There are many simple microcontroller architectures with only one register, usually called A (accumulator) or W (work register). You just need to constantly loa

There are many simple microcontroller architectures with only one register, usually called A (accumulator) or W (work register). You just need to constantly load memory to and from the register, nothing weird in that. Sometimes they will call their memory locations registers, in which case they have lots of registers - the terminology gets quite unclear when everything is on the same chip anyways. A true stack machine does not have any registers, so that would be 0.