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alu.sv
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enum [4:0]
{
//ALUOP_PASS_A,
//ALUOP_PASS_B,
ALUOP_ADD = 5'd0,
ALUOP_OR = 5'd1,
ALUOP_ADDC = 5'd2,
ALUOP_SUBC = 5'd3,
ALUOP_AND = 5'd4,
ALUOP_SUB = 5'd5,
ALUOP_XOR = 5'd6,
ALUOP_CMP = 5'd7,
ALUOP_ROL = 5'd8,
ALUOP_ROR = 5'd9,
ALUOP_ROLC = 5'd10,
ALUOP_RORC = 5'd11,
ALUOP_SHL = 5'd12,
ALUOP_SHR = 5'd13,
ALUOP_SHLA = 5'd14, // Does not exist, just ALUOP_SHL
ALUOP_SHRA = 5'd15,
ALUOP_INC = 5'd16,
ALUOP_DEC = 5'd17,
ALUOP_NEG = 5'd18
} AluOp;
enum [2:0]
{
ALU_FLAG_AC, // Auxiliary carry flag
ALU_FLAG_CY, // Carry flag
ALU_FLAG_V, // Overflow flag
ALU_FLAG_P, // Parity flag
ALU_FLAG_S, // Sign flag
ALU_FLAG_Z // Zero flag
} AluFlags;
module alu
(
input [4:0] alu_op,
input size,
input [15:0] A,
input [15:0] B,
output reg [15:0] R,
output reg [5:0] flags
);
function [15:0] rol
(
input byte_or_word,
input [15:0] x, input [15:0] s
);
if(byte_or_word == 0)
begin
case(s[3:0])
1:
rol = {x[14:0], x[7]};
2:
rol = {x[13:0], x[7:6]};
3:
rol = {x[12:0], x[7:5]};
4:
rol = {x[11:0], x[7:4]};
5:
rol = {x[10:0], x[7:3]};
6:
rol = {x[9:0], x[7:2]};
7:
rol = {x[8:0], x[7:1]};
default:
rol = x;
endcase
end
else
begin
case(s[4:0])
1:
rol = {x[14:0], x[15]};
2:
rol = {x[13:0], x[15:14]};
3:
rol = {x[12:0], x[15:13]};
4:
rol = {x[11:0], x[15:12]};
5:
rol = {x[10:0], x[15:11]};
6:
rol = {x[9:0], x[15:10]};
7:
rol = {x[8:0], x[15:9]};
8:
rol = {x[7:0], x[15:8]};
9:
rol = {x[6:0], x[15:7]};
10:
rol = {x[5:0], x[15:6]};
11:
rol = {x[4:0], x[15:5]};
12:
rol = {x[3:0], x[15:4]};
13:
rol = {x[2:0], x[15:3]};
14:
rol = {x[1:0], x[15:2]};
15:
rol = {x[0], x[15:1]};
default:
rol = x;
endcase
end
endfunction
function [15:0] ror
(
input byte_or_word,
input [15:0] x, input [15:0] s
);
if(byte_or_word == 0)
begin
case(s[3:0])
1:
ror = {x[15:8], x[0], x[7:1]};
2:
ror = {x[15:8], x[1:0], x[7:2]};
3:
ror = {x[15:8], x[2:0], x[7:3]};
4:
ror = {x[15:8], x[3:0], x[7:4]};
5:
ror = {x[15:8], x[4:0], x[7:5]};
6:
ror = {x[15:8], x[5:0], x[7:6]};
7:
ror = {x[15:8], x[6:0], x[7]};
default:
ror = x;
endcase
end
else
begin
case(s[4:0])
1:
ror = {x[0], x[15:1]};
2:
ror = {x[1:0], x[15:2]};
3:
ror = {x[2:0], x[15:3]};
4:
ror = {x[3:0], x[15:4]};
5:
ror = {x[4:0], x[15:5]};
6:
ror = {x[5:0], x[15:6]};
7:
ror = {x[6:0], x[15:7]};
8:
ror = {x[7:0], x[15:8]};
9:
ror = {x[8:0], x[15:9]};
10:
ror = {x[9:0], x[15:10]};
11:
ror = {x[10:0], x[15:11]};
12:
ror = {x[11:0], x[15:12]};
13:
ror = {x[12:0], x[15:13]};
14:
ror = {x[13:0], x[15:14]};
15:
ror = {x[14:0], x[15]};
default:
ror = x;
endcase
end
endfunction
function parity(input [15:0] x);
parity = ~(x[0] ^ x[1] ^ x[2] ^ x[3] ^ x[4] ^ x[5] ^ x[6] ^ x[7]);
endfunction
// @question: When size == 0, do we modify the contents of the upper byte?
