LAB 6: Elementary CPU Design

EEL 3701 
Department of Electrical & Computer Engineering 
Page 1/5 Revision 2
LAB 6: Elementary CPU Design: Fetching Instructions From a ROM
OBJECTIVE
The objective of this lab is to continue the design of an
elementary central processing unit (CPU) that was started in
Lab 4. In part 1 of this lab, a 2-bit instruction field will be
used to control a simple state machine that in turn will be
used to set the MUX lines in the ALU according to what type
of instruction is designated for execution. In part 2, a 3-bit
instruction field will be used. You will also add a program
counter (PC) and a memory module to store the instructions
of a “program” to be executed by the CPU.
MATERIALS
• Your entire 3701 kit including your PLD PCB Wires,
Resistor packages, Switches (including your debounced
switch circuit), LEDs, DAD/NAD
• VHDL ROM Alternative files:
o Instructions: VHDL_ROM.pdf
o Quartus archive file, Board_ROM.qar
o Excel spreadsheet, ROM_contents.xlsx
INTRODUCTION - LAB 4 ALU MUX SIGNALs
The ALU designed in Lab 4 consisted of four 4-input
MUXs on the inputs of REGA and four 4-input MUXs on
the inputs of REGB. The select lines for these MUXs
were designated MSA1:0 and MSB1:0, respectively. For
a quick review, the MUXs selected a bus as shown in
Table 1.
The outputs of REGA and REGB were then passed to a
combinatorial logic block and the results of this were then
passed to four 8-input MUXs. The select lines for these
four MUXs were designated as MSC2:0. For review
purposes, these (3) lines selected the functions shown in
Table 2.
Modify your lab 4 ALU design only if it did not work.
We’ll call this the Lab 4* ALU.
PART 1 INTRODUCTION: 1st ALU CONTROLLER
A state machine controller and Instruction Register (IR) are
now added to the Lab 4* ALU to facilitate the execution of
simple instructions. See Figure 1 for the total system
components of this section. The IR register contains 2 bits
that represent the four instructions shown in Table 3. Note
that the right shift is NOT sign-extended, i.e., a zero is
shifted into the most significant bit of REGA with the SRA
instruction. In this part of the lab, you will ultimately use
Quartus to make the project LAB6_Part1.
The flowchart (NOT an ASM) for the controller is shown in
Figure 2. All instructions execute in one cycle (plus one cycle
to load the IR register). I strongly encourage you to use
VHDL for the combinatorial part of the controller.
Instruction Register Design
The IR is clocked like a typical bank of D Flip-Flops,
however, it has a new feature; it can be loaded or not loaded
depending on “IR.LD”. When IR.LD is true, data is loaded
into the register and when IR.LD is false, new data is not
loaded into the register (hold condition). This register can be
simply realized with a 2-input MUX on the input of each flipflops of the IR. When a 2-input MUX select line is low,
select an IR output to pass through the MUX back into a DFF input; when the select line is high, an INPUT bus signal
should pass through a MUX and into a D-FF input.
PART 1 PRE-LAB REQUIREMENTS
1. Use the flowchart shown in Figure 2 to help you create
an ASM chart. The ASM’s outputs include the MUX
select signals (instead of the description of actions).
2. Create a next state truth table. If you use the graphic
design editor (block diagram/schematic file) for
schematic entry in Quartus to create your controller, you
must make K-maps and simplified logic equations for
the controller. If you use VHDL (and external flip-flops)
to create the controller, you do not need to make K-maps
or simplify the equations.
3. Using the block diagram/schematic editor in Quartus,
add the IR and controller circuitry to your Lab 4* ALU.
4. Simulate and test all instructions created in the controller
circuitry. As always, annotate your design simulation.
5. Turn in all the documents described above as stated in
the Lab Rules and Policies document. (Submit the
Quartus archive file LAB6_Part1.qar). Re-read, if
necessary. Documents must be submitted through
Canvas for every lab. All pre-lab material is to be
submitted BEFORE the beginning of your lab.
MSA1/
MSB1
MSA0/
MSB0
Bus Selected as Input
to REGA/REGB
0 0 INPUT Bus
0 1 REGA Output Bus
1 0 REGB Output Bus
1 1 OUTPUT Bus
Table 1. MUX A and B settings.
IR1:0 Action Instruction
00 Load INPUT bus = REGA LDAA #In
01 Move REGA contents = REGB TAB
10 Right Shift REGA 1 bit = REGA SRA
11 Sum REGA & REGB = REGA SUM_BA
Table 3. Part 1 instructions.
MSC2:0 Action
000 REGA Bus to OUTPUT Bus
001 REGB Bus to OUTPUT Bus
010 complement of REGA Bus to OUTPUT Bus
011 bit wise AND REGA/REGB Bus to OUTPUT Bus
100 bit wise OR REGA/REGB Bus to OUTPUT Bus
101 sum of REGA Bus & REGB Bus to OUTPUT Bus
110 shift REGA Bus left one bit to OUTPUT Bus
111
shift REGA Bus right one bit to OUTPUT Bus without
sign extension
Table 2. MUX C settings.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering 31-Oct-19
Page 2/5 Revision 2
LAB 6: Elementary CPU Design: Fetching Instructions From a ROM
PART 1 PRE-LAB QUESTIONS
1. Why did we require the new instruction register in this
design?
