Processor architecture

Organisation and architecture

Organisation and architecture involves the structure and properties of a computer system, but organisation is from the perspective of a hardware engineer and architecture from a software engineer.

Micro and macro instructions

Mnemonic assembly instructions are macro operations which consist of smaller, register-level operations called micro instructions. The CU knows control signals to send for each micro operations for a given macro instruction. For example, the ADD instruction has several micro instructions for copying registers to the ALU, setting control signals and copying the result.

Control unit design

The control unit is responsible for taking an instruction and sending the necessary signals to execute it. There are two approaches to CU design, hardwired and microprogrammed.

A hardwired (also called random logic) CU is a combinatorial logic circuit. This is a fast method but is complex and difficult to modify. It is more commonly used for RISC processors.

A microprogrammed CU functions like a very basic processor. Each machine instruction is turned into a sequence of microinstructions which are then executed. This is much more easy to design and implement and is also easier to reprogram, but is slower than hardwired CUs. It is more commonly used for CISC processors.

RISC and CISC

RISC and CISC are two types of instruction set. RISC uses fewer, more commonly used instructions whereas CISC has far more instructions which might not be used as often. RISC programs are usually more verbose but more efficient, whereas CISC programs are shorter but slower.