Classification of Instruction Sets

The instruction sets can be differentiated by

Operand storage in the CPU
Number of explicit operands per instruction
Operand location
Operations
Type and size of operands

The type of internal storage in the CPU is the most basic differentiation. The major choices are

a stack (the operands are implicitly on top of the stack)
an accumulator (one operand is implicitly the accumulator)
a set of registers (all operands are explicit either registers or memory locations)

The code segment C = A + B how it would appear on the classes of instruction sets
Stack	Accumulator	Register
PUSH A	Load A	Load R1,A
PUSH B	ADD B	ADD R1,B
ADD	Store C	Store C,R1
POP C

While most early machines used stack or accumulator-style architectures, all machines designed in the past ten years use a general purpose architecture. The reason is the registers are:

faster then memory
easier for a compiler to use
can be used more effectively

Primary advantages and disadvantages of each class of machine
Machine Type	Advantages	Disadvantages
Stack	Simple model of expression evaluation. Good code density.	A stack can't be randomly accessed. It makes it difficult to generate efficient code.
Accumulator	Minimizes internal state of machine. Short instructions	Since accumulator is only temporary storage, memory traffic is highest.
Register	Most general model for code generation	All operands must be named, leading to longer instructions.

Classification of General Purpose Register Machines

There are two major instruction set characteristics that divide GPR architectures. They concern

whether an ALU instruction has two or three operands

ADD R3, R1, R2
R3 <-R1 + R2               or ADD R1, R2
R1 <- R1 + R2

how many of the operands may be memory addressed in ALU instruction
Register- Register (Load/Store)
ADD R3, R1, R2     (R3 <- R1 + R2)
Register - Memory
ADD R1, A               (R1 <- R1 + A)
Memory - Memory
ADD C, A, B            (C <- A + B)