2. In this section of the lab you are setting the INPUT bus
by hand. If you wanted to read or fetch this value from
memory, what could you add to do this automatically for
you every CLK cycle?
3. How would you add more instructions (i.e., 8 instead of
4) to the controller?
PART 1 IN-LAB REQUIREMENTS
None. You do not need to build the circuit of part 1.
HELPFUL HINTS
Debug as you design for a much better chance of success.
When something goes wrong, i.e., when a design does not
work as expected, don’t panic! Think of some experiments
that you can do to break the problem down into pieces in
order to isolate the error. A useful tool for debugging a
design is to add outputs for some of the internal signals, i.e.,
signals that are neither outputs nor inputs of your design.
This will allow you to “peer inside” a design both in
simulation and with the actual hardware.
IR1:0
2
Figure 1. Block diagram of system components.
LAB 4* ALU
INPUT3:0
Controller
MSA1:0
MSB1:0
MSC2:0
IR
Clock
IR.LD
IR.LD
INPUT1:0
OUT3:0
RegA3:0
RegB3:0
Cin Cout
Note: When not specified, the
default actions for each state is to
“hold” REGA and REGB and
Output=REGA.
Figure 2. Controller flowchart (not an ASM)
IR.LD
(INPUT = next op code)
IR1, IR0 00 01 10 11
Set IR.LD to load IR
register with op code
at the next active
clock edge.
Decoded op codes
Set MUXs to move
REGA = REGB
TAB
(INPUT = data)
Set MUXs to load
INPUT Data =
REGA
LDAA #In
Set MUXs to sum
REGA+REGB =
REGA
SUM_BA
Set MUXs to right
shift
REGA = REGA
SRA
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering 31-Oct-19
Page 3/5 Revision 2
LAB 6: Elementary CPU Design: Fetching Instructions From a ROM
PART 2 INTRODUCTION: 2nd ALU CONTROLLER
The main difference between parts 1 and 2 of the lab is in the
way the inputs are generated. In part 1, you input the op
codes (i.e., 00, 01, 10, or 11) and data manually. The op
codes and inputs were entered between every active clock
transition with the switches at INPUT3:0. In this section
(part 2), the op code and data will be stored in memory. Your
controller will control the signals in such a manner that the
op code and data are automatically fetched from memory;
the outputs of this memory are inputs INPUT3:0. A program
counter (PC) will coordinate the sequencing of the
instruction by stepping through the addresses in an
appropriate manner. In this part of the lab, you will
ultimately use Quartus to make the project LAB6_Part2 and
then LAB6_Part2_VHDL_ROM).
SPECIFICATIONS
No changes will be made to MUXA, MUXB, MUXC, REGA
or REGB from the Lab 4* ALU.
1. As shown in Figure 3, a 32k × 8 EEPROM (or Flash) is
added. The instructions and data will be stored in this
EEPROM starting in location $3E70. (See Pre-Lab
question 1.)
2. A program counter (PC) is added. PC is a 4-bit upcounter with a synchronous count enable signal
(PC_INC). If PC_INC is TRUE, the counter will
increment by 1 at the next active clock transition. If
PC_INC is FALSE, the counter holds its current value.
The count after 1111 is 0000. Another synchronous
signal, PC_LD, is used to load the counter from the
INPUT bus. The 74’161 (with asynchronous clear) is
ideally suited to function as a 4-bit PC.
3. The instruction register is increased to 3 bits (IR2:0),
(from part 1 of this lab) with the following definition:
Changes to the ASM chart:
1. All the manual switching that you would need to do if
part 1 of this lab was built (e.g., setting the INPUT =
next op code or data) can be better accomplished by
incrementing the address on the ROM. The ROM will
have the information that (in part 1 of this lab) would
have come from switches. This is accomplished by
incrementing the PC register or “Inc PC,” as shown in
the Figure 4 flowchart.
2. Notice that an additional state is necessary for the
LDAA instruction in order to read the memory a second
time to obtain the data to placed in register A.
3. The instruction SUM_BA from part 1 is now spelled
ABA (which stands for add A to B and put the result into
A). Shift register A right (SRA) from part 1 is also
spelled differently here, now as SAR. In both parts, sign
extension is not used for this shift.
4. The first new instruction is called “JMP Addr”. JMP
Addr consists of two nibbles where the first is the
opcode and the second is an address. This instruction
forces the PC to load a 4-bit address read from memory
(2nd nibble).
5. The other new instruction is a shift register A left,
spelled SAL. A zero should be shifted into the right
most bit with this shift.
