Sunday, 30 June 2013
About the Processor.
There seem to be two popular 4-bit old Microcontroller families - the Texas Instruments TMS1x00 series, most famously in Simon, Merlin and the Hornby Railway system, and National Semiconductors' COP4 series - which was in Atari hardware and the Mattel sports hardware. There are a few others - the Entex Select-A-Game uses a Hitachi HD38800 processor for example, but not many I have come across. It is also in my all time favourite handheld, the utterly pointless Invisible Alien Neutraliser (IAN) which would have been the Retrochallenge if I could have come up with any use for it. (the actual use doesn't count IMO !)
This is one of the babies of the COP4 family - it has 512 bytes of ROM and 32 nibbles (e.g. 4 bit values) of data memory - about half the power of a TMS1000. It is fairly slow - about 50-60khz as a base CPU clock, but that's quite adequate for this kind of thing.
The design is fairly similar to that of the TMS1000 at heart - there is an Accumulator, a pair of registers which address memory, a Carry / Status flag and assorted I/O ports.
The instruction set is not unusual - with one exception. The microcontroller has an instruction XAD 3,15 (which swaps A with memory bank 3, location 15 directly). This is a bit out on its own (the COP444 for example can do this with every memory location).
I think this is to deal with the major problem with the SM-510 in the Watchman - without it it would have exactly the same problem regarding indirect writes (it is very difficult to get an SM-510 to write a value stored in one location to an address stored in a second .... see the Watchman blog if you want the gory details). What this instruction does is to sort of co-opt RAM(3:15) as another internal register in the MCU (even though it isn't).
It is unusual for these processors to have logic operations - unusually the processor has an XOR instruction (which is odd really, because an XOR instruction is the same as ADD but you don't carry .....). I think this is a hangover from the origin of these devices (most of them are converted calculator chips)