[v1.1] MECB 8088 CPU Card

[epaell]

Hi all,

I've been continuing on the theme of collecting various 8-bit bus CPUs and making boards for the MECB system (which I love because getting them to work not only gives them new life but also allows them to work with all of the devices that already exist for that system). Anyway, my latest edition is an 8088 board (512 KB RAM and 512 KB ROM and I/O connected to the I/O bus). It's my first time ever working with an Intel or Zilog family device (CPUs I never really grew up with or had access to when I was young) ... it showed it's first signs of life this morning with a really simple Hello World program

I still have a few minor adjustments to make - a couple of additional pull-up resistors to make the interrupt handling work more sensibly (I was a bit unclear how that worked with this CPU) but apart from that it seems to work OK chugging along at 8 MHz. Strangely, it's a CPU that is hard to find outside of a PC environment so there isn't a great deal of stand-alone software e.g. a lot of the "monitor" style programs seem to assume it is running in a PC e.g. assuming memory-mapped screen, etc. I may have to see if I can adapt one of these to work on the MECB platform.

[epaell]

I added the extra pull-up resistors that I was missing and have been able to confirm that the NMI and IRQ handing appears to be working. I tested it by setting up the ACIA to generate an interrupt whenever a character was received and this triggered the INT 0FFh interrupt vector to be called (I had to add pull up resistors on the data bus so that when it expects to see the vector presented it sees 0FFh by default). I'll add these to my next revision of the board.

[tarix76]

Nice, I assume your board would also work with the NEC V20 (D70108) since it's mostly a drop-in replacement for the 8088. AliExpress has a few if you wanted to try one.

[epaell]

Hmm, I'm not 100% certain but I assume it should work if the pinout is the same (as far as I can tell the timing/speed improvements are internal). I might have to give this a try.

[epaell]

Another update. I've just received my v1.0 board and all of the minor fixes have been included and the board appears to be functional. I've placed all the schematics, PCBs and software on my GitHub over here (in case it is of interest to anyone):

https://github.com/epaell/MECB/tree/main/MECB_8088

It turned out that the hardest part with respect to getting this board running was finding software to make it functional. It seems like everyone just assumed that if you had an 8088 that you must be running it in a PC or compatible and so had access to DOS or Windows. I ended up having to hack my own monitor (based on bits from software that I could fine and writing some from scratch). It's still not complete but at least allows you to modify/view memory, load programs and execute them. I made a start of including breakpoint functionality but haven't really taken it very far (I did find some code for a disassembler as well but I've been having trouble getting that to work as well so I left it out).

Even finding software to run, e.g. simple assembly programs, turned out to be quite a challenge. Again, I managed to find a DOS version of a Mandelbrot generator but had to adapt it to this system and also modify it to work with the MECB 9958 video graphics card.

[djrm]

Hi Emil, great achievement there, specially seeing you have the new cpu working with your existing cards.

[epaell]

Thanks

Yes, it's really nice being able to use a completely different processor with all of the existing hardware. Today I was playing with the 6800, 8088 and 68008 and in each instance only had to remove the CPU board to switch over (the same could also be done with the 6502 and 6809 card). I'm just missing a few processors now to complete my full deck of cards! I may start experimenting a bit more with MAME to see if I can at least get software compiled for some of the more esoteric CPUs first before attempting to find them on eBay and/or start building cards for them.

Ironically, it seems that as the processors advanced and became nicer to develop for (8088 vs 8080; 68008 vs 6800) less software seemed to be made available for them in assembly code. It seems that they arrived at a point where higher-level languages also took off and so most software for the 8088 and 68008 is written in C or PASCAL ... or in rare cases where they were written in assembler they were so tailored towards a particular system that it became difficult to make it more generic (or assumed you were running within an OS).

[djrm]

I understand what you mean there,in fact later cpus wee developed with support for high level languages built in. I used to have Sinclair QLs, I learnt C on it. But it also had a useful assembler which I used that before spending more time with the C language.

[Editor]

I've been continuing on the theme of collecting various 8-bit bus CPUs and making boards for the MECB system (which I love because getting them to work not only gives them new life but also allows them to work with all of the devices that already exist for that system). Anyway, my latest edition is an 8088 board (512 KB RAM and 512 KB ROM and I/O connected to the I/O bus). ...

Awesome work! p.s. I love the ceramic DIP you have on the board.

I've never played with Intel CPU's. My exposure to Intel was only via the PC, with my PC programming life starting with Borland Pascal.

You're really forging ahead with new MECB hardware! I hope to also allocate more time to hardware development, as soon as I can.
At the moment, all of my spare time is being consumed by administrative tasks. So much so, that it's taken me over a week to notice this topic.

But your hard work is definately inspiring me to get clear of the admin, so that I can get on with working on some projects!

[epaell]

The ceramic devices really are beautiful!

I also started PC programming with Pascal (back when it was still called "Turbo" Pascal) in first year of Uni but did some very rudimentary 8088 programming in my third year (I think) ... which left me with a bit of traumatic amnesia (as it was an awful processor with the segment registers and very specific registers for specific operations - ew!). Nonetheless, I'm glad I got to go through this process. There is still one weird thing that I've kind of swept under the carpet because I don't fully understand it ... for some reason there is an interrupt generated when I first start up the board but it goes away if I wait long enough. My dodgy fix was to introduce a memory check at the start in the monitor software and by the time that finishes the interrupt goes away. It's a very dissatisfying oddity but I can't work out why it is happening. It's like the PLD (which inverts the ECB_INT line) takes too long to do it - though that makes no sense to me whatsoever because it's not like there is a capacitor or anything delaying it. However, once it is up and running the interrupt seems to work fine (I tested it by triggering it via the ACIA whenever a character is received). I'm just wondering if maybe it is another situation of loading the USB-C power (I had a similar weirdness with the 68008 board if I had too many cards plugged in). I really should get a more reliable external power supply to check this against.

BTW, while looking up a few other things (in particular how one might conceivably connect a C64 SID chip) I found that there were some nice little inverters for converting voltages and it got me into thinking that maybe an 8080 option might still be on the cards (pun intended) Unfortunately the connecting of the SID chip might be more complicated than I would've liked as it requires a 1 MHz clock (and that is used as part of the data transfer logic) - so if it is an independent clock it might cause timing issues with whatever MECB processor is being used or if the system clock is used it may not necessarily be 1 MHz (and/or I'm not sure if the timing would be correct).

Oh, and admin stuff isn't fun - hopefully you get back to fun stuff soon!