Description
This lab develops the first building block for the Aubie CPU, the arithmetic-logic unit (ALU). It will be combined with the other data path components from later labs, resulting in a complete CPU by Lab 4.
Instructions
Develop VHDL for the following component. You should define an architecture for the ALU entity given below. You should test your architecture by developing simulation files for the entity. Your architecture should implement the functionality described in this document. To make the simulation results more readable, we will use a 32-bit datapath.
You should use the types from the package “dlx_types” (available on the files section of Canvas) and the conversion and math routines from the package “bv_arithmetic” in files bva.vhd, and bva-b.vhd on Canvas. The propagation delay through the ALU should be 15 nanoseconds.
Arithmetic-Logic Unit This unit takes in two 32-bit values, and a 4-bit operation code that specifies which ALU operation (eg. add, subtract, multiply, etc) is to be performed on the two operands. For non commutative operations like subtract or divide, operand1 always goes on the left of the operator and operand2 on the right.
The operation codes are
0000 = unsigned add
0001 = unsigned subtract
0010 = two’s complement add
0011 = two’s complement subtract
0100 = two’s complement multiply
0101 = two’s complement divide
0110 = logical AND
0111 = bitwise AND
1000 = logical OR
1001 = bitwise OR
1010 = logical NOT of operand1 (ignore operand2)
1011 = bitwise NOT of operand1 (ignore operand2)
1100-1111 = just output all zeroes
The unit returns the 32-bit result of the operation and a 4-bit error code. The meaning of the error code should be
0000 = no error
0001 = overflow
0010 = underflow
0011 = divide by zero
The ALU should be sensitive to changes on all the input ports. Note that the ALU is purely combinational logic, no memory, and that there is no clock signal.
The entity declaration is
entity alu is port(operand1, operand2: in dlx_word; operation: in
alu_operation_code;
result: out dlx_word; error: out error_code);
end entity alu;
Deliverables
Please turn in the following things for this lab:
1. The text of your VHDL source code.
2. Your simulation test file. Do not exhaustively test these designs since they take lots of input bits, but do test a reasonable number of things. For example, test both normal function and an error, if applicable (e.g. overflow for addu, nothing for and/or/not) for each of the alu operations.
3. Transcripts of tests or screenshots running your simulations. It should be clear what is being tested. If not, add text to explain it. You cannot test exhaustively but you should test each function code for correct operation and demonstrate how each error code is generated.
