Circuitos Digitales II The General Computer Architecture The CPU Control unit Semana No.9 Semestre Prof. Gustavo Patiño Prof. Eugenio Duque Departamento de Ingeniería Electrónica Facultad de Ingeniería

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia The Central Processor Unit (CPU)

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia The CPU Control Unit Having designed the ALU or datapath so that it can perform the necessary instructions, we now have to do the same thing for the control unit, which decodes instructions and provides direction to the CPU. The MIPS control unit decodes the six bits on either end of the 32-bit instruction word, that is, the op code and function code* fields, to determine each instruction sequence.

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Functionality of Control Unit The control unit determines ALU functions in each instruction and selects operands for the ALU. The operation code (the left six bits of the instruction) determines the type of operation and in some cases (such as jump instructions) the actual instruction itself. In the case of register-register instructions, the function code determines the instruction (for example, in the R/R instruction above, the function code 0x 20 means add).

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Functionality of Control Unit (…cont) The control unit is a collection of decoders and multiplexers. The decoded instruction fields tell The ALU what function to perform, What operands to use.

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Completing ALU Design

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Current Architecture The ALU control uses instruction bits 0-5 to obtain information about the ALU operation in register-to- register instructions. Note in the following diagram that some of the decoding is done in the register block, which has the decoding mechanism that identifies source and destination registers in load/store and register/register operations. The ALU control also has input control lines from the operation code decoder which decodes bits 26-31, and which will be shown later.

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia ALU Design with ALU Control Design shown

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia ALU Control Block

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Single-Cycle ALU Design with Full Control Block

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Stages on the MIPS processor

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Op Code Control Block Signal Identifier

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Function OpCode Control Signals

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia OpCode Control Block Circuitry

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Instruction Disposition Showing Destination Units

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Data/Control Signal Flow Examples The following diagrams illustrate the flow of control signals and data in some example MIPS instructions in the single cycle implementation. The single cycle implementation is just a stepping stone to the final MIPS design, but this simpler example has all the features of the more complex final design in terms of data routing and the way in which the control signals determine the specific operation for each given instruction. Note the data flow in these instructions.

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Start of R-Type Instruction

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Next Step of R-Type Instruction

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Third Step of R-Type Instruction

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Completion of R-Type Instruction

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Load Instruction

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Branch Instruction

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Jump Instruction Circuitry Added

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Jump Instruction Flow

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Drawbacks of the Single-Cycle Implementation We have now completed design of the basic MIPS CPU. Although a good basic design, it has a serious drawback: The processor is designed so that all instructions complete in one clock cycle. While this assures that there is sufficient time to complete any instruction, it also means that one clock period must be long enough to accommodate the longest and most complicated instruction. Thus, ALL instructions take as long as the longest instruction. Since many (most!) instructions in the MIPS architecture take less time to execute than the longest instructions (which are usually the lw memory reference instructions), this means that we are slowing execution of the CPU a large part of the time to accommodate instructions that occur substantially less frequently.

Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Departamento de Ing. Electrónica Circuitos Digitales II Universidad de Antioquia Comparative Instruction Timing