Instruction Set Principles - Computer Architecture: A Quantitative Approach - page 494

FIGURE A.1 Operand locations for four instruction set architecture classes . The arrows
indicate whether the operand is an input or the result of the arithmetic-logical unit (ALU) oper-
ation, or both an input and result. Lighter shades indicate inputs, and the dark shade indicates
the result. In (a), a Top Of Stack register (TOS) points to the top input operand, which is com-
bined with the operand below. The first operand is removed from the stack, the result takes
the place of the second operand, and TOS is updated to point to the result. All operands are
implicit. In (b), the Accumulator is both an implicit input operand and a result. In (c), one input
operand is a register, one is in memory, and the result goes to a register. All operands are re-
gisters in (d) and, like the stack architecture, can be transferred to memory only via separate
instructions: push or pop for (a) and load or store for (d).