By Philippe Dubois, Chairman.

For this fourth article, we're going to take out the main electronic cards in our Micral N to start referencing them and understanding how they work, and then we're going to dump (copy) its ROMs in a later article. In our example there are obviously several RAM cards, one or two ROM cards, a CPU card and several as yet unidentified I/O cards.

We began by extracting the cards one by one, noting the location of each one. At the time of the operation, we still had no idea of how the cards worked, how they linked together and how they communicated. Rigour is the order of the day!

A Micral N ROM card from the MO5 association

The first card looks interesting. It contains 8 ROMs, or rather EPROMS, recognisable by their small glass window. A ROM is a pre-programmed read-only memory, and the EPROM version can be reprogrammed as required after erasing the contents using ultra-violet light. According to Sylvain Glaize's analysis of the original Micral N documentation, there's a good chance that this card contains the machine's monitor. In other words, a set of functions that can be used to analyse what the Micral N is doing or what state it is in at any given moment. t.

The C1702 EPROMs used were the first to be made available and marketed by Intel in 1971.

Wikipedia page on the subject: https://en.wikipedia.org/wiki/EPROM.

You can also find documentation at this link: https://deramp.com/downloads/intel/C1702A.pdf

But it's with some horror that we realise that their protective glass windows, which should be blocked after programming, are not! We'll come back to this later.

An 8x256 byte buffer card for an external interface

The second card we're releasing is already more mysterious. Firstly, this one has two connection combs, on the right and left of the photo, which means that it's probably dedicated to communicating with the outside world. Then, if we look at the central gold chips, we find that the reference is to fast 8-byte SRAMs, MOSTEK 4007P, whose characteristics are as follows: https://usermanual.wiki.pdf

This may be a card used as a buffer to allow the microprocessor to handle data input/output faster than it can process it. This is a well-known mechanism in computing, also known as a FIFO (First In, First Out) queue or LIFO (Last In, First Out) stack.

A card with 2 KB of RAM and a 256-byte boot ROM (the machine's bootloader)

This third card is one of the easiest to understand. We are looking at a card with 2 KB of RAM (32 x 128 bytes in SRAM 2102) and support for a 256-byte EPROM chip which, according to our analysis, should contain the machine's bootloader code. In other words, the piece of code to be executed by the microprocessor as soon as the computer starts up. In the following article, we'll look at the dump, i.e. the archiving of its contents, which is very important for understanding how our Micral N works!

The CPU board of our Micral N, with its Intel 8008 at the bottom, a tiny gold chip

At last, here it is, the electronic board of our Micral N featuring the sacrosanct Intel 8008, the world's first 8-bit microprocessor. It's that tiny gold chip at the bottom of the card. No, you're not dreaming! It operates at around 500 kHz and the yellow dimmer blocks in the top right-hand corner allow you to adjust the frequency of the 'RC' oscillator (Resistor+Capacitor): there's no clock quartz like on a modern computer.

This microprocessor, a distant ancestor of the Intel x86 processors to come, takes up very little space because its outputs are multiplexed, meaning that the same pins can be used as inputs and outputs, either for addresses (on 14 bits) or for data on 8 bits.

You can find out more about this antediluvian microprocessor at this link: https://petsd.net/8008.php

A standard 2 Kilobyte RAM card

Here's a simpler card that simply corresponds to the equivalent of a RAM strip, in this case 2 KB of RAM made up of 16 128-byte SRAM chips.

2 KB of ROM on these 8 chips of 256 bytes contain the Micral N monitor.

Let's go back to the card with the multiple EPROMs probably containing the monitor. The monitor on our Micral N is very important, as it will undoubtedly contain functions that will enable us, as in the old days, to debug the programs that we will inject into the machine once it is up and running.

This monitor is activated via the Micral N's front panel, by stopping execution of the programme in progress and asking the Micral to 'jump' (JMP instruction) to a particular address, that of the monitor. The monitor then takes over and waits for instructions from the operator via the machine's serial interface.

The next step will be to check that the contents of these ROMs are still readable. This will be the subject of the next article.

See you soon, and don't forget to take part in the campaign to support our work! https://micral.mo5.com