Laboratory Exercise 9
A Simple Processor
Figure 1 shows a digital system that contains a number of 16-bit registers, a multiplexer, an adder/subtracter
unit, a counter, and a control unit. Data is input to this system via the 16-bit DIN input. This data can be loaded
through the 16-bit wide multiplexer into the various registers, such as R0, . . . , R7 and A. The multiplexer also
allows data to be transferred from one register to another. The multiplexer’s output wires are called a bus in the
figure because this term is often used for wiring that allows data to be transferred from one location in a system to
another.
Addition or subtraction is performed by using the multiplexer to first place one 16-bit number onto the bus
wires and loading this number into register A. Once this is done, a second 16-bit number is placed onto the bus,
the adder/subtracter unit performs the required operation, and the result is loaded into register G. The data in G
can then be transferred to one of the other registers as required.
16
16 16 16 16
A in
R0 in R7 in
R0 R7 A
Clock
16 AddSub
DIN Addsub
16
Multiplexers G in
G
8
16
Bus
9
G out DIN out
R0 out } R7 out
IR in
IR
9
Control unit
Run
Resetn
2
Clear Done
Counter
Figure 1. A digital system.
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The system can perform different operations in each clock cycle, as governed by the control unit. This unit
determines when particular data is placed onto the bus wires and it controls which of the registers is to be loaded
with this data. For example, if the control unit asserts the signals R0 out and Ain , then the multiplexer will place
the contents of register R0 onto the bus and this data will be loaded by the next active clock edge into register A.
A system like this is often called a processor. It executes operations specified in the form of instructions.
Table 1 lists the instructions that the processor has to support for this exercise. The left column shows the name
of an instruction and its operand. The meaning of the syntax RX ← [RY] is that the contents of register RY are
loaded into register RX. The mv (move) instruction allows data to be copied from one register to another. For
the mvi (move immediate) instruction the expression RX ← D indicates that the 16-bit constant D is loaded into
register RX.
Operation Function performed
mv Rx,Ry Rx ← [Ry]
mvi Rx,#D Rx ← D
add Rx, Ry Rx ← [Rx] + [Ry]
sub Rx, Ry Rx ← [Rx] − [Ry]
Table 1. Instructions performed in the processor.
Each instruction can be encoded and stored in the IR register using the 9-bit format IIIXXXYYY, where III
represents the instruction, XXX gives the RX register, and YYY gives the RY register. Although only two bits
are needed to encode our four instructions, we are using three bits because other instructions will be added to the
processor in later parts of this exercise. Hence IR has to be connected to nine bits of the 16-bit DIN input, as
indicated in Figure 1. For the mvi instruction the YYY field has no meaning, and the immediate data #D has to be
supplied on the 16-bit DIN input after the mvi instruction word is stored into IR.
Some instructions, such as an addition or subtraction, take more than one clock cycle to complete, because
multiple transfers have to be performed across the bus. The control unit uses the two-bit counter shown in Figure 1
to enable it to “step through” such instructions. The processor starts executing the instruction on the DIN input
when the Run signal is asserted and the processor asserts the Done output when the instruction is finished. Table
2 indicates the control signals that can be asserted in each time step to implement the instructions in Table 1. Note
that the only control signal asserted in time step 0 is IR in , so this time step is not shown in the table.
T1 T2 T3
(mv): I0 RYout , RXin ,
Done
(mvi): I1 DINout , RXin ,
Done
(add): I2 RXout , Ain RYout , Gin Gout , RXin ,
Done
(sub): I3 RXout , Ain RYout , Gin , Gout , RXin ,
AddSub Done
Table 2. Control signals asserted in each instruction/time step.
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Part I
Design and implement the processor shown in Figure 1 using VHDL code as follows:
1. Create a new Quartus II project for this exercise.
2. Generate the required VHDL file, include it in your project, and compile the circuit. A suggested skeleton
of the VHDL code is shown in parts a and b of Figure 2, and some subcircuit entities that can be used in this
code appear in parts c and d.
