simple processor control unit
DESCRIPTION
Simple Processor Control Unit. Instructor: Oluwayomi Adamo Digital Systems Design. Control Unit. Design a control unit for picking up instructions from memory address given by the program counter (PC). Interpret the instruction, Fetch the operands and feed them to the ALU, - PowerPoint PPT PresentationTRANSCRIPT
Instructor: Oluwayomi AdamoDigital Systems Design
Design a control unit for picking up instructions from memory address given by the program counter (PC).
Interpret the instruction, Fetch the operands and feed them to the
ALU, Store the result in destination registers Load the pc with destination address in case
of branch instruction, Contents of destination will be forwarded to
the LED or 7 segment display for display.
Op CC SRC DST
Register Instruction
01001011
Branch Instruction
11 CC ADDRESS
Halt and I/O Instruction
1100 L H DST
1100101111000110
11100011
CONT UNIT D MUX
MUX
MUX
PC
MUX A
MUX B
ALU
000G
DISPLAY
4 BITS
RCAOP
DMUXA
0 OR 1
SWICTH
MEMMUX R
EG
CC
DSTSRC
The control unit is like computer’s traffic cop. It coordinates and controls all operations occurring within
the processor. The control unit does not input, output, process, or store
data, it initiates and controls the sequence of these operations. Controls Data Movements in an Operational Circuit by
Switching Multiplexers and Enabling or Disabling Resources Follows Some ‘Program’ or Schedule Often Implemented as Finite State Machine or collection of
Finite State Machines
Any Circuit with Memory could be called a Finite State Machine◦ Even computers can be viewed as huge FSMs
Design of FSMs Involves◦ Defining states◦ Defining transitions between states◦ Optimization / minimization
Above Approach Is Practical for Small FSMs Only
pr_state nx_state
Combinational logic
Sequential logic
input
clock
rst
Output Is a Function of a Present State Only TYPE state IS (S0, S1, S2); SIGNAL Moore_state: state;
U_Moore: PROCESS (clock, reset) BEGIN IF(reset = ‘1’) THEN Moore_state <= S0; ELSIF (clock = ‘1’ AND clock’event) THEN CASE Moore_state IS WHEN S0 => IF input = ‘1’ THEN Moore_state <= S1; ELSE Moore_state <= S0;
END IF;
reset
WHEN S1 =>
IF input = ‘0’ THEN Moore_state <= S2; ELSE Moore_state <= S1; END IF; WHEN S2 => IF input = ‘0’ THEN Moore_state <= S0; ELSE Moore_state <= S1; END IF; END CASE; END IF; END PROCESS;
Output <= ‘1’ WHEN Moore_state = S2 ELSE ‘0’;
Output Is a Function of a Present State and Inputs
TYPE state IS (S0, S1); SIGNAL Mealy_state: state;
U_Mealy: PROCESS(clock, reset) BEGIN IF(reset = ‘1’) THEN Mealy_state <= S0; ELSIF (clock = ‘1’ AND clock’event) THEN CASE Mealy_state IS WHEN S0 => IF input = ‘1’ THEN Mealy_state <= S1; ELSE Mealy_state <= S0; END IF;
WHEN S1 => IF input = ‘0’ THEN Mealy_state <= S0; ELSE Mealy_state <= S1; END IF; END CASE; END IF; END PROCESS;
Output <= ‘1’ WHEN (Mealy_state = S1 AND input = ‘0’) ELSE ‘0’;
Fetch -> Decode -> Execute
Fetch Sequence t1: MAR <- (PC) t2: MBR <- (memory) PC <- (PC) +1 t3: IR <- (MBR) (tx = time unit/clock cycle)
Good Luck!!!