By Philippe Dubois, President. Translation by Sébastien Marty.
In this fourth article, we are going to take out the main electronic boards of our Micral N to start referencing them and understanding how they work, then we will dump (copy) its read-only memories (ROMs) in a future article. Our unit visibly contains several RAM boards, one or two ROM boards, a CPU board and several as yet unidentified input/output boards.
First of all, we started extracting the boards one by one, taking note of each one's location. We had no idea at the time how they worked, how they are linked and how they communicate. Rigour is key!
A first board looks interesting to us. It contains 8 ROM – or rather EPROM – chips, recognizable by their small glass windows. A ROM is a read-only, pre-programmed memory, and the EPROM version can be reprogrammed as needed after erasing its contents with ultraviolet light. According to Sylvain Glaize's analysis of the original Micral N documentation, there is a chance that this board contains the machine's monitor. That is to say a set of functions that allow you to analyze what the Micral N is doing or in what state it is at any given moment.
Regarding the EPROMs used, they are C1702 chips, which are the first production runs available and put on the market in 1971 by Intel.
Here's the Wikipedia page on the subject: https://en.wikipedia.org/wiki/EPROM.
You can also find documentation via this link: https://deramp.com/downloads/intel/C1702A.pdf
But it is with some dread that we realized their protective glass windows, which should be obscured after programming, are not! We will come back to this later.
The second board we took out is already a bit more mysterious. Firstly, as shown in the photo, it has two connection combs – one to the right and one to the left –, which means that it is probably dedicated to communicating with the outside world. Then, focusing on the central golden chips, we find 8-byte SRAMs referenced as MOSTEK 4007P, whose characteristics are described there: https://usermanual.wiki.pdf
It may be a board used as a buffer, to allow the microprocessor to manage faster data inputs/outputs than it can process. This is a well-known mechanism in computer design, which also takes the name of FIFO queue (First In, First Out) or LIFO stack (Last In, First Out).
This third board is one of the easiest to understand. We are in front of a board comprising 4 KB of RAM (32 x 128 bytes in SRAM 2102 chips) as well as a 256-byte EPROM chip which certainly contains, according to our analysis, the boot code (bootloader) of the machine. That is to say the piece of code to be executed by the microprocessor as soon as the computer starts. Don't miss our next article, in which we'll explain how we dumped it – i.e. how we archived its content, which is very important to understanding how our Micral N works!
Finally here it is: the electronic board housing the famous Intel 8008, the world's first 8-bit microprocessor. It is that tiny golden chip at the bottom of the board. No, you are not dreaming! The processor operates at around 500 kHz and a set of yellow trimmer pots at the top right let you adjust the frequency of the "RC" oscillator (Resistor+Capacitor): there was no clock quartz yet as on a modern computer.
This microprocessor, a distant ancestor of future Intel x86 processors, takes up little space because its outputs are actually multiplexed, i.e. the same pins can be used as inputs and outputs, either for addresses (on 14 bits), or for data on 8 bits.
You will find a lot of information on this ancient microprocessor by consulting this link in particular: https://petsd.net/8008.php
Here is a simpler board which is similar to a RAM stick, with 2 KB of RAM using 16 SRAM chips of 128 bytes each.
Let's go back to the board with the multiple EPROM chips probably containing the monitor. Our Micral N's monitor is very important, since it certainly contains functions which will allow, as they did back in the days, to debug the programs we shall inject into the machine once fully functional.
This monitor is activated via the front panel of the Micral N, by halting the execution of the current program and asking the Micral to "jump" (JMP instruction) to a specific address (the monitor's address). The monitor then takes over and waits for the operator's instructions via the machine's serial interface.
The next step will be to verify that the contents of these ROMs are still readable. Which will be the subject of the next article.
See you soon and meanwhile, remember to take part in the campaign to support our work! https://micral.mo5.com