3. Use functional simulation to verify that your code is correct. An example of the output produced by a
functional simulation for a correctly-designed circuit is given in Figure 3. It shows the value (2000) 16 being
loaded into IR from DIN at time 30 ns. This pattern represents the instruction mvi R0,#D, where the value
D = 5 is loaded into R0 on the clock edge at 50 ns. The simulation then shows the instruction mv R1,R0
at 90 ns, add R0,R1 at 110 ns, and sub R0,R0 at 190 ns. Note that the simulation output shows DIN as a
4-digit hexadecimal number, and it shows the contents of IR as a 3-digit octal number.
4. Create a new Quartus II project which will be used for implementation of the circuit on the Altera DE2
board. This project should consist of a top-level entity that contains the appropriate input and output ports
for the Altera board. Instantiate your processor in this top-level entity. Use switches SW 15−0 to drive the
DIN input port of the processor and use switch SW 17 to drive the Run input. Also, use push button KEY 0 for
Resetn and KEY1 for Clock. Connect the processor bus wires to LEDR 15−0 and connect the Done signal to
LEDR17 .
5. Add to your project the necessary pin assignments for the DE2 board. Compile the circuit and download it
into the FPGA chip.
6. Test the functionality of your design by toggling the switches and observing the LEDs. Since the processor’s
clock input is controlled by a push button switch, it is easy to step through the execution of instructions and
observe the behavior of the circuit.
LIBRARY ieee; USE ieee.std_logic_1164.all;
USE ieee.std_logic_signed.all;
ENTITY proc IS
PORT ( DIN : IN STD_LOGIC_VECTOR(15 DOWNTO 0);
Resetn, Clock, Run : IN STD_LOGIC;
Done : BUFFER STD_LOGIC;
BusWires : BUFFER STD_LOGIC_VECTOR(15 DOWNTO 0));
END proc;
ARCHITECTURE Behavior OF proc IS
. . . declare components
. . . declare signals
BEGIN
High <= ’1’;
Clear <= . . .
Tstep: upcount PORT MAP (Clear, Clock, Tstep_Q);
I <= IR(1 TO 3);
decX: dec3to8 PORT MAP (IR(4 TO 6), High, Xreg);
decY: dec3to8 PORT MAP (IR(7 TO 9), High, Yreg);
Figure 2a. Skeleton VHDL code for the processor.
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controlsignals: PROCESS (Tstep_Q, I, Xreg, Yreg)
BEGIN
. . . specify initial values
CASE Tstep_Q IS
WHEN "00" => - - store DIN in IR as long as Tstep_Q = 0
IRin <= ’1’;
WHEN "01" => - - define signals in time step T1
CASE I IS
...
END CASE;
WHEN "10" => - - define signals in time step T2
CASE I IS
...
END CASE;
WHEN "11" => - - define signals in time step T3
CASE I IS
...
END CASE;
END CASE;
END PROCESS;
reg_0: regn PORT MAP (BusWires, Rin(0), Clock, R0);
. . . instantiate other registers and the adder/subtracter unit
. . . define the bus
END Behavior;
Figure 2b. Skeleton VHDL code for the processor.
LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.std_logic_signed.all;
ENTITY upcount IS
PORT ( Clear, Clock : IN STD_LOGIC;
Q : OUT STD_LOGIC_VECTOR(1 DOWNTO 0));
END upcount;
ARCHITECTURE Behavior OF upcount IS
SIGNAL Count : STD_LOGIC_VECTOR(1 DOWNTO 0);
BEGIN
PROCESS (Clock)
BEGIN
IF (Clock’EVENT AND Clock = ’1’) THEN
IF Clear = ’1’ THEN
Count <= "00";
ELSE
Count <= Count + 1;
END IF;
END IF;
END PROCESS;
Q <= Count;
END Behavior;
Figure 2c. Subcircuit entities for use in the processor.
