EMU 1 : First attempt to write a program for the EMU1 motherboard

As I indicated in my previous post, I now need to know if the DRAM on the EMU1 motherboard is fully functional or not.

As a reminder, when I had this EMU1, a few years ago, I was able to load a sound disk without the slightest problem. Then the machine smoked. Nothing serious, a few tantalum capacitors shorted. I replaced these capacitors and reassembled the machine without ever being able to restart it. Since then, it has never managed to read a diskette. So I had tested all the DRAM components with a small tester which had not indicated any faulty components.

Due to the multiplexing mechanism of the DRAM address bus, using 'criminal' timings performed with resistors/capacitors, I am therefore trying to get an idea of how this system currently works.

To do this, I will write a very simple program whose goal will be to represent on a series of LEDs connected to an output port of the EMU1 motherboard, the evolution of a simple 8-bit counter.

This program, of rare complexity, simply increments an Index whose value will be sent to the IO address space 0xC0 (__sfr __at 0xC0), in a loop slowed down by a timer consisting of two nested empty loops. There is no prior initialization of the Z80 registers. I do not use IRQs or the memory stack.

This simple program requires 5 bytes of RAM memory. If it works, it will already indicate that the DRAM components are being accessed 'relatively' correctly. If this program works, I will modify it to test the entire 128KB of RAM.

I compile this program using SDCC and the command:

sdcc -mz80 --no-std-crt0 --vc --code-loc 0x0000 --data-loc 0x400 TestRam.c

No special comments. The code starts at address 0x0000 and continues and uses only a few bytes of program area. The data in RAM will be positioned from address 0x0400, just at the beginning of the RAM space, just behind the first KB of code.

And now??? Well, we need to encode the content of the ROM code obtained after compilation and conversion into a binary file to make it compatible with the wiring system made by EMU (thanks EMU)!

Even for a few bytes of ROM, I preferred to write a small program under Windows to automate the task. Because it is necessary to encode not only the data but also the addresses. So as I plan to expand this small test program, I might as well make this encoding process as automatic as possible, even if it means 'taking' a little time to write the Windows application.

I hate using Microsoft's Visual C++. A gas factory, a big piece of crap (for me, because there are thousands of programmers who love Visual C++), GB of data to load and never knowing where we're going! So I tried it with QT. Its mode of operation is much more logical/simpler/efficient for me. Starting with no knowledge of QT, I 'only' needed a short day (I program embedded and am very bad with OS applications) of discovery to be able to write a minimal application that suits me.

So I know that the original EMU executable is 60 bytes. It's 'big' for such a small program, but I coded it in 'C' and not directly in assembler.

And for comparison, here is what the contents of the original binary file look like :

And now the binary encoded in the EMU 'way':

Well, if I understood correctly how the EMU1 motherboard works, if I also understood correctly the ROM coding system and if the DRAM multiplexing system is correct, it is not impossible that I can see something on the LED connector. Maybe not the progress of the counter if my temporisation is not sufficient, but still something...

Now I just have to program a 2732, put it on the motherboard, connect an LED and see if anything happens.

The result of my investigations in a future post...