ECS 154A: Homework 4 Solved

Logisim
    1. Use the version from the class Google Drive of Logisim Evolution. Other versions may not work correctly.  
    2. Do not rename the files you receive. If you do so you will automatically fail the tester when you submit.
    3. Put your solution for each problem into implementation subcircuit 
    4. Do not rename the implementation subcircuit anything else. If you do so you will automatically fail the tester when you submit.
    5. Do not change the appearance of the implementation subcircuit from what it is set as. Doing so will cause you to automatically fail the tester when you submit. 
        a. That is this field right here 
    6. Do not move the pins inside of the implementation subcircuit as that affects the appearance of the circuit on the outside as you saw in discussion. Doing so will cause you to automatically fail the tester when you submit. 
        a. If you want to “move the pins” instead connect tunnels to the pins and move the tunnels around. 
    7. Do not name any of the subcirucits in your solution main. Doing so will cause you to automatically fail the tester when you submit. 
    8. You can create as many other subcircuits as you want in your solution. Just make sure your solution ends up in the implementation subcircuit 

Restrictions
For all problems in this homework, you may only use
    • All of the components under Wiring
    • All of the components under Gates EXCEPT for Controlled Buffer, Controlled Inverter, PLA
    • All of the components under Plexers
    • All of the components under Arithmetic
    • All of the components under Memory EXCEPT for RAM, ROM, and Random Generator
Unless a problem specifies otherwise.

You have been provided a Register File circuit in the starting circuit. The only outputs of the register file that you are allowed to use are A_Out and B_Out. The rest are for testing purposes and should not be used. Using them will result in a 50% penalty in your grade. 
Problem 1: CPU.circ (100 points)
Build a 4-bit single cycle CPU that can implement the given instructions.

Instruction Format
Our CPU will be using fixed length instructions. Our CPU will also have two types of instruction formats: R-type and I-type. In R-type instructions both operands come from registers. In I-type instructions, the first operand comes from a register and the second will be contained within the instruction.
R-Type

Name
Bits
Description
OpCode
15 - 12
Determines what operation should be performed
C
11 - 8
The destination register. The C in RegC = RegA OP RegB 
A
7 - 4
The first source register. The A in RegC = RegA OP RegB
B
3 - 0
The second source register. The B in RegC = RegA OP RegB


I-Type

Name
Bits
Description
OpCode
15 - 12
Determines what operation should be performed
C
11 - 8
The destination register. The C in RegC = RegA OP Imm 
A
7 - 4
The first source register. The A in RegC = RegA OP Imm
Immediate
3 - 0
The second source register. The Imm in RegC = RegA OP Imm

Instructions

Operation
Encoding (The value in the OpCodeField)
Description
STOP
0000
The CPU ceases execution
NOP
0001
Do nothing
LOAD
0010
RegC = Immediate
MOVE
0011
RegC = RegA
ANDR
0100
RegC = RegA AND RegB
ANDI
0101
RegC = RegA AND Immediate
ORR
0110
RegC = RegA OR RegB
ORI
0111
RegC = RegA OR Immediate
XORR
1000
RegC = RegA XOR RegB
XORI
1001
RegC = RegA XOR Immediate
NOT
1010
RegC = NOT RegA
NEGATE
1011
RegC = -RegA 
ADDR
1100
RegC = RegA + RegB
ADDI
1101
RegC = RegA +Immediate
SUBR
1110
RegC = RegA - RegB
SUBI
1111
RegC = RegA - Immediate


Inputs
Pin
Size (in bits)
Explanation
Instruction
16
The instruction located at Instruction_Address
ClkIn
1
The Clock. Connect this to the clock ports of your registers/flip-flops. Do nothing else with this.
Outputs
Pin
Size (in bits)
Explanation
Instruction_Address_Out
5
The address of the instruction you want to execute
Reg0-15
4
The values in the register file. This has already been connected for you
CPU Components
Your CPU should have 
    • A Program Counter (PC)
        ◦ This stores and keeps track of what instruction you are on
    • Instruction Decoder
        ◦ This is a bunch of combinational logic that sets the control signals inside of your CPU
    • Register File
        ◦ A bunch of registers as well as ways to specify which ones you want. This has already been created for you. 
        ◦ The only outputs of the register file that you are allowed to use are A_Out and B_Out. The rest are for testing purposes and should not be used. Using them will result in a 50% penalty in your grade.  