MIF FILE CREATION INFORMATION
When you write code in pre-lab part 4, below, you will hand
assemble your code and put it into the “rom_32k.mif” file. A
sample mif file (rom_8k.mif) can be found on the website and
another mif file, rom_1k.mif, in the tutorial. These files can
be used as templates to generate your own file. Key points
related to these files are:
1. The comments are surrounded by “%”symbols. The left
most number represents address space followed by the
hex value to the right. For example:
“memory address” :”memory value” %comment%
2. The last line of code in the “rom_32k.mif,” file (after
your program) should zero out the remaining data in
ROM. In the rom_8k.mif file, the end of memory was
filled with $FF using the line below:
[8A..1FFF] : 00;
Your last line will be
[XX..7FFF] : 0;
In the above, XX represents the next address after the last
address of your code. This will initialize all your
remaining unused memory to a known value of zero.
PRE-LAB REQUIREMENTS
1. Create an ASM chart using the Figure 4 flowchart as an
aide; i.e., put in the actual signals to control the PC, IR,
and the MUXs. Complete the ASM diagram, by also
including the required elements for the SAR and SAL
instructions.
2. Create a next state truth table. If you use the block
diagram/schematic editor in Quartus to create your
controller, you must make K-maps and use simplified
logic equations for the controller. If you use VHDL
(and external flip-flops) to create the controller, you do
not need to make K-maps or simplify the equations.
IR2:0 Instruction Function
000 TAB Copy A to B (transfer A to B)
001 LDAA #data Load A with input data
010 SAR Shift A right 1 bit , store in A
(no sign extension)
011 SAL Shift A left 1 bit, store in A
100 ABA Load A with A plus B plus Cin;
update Cout
101 JMP Addr Load PC with input address
110 Future use
111 Future use
Table 4. Part 2 instructions.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering 31-Oct-19
Page 4/5 Revision 2
LAB 6: Elementary CPU Design: Fetching Instructions From a ROM
3. Add an active-low asynchronous RESET signal to all
registers and counters. You should use this to initialize
all flip-flops to zero before beginning your testing.
(You should design every state machine so that you can
start it in a known state. For this lab, the known state
has state bits of all zeros.)
4. Hand assemble the below program and complete the
table. The successive columns of A and B are for the
changing values of RegA and RegB as the program is
executed and the loop causes the code to repeat.
Addr Mach
Codes A B A B A B A B
$3E70 LDAA #7
TAB
LDAA #3
SAL
ABA
SAR
TAB
JMP 5
LDAA $F
Implement the design in Quartus (LAB6_Part2) and
simulate the execution of this program. You should
start your simulation with a reset.
You can simulate the flash memory portion of your
design in Quartus. Read through the documentation on
ROM Creation Tutorial on our website. Also see the
below section on MIF file creation.
You must make a MIF file for this program. If you did
not simulate a ROM (or flash memory), you would need
to input the op code and data in the *.vwf simulation
waveform file (just as you did in Lab 4), which is much
harder than making the ROM and MIF file. You cannot
compile an entire 32k × 8 EEPROM with any devices
in MAX X family, but you can compile and simulate a
32k x 8 EEPROM if you select the Cyclone V family
and the device at the bottom of the list in Quartus in the
Assignments | Device menu. Use a functional
compilations and simulation. As always, annotate
your design simulation. Outputs should include the
state bits, the registers, INPUT3:0, OUT3:0, IR.LD, IR,
PC, PC_LD, and PC_INC. During debugging, you
might also need to add the MUX select lines.
Note that the memory clock should be at least twice as
fast as the state machine clock in order to assure that
the ROM data is available at the proper time. (See the
ROM Creation Tutorial for more information.)
The simulation technique used above will be used again
in Lab 7.
Although there is a flash ROM on your OOTB PLD PCB, we
will instead use the VHDL ROM alternative procedure
described on the website and put the entire design in Quartus
(LAB6_Part2_VHDL_ROM). In addition to the one done in
part 4, above, you must also do a second simulation with the
VHDL ROM alternative.
5. Download your design to the PLD board. Warning:
Tri-state all unused PLD pins, as described in the
Quartus Tutorial.
6. Appropriately connect the flash, PLD PCB, LEDs, and
switches on your bread board; verify that it functions as
specified in the Pre-Lab Requirements. You will need a
debounced switch for the CLK input.
7. Run the program.
Pre-lab Simulation and Programming Summary:
• Create a 32k ×8 ROM with the program’s machine codes,
using a Cyclone V device.
o Create a MIF file with these machine codes.
o Simulate this design.
• Use the VHDL ROM alternative for the ROM with your
MAX 10. Simulate this design.
o Program this design to your MAX 10 PLD
PART 2 PRE-LAB QUESTION
1. Why do we need the extra states in the LDAA and JMP
instruction paths?
2. What do you need to do to the address lines to get your
program to start at address $4370 (instead of $3E70)?
PART 2 IN-LAB REQUIREMENTS
1. Demo the program (step 12 in the prelab).
2. Write a new program (given to you by your Alpha);
handle assemble the program; program your flash; run
the program.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering 31-Oct-19
Page 5/5 Revision 2
LAB 6: Elementary CPU Design: Fetching Instructions From a ROM