" and "HOW?").
ROM "B" of home computer Galaksija is a 2732 EPROM chip that contains extensions to original Galaksija BASIC available in computer's base ROM ("A"). It was labeled as "B" by the creator of Galaksija, Voja Antonić, but was commonly referred to as "ROM 2".
ROM "B" contained additional Galaksija BASIC commands and functions (mostly trigonometrical) as well as Z80 assembler and machine code monitor. This ROM was not required and was an optional upgrade. Although planned on the mainboard, the content of ROM "B" was not automatically initialized during booting. Instead, users had to execute a Galaksija BASIC command to run a machine code program from ROM "B" before they can gain additional features. This also meant that even Galaksijas with ROM "B" plugged in can behave entirely as base models.
Character ROM of home computer Galaksija is a 2716 EPROM chip that contains graphical definitions of Galaksija's character set. It had no special name and was labeled "2716" after the type of 2 KB EPROM required.
Galaksija had a slightly modified (localized) ASCII character set:
- There were no lowercase characters
- Codes 91 to 94 represented Serbian language characters Č, Ć, Ž and Š, respectively. Letter "Đ" was not present in original version and was commonly replaced with "DJ".
- It contained 64 pseudo-graphics characters, having different combinations of dots in 2x3 matrix.
- Character codes 64 and 39 are used for two halves of the logo of Elektronika Inženjering company (they can be seen in "READY" prompt)
Each character was represented as 8x13 matrix of pixels. In this ROM, 8-pixel rows of each character are represented as 8 bits of one byte.
Galaksija did not have a dedicated video circuitry. Its Z80A CPU was directly responsible for modulating the monochrome video signal with a little help of a shift register. The CPU would write a single byte to the shift register, which would, in turn, serialize its 8 bits one by one to the video output.
Since Galaksija had little RAM, a portion of it was taken not to store information for each pixel separately but character codes. CPU then had to look up character definitions in its character definitions ROM to find the values to send to the shift register.
This was standard operation available in Galaksija's ROM. Some software, however, reportedly took the responsibility for driving the shift register (and thus generating video) and was able to use various tricks to achieve what appears as high-resolution graphics, such as with user defined graphic characters.
It was thus possible for Galaksija with sufficient RAM upgrade (having at least 6144 bytes available for video memory) to achieve 256x208 graphics without any specialized circuitry. Other resolutions were also possible while maintaining only pixel width, by changing the amount of the active area of video picture and/or handling two interlaced video fields separately. Changing the number of active lines would have also altered the CPU usage for video generation and ratio of it available for other use.
Galaksija used cassette tape
as secondary storage
. It featured a 5-pin DIN connector
used to connect the computer to a cassette tape recorder. Tape interface circuitry was rudimentary - other than few elements controlling the levels it was essentially one-bit digital equivalent to the one in the ZX Spectrum
. The input signal was routed to the integrated circuit
otherwise responsible for keyboard, so the CPU would "see" the input signal as a series of very fast key presses of varying lengths and gaps between them.
It is normally stated that original Galaksija does not have any dedicated (separate) audio ports and most of the programs were written as silent. It was, however, possible to utilize the cassette tape port as an audio output as well like it is done in ZX Spectrum (its "EAR" connector). The only technical difference between ZX Spectrum and Galaksija in regards to existence of audio is that ZX Spectrum has a built-in beeper, while Galaksija's plans do not include any kind of a speaker.
- Approximately 70% of CPU time was used just to generate video, which made Galaksija relatively slow in normal operation. This was unacceptable while saving or loading data from the tape so video generation was disabled during tape operations. There is also a way to disable (and re-enable) video generation from BASIC when "fast computation" is required. With video disabled, built-in BASIC interpreter was in many instances able to outperform interpreters of other home computers of the time.
- Galaksija's BASIC has slightly different names for standard commands (e.g. BYTE instead of PEEK and POKE, OLD instead of LOAD, etc). This was done to assure that most commands begin with different letters. Not only this helps the interpreter in recognizing the command but also allowed all commands to be abbreviated to a single letter followed by a dot (e.g. "P." instead of "PRINT") and, thus, take less memory for the same program, albeit with reduced readability.
- Galaksija's interpreter can report only a few error messages - "WHAT?", "HOW?" and "SORRY". This made more ROM space available to code.
- "READY" prompt (bytes in ROM representing it) is also used as code in video generation routine.
- To simplify "do-it-yourself" building and reduce cost, the Printed circuit board was designed as single-layer (single-side) board. This resulted in a relatively complicated design requiring many components-side connections to be made using wires.
- Galaksija's case was not pre-built. Instead, the guide suggested it to be built out of the Printed Circuit Board material used for the mainboard. Thus, the top, sides and reinforcements were soldered together to form the "lid". acrylic glass was recommended for the underside. The guide included instructions on cleaning, painting and even decorating the assembled case. The name "GALAKSIJA" and decorative border were to be added using Letraset transfer letter sheets after the first (white) coat of paint but prior to the second coat of final colour. After the paint dried, transferred decorations were supposed to be scratched off, exposing underlying white paint.
- Among standard ways of distributing software, Galaksija programs used to be broadcast over FM Radio "Beograd 202" in a show Ventilator 202, hosted by Zoran Modli. At 280 bit/s, a mere 2 KB program would take a whole minute of characteristic noise that only computer enthusiasts enjoyed.
- "Dots" of pseudographic mode were not all of the same height. They were all 3 pixels wide and 1 pixel gap existed between them (both horizontally and vertically). In a single character there were two dots horizontally and three vertically. The top and bottom dots in a character were 3 pixels tall, and the middle ones were 4 pixels tall.
- Generated video signal was made for PAL equipment, but does not fully conform to its timings. Instead of 15,625 lines per second (625 lines 25 times per second) Galaksija generated full 16,000 because it was easier to fit in CPU timings (exactly 192 clock ticks) per line, while keeping the 25 frames (50 fields) per second. Instead of 312.5 lines per field, this results in exactly 320 fields. Due to a missing half-line to facilitate interlacing, Galaksija's video output was essentially progressive, albeit because it never comes to the "second field". This did not create any usage problems as it was well within tolerance of the video equipment.
- The keyboard is laid out such that keys have their own memory-mapped addresses that, in most cases, follow the same order as ASCII code of the letter on the key. This saved the ROM space by reducing lookup tables but significantly increased the complexity of single-layer keyboard PCB such that it alone required 35 jumpers.
- The original Galaksija's ROM lacks support for auto-starting programs after they have been loaded from the tape (a feature common in similar computers of the time). Users had to start each program manually after the loading with a "RUN" command. This was an intentional attempt to prevent copy-protection and to encourage sharing of software between users.
- Computers in your home - short overview by Dejan Ristanović, the author of Računari u vašoj kući magazine issue, in English language
- 1983: Galaksija - how it all started, by Galaksija's creator Voja Antonić himself (in Serbian language)
- Computer Galaksija - detailed description of computer operation for those planning to build it, as published in the Računari u vašoj kući magazine issue. Written by creator Voja Antonić, in Serbian language.
- Uputstvo za upotrebu - complete, original, user manual on-line, in Serbian language.
- Magazine Scans - scans of original magazine pages containing schematic diagrams, building and other instructions and programs for Galaksija (text in Serbian language)
- Računar Galaksija by Dejan Ristanović, the author of Računari u vašoj kući magazine issue, in Serbian language