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Updated alu to feed only positive values to multiply block

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When both values are positive/negative the positive result from the multiply block is correct. When only one is negative, the result is inverted.
This commit is contained in:
Aadi Desai 2020-06-09 20:34:29 +01:00
parent 12da9b94a5
commit fefcad13ce
1 changed files with 49 additions and 13 deletions
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62
alu.v
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@ -1,4 +1,4 @@
module alu (enable, Rs1, Rs2, Rd, opcode, mulresult, exec2, stackout, mul1, mul2, Rout, jump, carry);
module alu (enable, Rs1, Rs2, Rd, opcode, mulresult, exec2, stackout, mul1, mul2, Rout, jump, carry, jumpflags);
input enable; // active LOW, disables the ALU during load/store operations so that undefined behaviour does not occur
input signed [15:0] Rd; // input destination register
@ -14,6 +14,7 @@ output reg signed [15:0] mul2; // second number to be multiplied
output signed [15:0] Rout; // value to be saved to destination register
output jump; // tells decoder whether Jump condition is true
output reg carry; // Internal carry register that is updated during appropriate opcodes, also provides output for debugging
output [7:0] jumpflags;
reg signed [16:0] alusum; // extra bit to hold carry from operations other than Multiply
assign Rout = alusum [15:0];
@ -30,8 +31,9 @@ assign JC5 = (Rs1 >= Rs2);
assign JC6 = (Rs1 <= Rs2);
assign JC7 = (Rs1 != Rs2);
assign JC8 = (Rs1 < 0);
assign jumpflags = {JC1, JC2, JC3, JC4, JC5, JC6, JC7, JC8};
always @(*)
always @(opcode, mulresult)
begin
if(!enable) begin
case (opcode)
@ -87,31 +89,65 @@ always @(*)
6'b011100: begin // MUL Multiply (Rd = Rs1 * Rs2)
if(!exec2) begin
mul1 = Rs1;
mul2 = Rs2;
if(Rs1[15]) begin
mul1 = ~Rs1 + {16'h0001};
end
else begin
mul1 = Rs1;
end
if(Rs2[15]) begin
mul2 = ~Rs2 + {16'h0001};
end
else begin
mul2 = Rs2;
end
alusum = 17'b00000000000000000;
carry = (Rs1[15]^Rs2[15]) ? 1'b1 : 1'b0;
end
else begin
alusum[16] = 1'b0;
{mulextra, alusum[15:0]} = mulresult;
{mulextra, alusum[15:0]} = (carry) ? ~mulresult + 32'h00000001 : mulresult;
end
end
6'b011101: begin // MLA Multiply and Add (Rd = Rs2 + (Rd * Rs1))
if(!exec2) begin
mul1 = Rs1;
mul2 = Rs2;
if(Rd[15]) begin
mul1 = ~Rd + {16'h0001};
end
else begin
mul1 = Rd;
end
if(Rs1[15]) begin
mul2 = ~Rs1 + {16'h0001};
end
else begin
mul2 = Rs1;
end
alusum = 17'b00000000000000000;
carry = (Rs1[15]^Rs2[15]) ? 1'b1 : 1'b0;
end
else begin
alusum[16] = 1'b0;
{mulextra, alusum[15:0]} = mulresult + {16'h0000, Rs2};
{mulextra, alusum[15:0]} = (carry) ? ~mulresult + 32'h00000001 + {16'h0000, Rs2} : mulresult + {16'h0000, Rs2};
end
end
6'b011110: begin // MLS Multiply and Subtract (Rd = Rs2 - (Rd * Rs1)[15:0])
if(!exec2) begin
mul1 = Rs1;
mul2 = Rs2;
if(Rd[15]) begin
mul1 = ~Rd + {16'h0001};
end
else begin
mul1 = Rd;
end
if(Rs1[15]) begin
mul2 = ~Rs1 + {16'h0001};
end
else begin
mul2 = Rs1;
end
alusum = 17'b00000000000000000;
carry = (Rs1[15]^Rs2[15]) ? 1'b1 : 1'b0;
end
else begin
alusum = {1'b0, Rs2 - mulresult[15:0]};
alusum = (carry) ? {1'b0, Rs2 - (~mulresult[15:0] + 16'h0001)} : {1'b0, Rs2 - mulresult[15:0]};
end
end
6'b011111: alusum = mulextra; // MRT Retrieve Multiply MSBs (Rd = MSBs)