The Control Logic

Andreas KlappeneckerCPSC321 Computer

Architecture

Verilog HDL

Verilog Programming Suppose that we have a module foo Our goal is to instantiate and wire a

module foon that contains n copies of the module foo.

Many textbooks on Verilog suggest to write down the n instance explicitly if n = 1024 then this is not a viable option reusability of the code would be limited

Example 1

foo foo foo foofoo foofoo foo

foon

=

Example 1

module foo(out, a,b);

output out;

input a, b;

xor (out, a, b);

endmodule

module foon(out, a, b);

parameter n = 8;

output [n-1:0] out;

input [n-1:0] a, b;

foo foon[n-1:0](out,a,b);

endmodule

Example 2

foo foo

foon

=

foofoofoofoofoofoo

Example 2

module foo(out, a,b);

output out;

input a, b;

xor (out, a, b);

endmodule

module foon(out, a, enable);

parameter n = 8;

output [n-1:0] out;

input [n-1:0] a;

input enable;

foo foon[n-1:0] (out, a,

{out[n-2:0],enable});

endmodule

Remarks The module vector operator is not available in

Icarus Verilog nor in Veriwell The features shown work in vcs

foo foon[n-1:0](out,a,{out[n-2:0],enable}); out and a range over [n-1:0] {out[n-2:0], enable} gives vector [n-1:0]

The suggested for-loop construction does not work

Control

MIPS Multicycle DatapathIncomplete (branch and jumps…)

Control What are the control signals? Finite state machine control

Instruction fetch instruction decode

memory reference R-type branch jump

Multicycle Datapath and Control Lines

High-Level Picture What happens precisely during each

step of fetch/decode/execute cycles Construct the finite state control

machine High-level view

Instruction Fetch/Decode/Execute

Step nameAction for R-type

instructionsAction for memory-reference

instructionsAction for branches

Action for jumps

Instruction fetch IR = Memory[PC]PC = PC + 4

Instruction A = Reg [IR[25-21]]decode/register fetch B = Reg [IR[20-16]]

ALUOut = PC + (sign-extend (IR[15-0]) << 2)

Execution, address ALUOut = A op B ALUOut = A + sign-extend if (A ==B) then PC = PC [31-28] IIcomputation, branch/ (IR[15-0]) PC = ALUOut (IR[25-0]<<2)jump completion

Memory access or R-type Reg [IR[15-11]] = Load: MDR = Memory[ALUOut]completion ALUOut or

Store: Memory [ALUOut] = B

Memory read completion Load: Reg[IR[20-16]] = MDR

Instruction Fetch & Decode FSM

Memory-Reference FSM

• Address calculation

• Load sequence

• read from memory

• store to register

• Access memory

• Store sequence write

R-type Instruction

• Execution of instruction

• Completion of instruction

Branch Instruction

Implementation of FSM

A FSM can be implemented by a register holding the state and a block of combinatorial logic

Task of the combinatorial logic:

• Assert appropriate signals

• Generate the new state to be stored in the register