Monday, 17 December 2012

Is this the worst console ever (Part 2) ?

The Milton Bradley Microvision. The first real hand held console machine that had interchangeable cartridges. (You could make a case for the Entex Select-a-Game)  It's bigger than this picture makes you think - it's about a foot long. I've actually got a collection of these.

What makes it terrible ? Well, you can see the graphics resolution on this picture. It's 16 x 16 pixels, which makes games look like a multiplication table that's been shaded in. It's sound is a single channel beeper. It does have a rotary controller and a passable keyboard which wears out very rapidly. (Though this German one looks more solid.I think "Das Computer Spiel-System" is German for "A heap of old junk")

The games are very vulnerable to static so these aren't very robust. There aren't very many games to choose from. The cartridges are massive (the whole grey bit in the picture is the cartridge)

I had considered writing an MB Homebrew game for this Retrochallenge (maybe the next one). For such a simple machine it's actually difficult to program. The upside is the graphics are easy :)

To start with, it uses a TMS1100 4 Bit Microcontroller which is errr..... interesting to program. I did an entry for this retrogaming competition on a Simon (the Milton Bradley sequence toy) which is a TMS1000 (the TMS1100 is a double size TMS1000 with 2k ROM and 128 nibbles (4 bit units) of RAM) and it is challenging. The Microvision is tougher because the screen has to be continually refreshed otherwise it won't work - you don't just say "turn this pixel on", you have to tell the LCD Controller thirty times a second you want these pixels on. Then you have to do it with the opposite polarity on the LCD and if you get it wrong static builds up and it might stop working. (Hats off to the people who managed to get the simple Microvision games working. Simple things like (say) displaying a two digit score on the screen is hard)

A TMS1100 is so slow (about 50,000 instructions/second) that it must spend most of its time doing this. I don't think it's as hard as the Atari 2600, but it runs it close, and the 6502 is quite easy to program. Programming the TMS1x00 series requires you to think differently - you've got one level of subroutine call and its instruction set appears to be half missing. Better than the TMS1000 when you can ADD 1,6,8,10,15 as a constant but not 7 :)

The Microvision originally used the Intel 8021 (a predecessor of the well known Intel 8048) but changed because they couldn't get enough of them I think. This is much easier to code for than a TMS1100 but then what would be the fun if it was easy. Just to make things complicated again, the wiring for the rotary control appears to be different on different cartridges.

I did spend an afternoon trying to bully the ROM images out of a Pocket Simon - the TMS1000 is supposed to have a 'debug' mode that allows you to dump the ROM contents. which is vaguely described by its patent. Wiring it up to an Arduino worked (you could pulse the clock and watch the chip operate) but I couldn't persuade it to give up its secrets.  So not much chance of dumping the Microvision ROMs unless we find an expert (it definitely can be done)

But .... this isn't the worst console ever either.

Next time : the machine usually labelled the worst console ever. 

But it isn't.

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