CDP1802
The RCA CDP1802 (aka RCA COSMAC) is an 8-bit low power microprocessor introduced by RCA around mid 1970's. It was used by many popular early computers (like all the COSMAC ELF variants & VIP in the USA and Telmac in Finland) and has also gone to space controlling satellites (military version is highly resistant to radiation and ESD).
1802 is a very simple CPU and also somewhat different than the competitors of the age. For example the I/O handling is pretty unique. Normally (if there is such a thing) I/O is handled by moving data from(to) I/O devices (either mapped to memory or separate address space) to a register (like accumulator). 1802 does this differently. There does exist special opcodes for I/O transfer, but instead of moving data from I/O device to a CPU register, data is moved directly between the I/O device and memory, kind of fly-by bus access (the CPU drives memory and I/O active at the same time), cool. At first this was difficult to understand after spending a lot of time with 8080 and Z80.
It also has a large set of registers (16pcs, each 16bits) reminding me of more modern RISC-processors.
Third speciality is the way sub-routines are called, there is no familiar CALL-RET procedures using stack pointer as most other CPUs do.
