Preparation for Midterm Binary Data Storage (integer, char, float pt) and Operations, Logic, Flip Flops, Switch Debouncing, Timing, Synchronous / Asynchronous Circuits, Memory Organization, Finite State Machines (FSM) RISC / CISC Computers, MIPS Organization, MIPS Instructions, MIPS Addressing, MIPS Frames / Context Switching, MIPS Assembly/Machine Programming Hamming Code Developing MIPS Datapaths, MIPS FSM Implementations, and Alternative Microprogramming

Preparation for Midterm Midterm Questions/Problems will attempt to evaluate understanding of concepts and ability to use them, e.g. in design. Review Class Slides – use them to test your understanding. Review HW assignments and Projects. Midterm will be Closed Book, Open Minds. Understand material on MIPS Reference Data Card. Bring it with you for the midterm. Midterm will be designed to take approximately ½ of the period. You will have the entire period as needed.

Appendix C Mapping Control to Hardware For a Single Clock Cycle Implementation For a Multiple Clock Cycle Implementation

Implementing the Simple MIPS Machine I-Type: oprsrt address offset R-type: oprsrtrdfunctshamt 10 J-type: optarget address MIPS has 3 Instruction Types: The Simple Machine Implements: R-Types: ADD, SUB, AND, OR, NOR, SLT I-Types: LW, SW, & BEQ J-Types: J

Single Cycle Datapath with Control Unit Read Address Instr[31-0] Instruction Memory Add PC 4 Write Data Read Addr 1 Read Addr 2 Write Addr Register File Read Data 1 Read Data 2 ALU ovf zero RegWrite Data Memory Address Write Data Read Data MemWrite MemRead Sign Extend 1632 MemtoReg ALUSrc Shift left 2 Add PCSrc RegDst ALU control ALUOp Instr[5-0] Instr[15-0] Instr[25-21] Instr[20-16] Instr[15 -11] Control Unit Instr[31-26] Branch

Generating the ALU Control Signals ALUOp1 & ALUOp2 are generated from the 6 bit OPCODE: 00 for LW & SW 01 for BEQ 10 for R-Type (ADD, SUB, AND, OR, NOR, SLT)

Generating the ALUOp1 & ALUOp0 Signals ALUOp1 & ALUOp0 are generated from the 6 bit OPCODE: Instruction Types: Instructions (Opcodes)  ALUOp1 &ALUp0 R-Type: ADD, SUB, AND, OR, NOR, SLT (000000)  10 I-Type: LW (100010) & SW (101011)  00 I-Type: BEQ (000100)  01 J-Type: J (000010)  XX (Doesn’t use the ALU) OpCodes | ALUOp1 ALUOp | | | | | X X

Generating the ALU Control Signals

Generating the One Clock Control Signals

Generating the One clock Control Signals

Implementing the Multiple Clock Cycle Machine The Multiple Cycle Machine cannot be implemented simply as combinational logic. Why? Because each instruction requires 3 -5 states (clock cycles) to complete. It will require a Finite State machine (FSM). A set of states. How many? Combinational logic for control signals. Combinational logic for next state inputs. It can be implemented with a PLA(s).

The Multicycle Datapath with Control Signals Address Read Data (Instr. or Data) Memory PC Write Data Read Addr 1 Read Addr 2 Write Addr Register File Read Data 1 Read Data 2 ALU Write Data IR MDR A B ALUout Sign Extend Shift left 2 ALU control Shift left 2 ALUOp Control IRWrite MemtoReg MemWrite MemRead IorD PCWrite PCWriteCond RegDst RegWrite ALUSrcA ALUSrcB zero PCSource Instr[5-0] Instr[25-0] PC[31-28] Instr[15-0] Instr[31-26] 32 28

Multiple Clock Cycle Finite State Machine

Comparing Single and Multiple Clock Machines

Generating the Next State Equations

PLA Implementation of Multi Clock PLA

PLA Implementation of Multi Clock FSM

An alternative to a FSM implementation. Most Computers today are at least partially implemented with Micro-program control The concept is to build a simpler internal computer (micro-computer) that implements the control sequences (micro-instructions) that are stored in a micro-computer memory (ROM) The advantage is ease of design, flexibility, and adaptability to “families of computers”. Particularly useful in CICS Machine Implementation. Micro-Programmed Control

Control Unit Organization Tasks of Control Unit: Microinstruction sequencing Microinstruction execution The Control Memory contains sequences of microinstructions that provide the control signals to execute instruction cycles, e.g. Fetch, Indirect, Execute, and Interrupt.

Typical Microinstruction Formats

Microinstruction Encoding: Direct Encoding

Horizontal Micro-programming Wide control memory word High degree of parallel operations possible Little encoding of control information Relatively Fast

Microinstruction Encoding: Indirect Encoding

Vertical Micro-programming Word width can be much narrower Control signals encoded into function codes – need to be decoded More complex, more complicated to program, less flexibility More difficult to modify Slower

Next Address Decision ? Like a normal computer, we want to allow branches. Why? Target address depends upon “ALU flags” and control buffer register bits: Get next instruction –Add 1 to control address register Jump to machine instruction routine –Load control address register based on opcode in IR Jump to other new routine based on jump microinstruction –Load address field of control buffer register into control address register

Branch Control: Two Address Fields Branch based upon: Instruction Opcode Address 1 Address 2 Does require a wide microinstruction, but no address calculation is needed

Branch Control: Single Address Field Branch based upon: Next instruction Address Opcode Does require more circuitry, e.g. adder

Branch Control: Variable Format One bit determines microinstruction format: Control signal format Branch format Does require even more circuitry, and is slowest.

Skeleton of a MIPS Microprogram Implementation

Reflection on Microprogramming Control The advantage is ease of design, flexibility, and adaptability to “families of computers”. The disadvantage is speed. Particularly useful in CICS Machine implementation, since they have a lot of complex instructions and hence complex design requirements. One of the advantages of RISC machines is that they intrinsically simple and don’t need a lot of complex options and huge numbers of states to implement. In reality, our view of the MIPS RISC machine is distorted by the simplicity of the subset of the MIPS machine that we looked at.