Testing
Testing for this problem is different than for previous assignments but is more similar to sequential circuits than combinational circuits. After you finish building your circuit and are ready to test it
    1. Open the associated grader circuit
    2. Scroll down on the left and side until you find your circuit. Right-click on it and select Reload Library
        a. 
    3. Use cntrl+t to tick the clock and check that the outputs of the registers are what they are supposed to be as based on the test program below. 
 

The Test Program

Instruction
Meaning
Result
LOAD REG0, 3 
Reg0 = 3
Reg0 = 3
LOAD REG1,6 
Reg1 = 6
Reg1 = 6
NOP
Do Nothing
No Change
MOVE REG2, Reg1
Reg2 = Reg1
 Reg2 = 6 
ANDR REG3, REG0, REG1
REG3 = REG0 AND REG1
REG3 = 3 & 6 = 2
ANDI REG4, REG3,3 
REG4 = REG3 AND 3
REG4 = 2 & 3= 2
ORR REG5, Reg2, REG0
REG5 = Reg2 OR REG0
REG5 = 6 |3 = 7
ORI REG6, Reg3, 12
REG6 = Reg3 OR  12
REG6 = 2 OR  12 = 14
XORR REG7, Reg2, REG0
REG7 = Reg2 XOR REG0
REG7 = 6 ^ 3 = 5
XORI REG8, Reg6, 15
REG8 = Reg6 XOR 15
REG8 =14 ^15 = 1
NEG REG9, REG3
REG9 = -REG3
REG9 = -2
ADDR REG10, Reg7, REG7
REG10 = Reg7 + REG7
REG10 = 5 + 5 = 10
ADDI REG11, Reg1, 3
REG11 = Reg1 + 3
REG11 = 6 + 3 = 9
SUBR REG12, Reg6, REG2
REG12 = Reg6 - REG2
REG12 = 14 - 6 = 8
SUBI REG13, Reg8, 5
REG13 = Reg8 - 5
REG13 = 1 - 5 = -4
NOT REG15, REG8
REG15 = NOT REG8
REG15 = ~1 = 14
ANDR REG1, REG12, REG13
REG1 = REG12 AND REG13
REG1 = 8 & 12 = 8
NEG REG5, REG5
REG5 = -REG5
REG5 = -7
NOP
Do Nothing
No Change
ADDR REG14, REG5, REG2
REG14 = Reg5 + REG2
REG14 = -7 + 6 = -1
XORR REG7, REG7, REG14
REG7 =  Reg7 ^ REG14
REG7 = 5 ^ -1 = 10
ORI REG2, REG2, 3
REG2 = Reg2 | 3
REG2 = 6 | 3 = 7
SUBI REG13, REG13, 12
REG13 = REG13 - 12
REG13 = 12 - 12 = 0
STOP
CPU should cease execution
CPU should cease execution
LOAD REG15, 13
This line should not be executed because the CPU should have STOPped already
This line should not be executed because the CPU should have STOPped already

The test program is just an example program. Your CPU should function on any program given to it. 

Making Fixes to Your Solution
After you make changes to your solution you will need to reload your circuit in the grader circuit. If you don’t it won’t see the updates. To reload your circuit, select your circuit in the grader, right-click it and select Reload Library.
Submitting
Submit to
Logisim Homework 3 on GradeScope. 
What to Submit

Submit a zip file that contains the following .circ files
    1. CPU.circ

Inside of each .circ file leave a comment with you and your partner’s names. 

Make sure that you submit a zip that contains the files and NOT the folder containing the files. Check out the animation below for what to submit.