// Does it matter at all if we write back only the lower byte anyway?
// I would guess not.
// @question: Is it better to use non-blocking assigns and set flags based
// strictly on the input data, or using blocking assignments with extended
// by-1-bit data and use the result for the carry?
// @todo: What's the correct way to handle 0 shifts?
wire [3:0] msb = (size == 0)? 4'd7: 4'd15;
always_comb
begin
case(alu_op)
ALUOP_AND:
begin
R = B & A;
flags[ALU_FLAG_CY] = 0;
flags[ALU_FLAG_V] = 0;
flags[ALU_FLAG_Z] = (R == 0);
flags[ALU_FLAG_P] = parity(R);
flags[ALU_FLAG_S] = R[msb];
end
ALUOP_INC,
ALUOP_ADD:
begin
if(alu_op == ALUOP_ADD)
{flags[ALU_FLAG_CY], R} = {1'b0, B} + {1'b0, A};
else
R = B + A;
flags[ALU_FLAG_V] = (A[msb] == B[msb]) && (R[msb] != B[msb]);
// can we do this? flags[ALU_FLAG_V] = (R[msb] == flags[ALU_FLAG_CY]);
flags[ALU_FLAG_Z] = (R == 0);
flags[ALU_FLAG_P] = parity(R);
flags[ALU_FLAG_S] = R[msb];
end
ALUOP_DEC,
ALUOP_CMP,
ALUOP_SUB:
begin
R = B - A;
if(alu_op != ALUOP_DEC)
flags[ALU_FLAG_CY] = (B < A);
flags[ALU_FLAG_V] = (A[msb] != B[msb]) && (R[msb] != B[msb]);
flags[ALU_FLAG_Z] = (R == 0);
flags[ALU_FLAG_P] = parity(R);
flags[ALU_FLAG_S] = R[msb];
end
ALUOP_XOR:
begin
R = B ^ A;
flags[ALU_FLAG_CY] = 0;
flags[ALU_FLAG_V] = 0;
flags[ALU_FLAG_P] = parity(R);
flags[ALU_FLAG_Z] = (R == 0);
flags[ALU_FLAG_S] = R[msb];
end
ALUOP_ROL:
begin
R = rol(size, B, A);
flags[ALU_FLAG_CY] = R[msb];
if(A[msb] == R[msb]) flags[ALU_FLAG_V] = 0;
end
ALUOP_ROR:
begin
R = ror(size, B, A);
flags[ALU_FLAG_CY] = R[0];
if(A[msb] == R[msb]) flags[ALU_FLAG_V] = 0;
end
ALUOP_SHL:
begin
if(A == 1)
begin
R = {B[14:0], 1'b0};
flags[ALU_FLAG_CY] = B[msb];
if(B[msb] == B[msb-1]) flags[ALU_FLAG_V] = 0;
end
else
begin
R = (B << A[4:0]);
if(A > 0) flags[ALU_FLAG_CY] = B[msb - A[4:0] + 1];
end
flags[ALU_FLAG_Z] = (R == 0);
flags[ALU_FLAG_S] = R[msb];
flags[ALU_FLAG_P] = parity(R);
end
ALUOP_SHR:
begin
if(A == 1)
begin
R = (size == 0)?
{B[15:8], 1'b0, B[7:1]}:
{1'b0, B[15:1]};
flags[ALU_FLAG_CY] = B[0];
if(B[msb] == 0) flags[ALU_FLAG_V] = 0;
end
else
begin
R = (B >> A[4:0]);
if(A > 0) flags[ALU_FLAG_CY] = B[A[4:0]-1];
end
flags[ALU_FLAG_Z] = (R == 0);
flags[ALU_FLAG_S] = R[msb];
flags[ALU_FLAG_P] = parity(R);
end
default:
R = 16'hFACE;
endcase
end
endmodule