Logisim 4

Logisim 4
Points: 16
In this problem you will be implementing a 4 bit CPU. This means that the size of the data path will be
4 bits big.
Instruction Format
We will be using fixed length instruction formats for this problem. This means that all instructions will
be the same size. We will be using 2 different instruction formats for this problem, R-type and I-type.
In R-type instructions both operands will come from the registers. In I-type instructions the first
operand will come from the register file and the second will be contained in the instruction.
R-Type
Name Bits Description
OpCode 12 - 15 Determines what operation
should be performed
C 8 - 11 The destination register. The C in
the formula C = A OP B
A 4 - 7 The first source register. The A
in the formula C = A OP B
B 0-3 The second source register. The
B in the formula C = A OP B
I-Type
Name Bits Description
OpCode 12 - 15 Determines what operation
should be performed
C 8 - 11 The destination register. The C in
the formula C = A OP Imm
A 4 - 7 The first source register. The A
in the formula C = A OP Imm
Immediate 0-3 The immediate value. The Imm
in the formula C = A OP Imm
Instructions
Operation Encoding(The value in the
OpCode field)
Description
HALT 0000 The CPU ceases execution
NOP 0001 Do nothing
LOAD 0010 RegC = Immediate
MOVE 0011 RegC
= Reg A
ANDR 0100 RegC
= Reg A
AND
Reg B
ANDI 0101 RegC
= Reg A AND
Immediate
ORR 0110 RegC
= Reg A
OR
Reg B
ORI 0111 RegC
= Reg A
OR
Immediate
XORR 1000 RegC
= Reg A
XOR
Reg B
XORI 1001 RegC = Reg A
XOR
Immediate
NOT 1010 RegC
= NOT Reg A
NEGATE 1011 RegC
= - Reg A
ADDR 1100 RegC
= Reg A
+ Reg B
ADDI 1101 RegC
= Reg A
+
Immediate
SUBR 1110 RegC
= Reg A
- Reg B
SUBI 1111 RegC
= Reg A
- Immediate
CPU Components
1. Memory.
1. We will be using a ROM module to store our instruction. The address bit width will be 5
bits wide and the data bit width will be 16 bits wide
2. This is already given to you and comes preloaded with the testing program
2. Register File
1. A collection of 16 registers numbered 0 through 15.
2. This has already been given to you so you don't have to implement it
3. ALU
1. This is where you actually execute the instruction
4. Decoder
1. A bunch of combinational logic that will enable or disable the appropriate control signals in
your CPU
Testing
You have also been give a file called CPU_Inputs.py. This contains the program that you can use to
create test programs. If you look inside it you will find the solutions for the testing program that is
being used. We will check your answers by looking inside your register file and making sure they have
the right values.