zaterdag 17 oktober 2020

My first computer and Forth

When I needed to select my career it was difficult to choose between chemistry and electronics. I decided to go for (analytical) chemistry and kept electronics as a hobby. In my last years at school computers were just coming. The last years i went to school there where 4 terminals (with one printer) connected (i expect via a phone line) to a computer somewhere else and we had to write some computer programs in Basic as exercise.   

There I became fascinated by computers, did know a little bit of electronics, and after finishing school I started working in a pharmaceutical company. 

I wanted to know more about the link between the electronics and computers. In the Popular Electronics Magazine i found an article about the 1802 microprocessor. That was the first article about a microprocessor where I at least thought that I understood something a little bit. Magazines spread interesting articles over multiple editions, so a month later i also needed to buy the next edition as, i expect for a business model, most interesting articles were spread over multiple editions. After this I did read articles about other microprocessors, however at that time a lot of it was very complicated for me.
As the desire to have a real computer for my own grew I started investigating options. I already had a programmable calculator. Buying a ready computer was, at that time, not an option. The only little affordable options were some microprocessor kits and I decided to buy an ELF II with an 1802 processor.

For me important features were that it had a graphic chip so you could create some (black and white) graphic output to display on a TV and the architecture and instruction set of the 1802 was easier for me to understand than that of other processors. I ordered a kit and after soldering I got the ELF II with its 256 byte (¼ k) ram, the graphic processor, a hexadecimal display, a hex keyboard working and started leaning more.

Very soon I also needed the expansion board with a 256 byte containing a small monitor program. The most important in the 256 byte rom was a program to read and write memory to an audio cassette recorder using simple hardware interfaces on this board.

Some of the 256 byte memory was also used as memory for graphics on the TV. 256 byte (¼ K) is not much so I kept looking at the slowly decreasing price of additional memory.
At some point I decided not to for the 4K ram memory board and made a big jump by buying a 64K ram board with 48K ram. I expect it was around that time that my boss told me that it was a little crazy that I had a computer. However he understood it as i was only living during the weekends with my parents a little more than 200 km away from my work. From Monday till Friday I lived in a small room near my work and could enjoy my computer.

And with the huge amount of memory it became possible using programming languages on the ELF II 1802 processor. However often the programming languages used a terminal on the RS232 interface and that was, after already spending more than a month's salary on my computer, above my budget. However i did buy a kit for an ASCII keyboard over the parallel port. 

There was Chip8, Pilot and from a magazine i did type over a terminal program using the video chip using black and white characters. I could not use Basic as this language needed an external terminal on the RS232 port and there was no info or source code about its internal working.

And then there came Forth. I got a cassette tape and a print of the 1802 programming code for Forth also using the RS232 interface. However the standard Forth interface uses only 3 important routines. ?KEY and KEY to check and get input from the keyboard and EMIT to output one character. Making ?KEY and KEY work was not that difficult and in my first Forth experiments i used the hexadecimal display with a button to go to the next character after reading the HEX Ascii value and decoding it to the ASCII value.

My plan was interfacing Forth with the 1802 terminal program as that seemed possible as the source code of these two programs was available. This was my first real big IT project and I am still proud that I, on my own, could make this work.

The first,and easiest step was to move the terminal program (and video memory) to another part in memory. After that came the bigger challenge. The 1802 processor has 16, almost identical, registers of 16 byte. Each of these registers can be assigned as a program counter, stack pointer, a pointer somewhere in memory or to store a bite in the high or low part. These registers are also used in the Forth implementation for the 1802 processor.
However these 16 registers ( 0 to F) are almost identical, not completely!
R0 was (also) used for DMA, direct memory access for the graphical display chip.
After a hardware interrupt (also used for the video display) R1 became the program counter and R2 became (assembler) stack pointer. Also some of the registers were used in the terminal display program.
As these registers were also used in the 1802 Forth code i needed to change this in the Forth code. Nowadays this would be easy. At that time I did not have a working assembler. Only the hex keyboard, a hexadecimal display, a simple monitor program in the rom to inspect and change a hex address using the limited I/O. And after making changes I needed to store my work on an audio cassette tape to continue on another day.  Printing a version with changes was not possible as a printer was not available. So i used a paper notebook to keep track of changes to do and done.

The video display worked using the interrupt code and DMA (direct memory access). Every time to display memory data on the TV screen the video chip gave an interrupt.. It did set R0 to the beginning of the video memory and some other housekeeping. As Forth and the Terminal program used the registers. Also a small assembler program that saved and retrieved some of the 16 registers in memory when going from Forth to the Terminal display program and vice versa was added.

After many evenings of work the changes in the code where ready and the Forth ok prompt appeared on my black and white television and my adventure could continue on a higher level. 

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