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LIBRARY ieee;
USE ieee.std_logic_1164.all;
ENTITY dec3to8 IS
PORT ( W : IN STD_LOGIC_VECTOR(2 DOWNTO 0);
En : IN STD_LOGIC;
Y : OUT STD_LOGIC_VECTOR(0 TO 7));
END dec3to8;
ARCHITECTURE Behavior OF dec3to8 IS
BEGIN
PROCESS (W, En)
BEGIN
IF En = ’1’ THEN
CASE W IS
WHEN "000" => Y <= "10000000";
WHEN "001" => Y <= "01000000";
WHEN "010" => Y <= "00100000";
WHEN "011" => Y <= "00010000";
WHEN "100" => Y <= "00001000";
WHEN "101" => Y <= "00000100";
WHEN "110" => Y <= "00000010";
WHEN "111" => Y <= "00000001";
END CASE;
ELSE
Y <= "00000000";
END IF;
END PROCESS;
END Behavior;
LIBRARY ieee;
USE ieee.std_logic_1164.all;
ENTITY regn IS
GENERIC (n : INTEGER := 16);
PORT ( R : IN STD_LOGIC_VECTOR(n-1 DOWNTO 0);
Rin, Clock : IN STD_LOGIC;
Q : BUFFER STD_LOGIC_VECTOR(n-1 DOWNTO 0));
END regn;
ARCHITECTURE Behavior OF regn IS
BEGIN
PROCESS (Clock)
BEGIN
IF Clock’EVENT AND Clock = ’1’ THEN
IF Rin = ’1’ THEN
Q <= R;
END IF;
END IF;
END PROCESS;
END Behavior;
Figure 2d. Subcircuit entities for use in the processor.
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Figure 3. Simulation of the processor.
Part II
In this part you are to design the circuit depicted in Figure 4, in which a memory module and counter are connected
to the processor from Part I. The counter is used to read the contents of successive addresses in the memory, and
this data is provided to the processor as a stream of instructions. To simplify the design and testing of this circuit
we have used separate clock signals, PClock and MClock, for the processor and memory.
Processor
Memory
Counter
16
n Bus Bus
16
addr data DIN
Done Done
Resetn
Run
MClock
PClock
Resetn
Run
Figure 4. Connecting the processor to a memory and counter.
1. Create a new Quartus II project which will be used to test your circuit.
2. Generate a top-level VHDL file that instantiates the processor, memory, and counter. Use the Quartus II
MegaWizard Plug-In Manager tool to create the memory module from the Altera library of parameterized
modules (LPMs). The correct LPM is found under the storage category and is called ALTSYNCRAM. Follow
the instructions provided by the wizard to create a memory that has one 16-bit wide read data port and is 32
6
words deep. The first screen of the wizard is shown in Figure 5. Since this memory has only a read port,
and no write port, it is called a synchronous read-only memory (synchronous ROM). Note that the memory
includes a register for synchronously loading addresses. This register is required due to the design of the
memory resources on the Cyclone II FPGA; account for the clocking of this address register in your design.
To place processor instructions into the memory, you need to specify initial values that should be stored in
the memory once your circuit has been programmed into the FPGA chip. This can be done by telling the
wizard to initialize the memory using the contents of a memory initialization file (MIF). The appropriate
screen of the MegaWizard Plug-In Manager tool is illustrated in Figure 6. We have specified a file named
inst_mem.mif, which then has to be created in the directory that contains the Quartus II project. Use the
Quartus II on-line Help to learn about the format of the MIF file and create a file that has enough processor
instructions to test your circuit.
3. Use functional simulation to test the circuit. Ensure that data is read properly out of the ROM and executed
by the processor.
4. Make sure your project includes the necessary port names and pin location assignments to implement the
circuit on the DE2 board. Use switch SW 17 to drive the processor’s Run input, use KEY 0 for Resetn, use
KEY1 for MClock, and use KEY 2 for PClock. Connect the processor bus wires to LEDR 15−0 and connect
the Done signal to LEDR 17 .
5. Compile the circuit and download it into the FPGA chip.
6. Test the functionality of your design by toggling the switches and observing the LEDs. Since the circuit’s
clock inputs are controlled by push button switches, it is easy to step through the execution of instructions
and observe the behavior of the circuit.
Figure 5. ALTSYNCRAM configuration.
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Figure 6. Specifying a memory initialization file (MIF).
Enhanced Processor
It is possible to enhance the capability of the processor so that the counter in Figure 4 is no longer needed, and
so that the processor has the ability to perform read and write operations using memory or other devices. These
enhancements involve adding new instructions to the processor and the programs that the processor executes are
therefore more complex. Since these steps are beyond the scope of some logic design courses, they are described
in a subsequent lab exercise available from Altera.
Copyright 2006
c Altera Corporation.
