Classification of Architecture

The HC11 is a member of what's called the register-memory family of architectures. In general (there are exceptions - there are always exceptions) an arithmetic operation on the HC11 requires one operand to be in a register, and the other operand to be in memory. The operation works something like

reg <- reg op mem

So the operation is performed on the old contents of the register and the memory, with the result written back into the register. This is a restricted form of a two-operand computer; a two-operand computer is one that specifies two operands (like the HC11), but can also have more flexibility in the source and destination operands locations.

Looking at some other families of computers, the Intel family is a two-operand family in which either the source or the destination can be in memory, but not both. The DEC PDP-11 was a two-operand family in which either operand can be in either registers or memory.

Since it only has two arithmetic registers, A and B, the HC11 is almost an accumulator machine. An accumulator machine only has one register, so the destination of all arithmetic instructions must go to that one register.

As is the case with any real computer, there are exceptions to the rules regarding what can be done with registers and with memory, to handle special circumstances that can arise.