Add basic ALU skeleton

Signed-off-by: Warrick Lo <wlo@warricklo.net>

2 files changed, 89 insertions, 0 deletions

diff --git a/rtl/alu.sv b/rtl/alu.sv
new file mode 100644
index 0000000..f5dde19
--- /dev/null
+++ b/rtl/alu.sv
@@ -0,0 +1,61 @@
+module alu
+  import montreal_pkg::*;
+#(
+  parameter int unsigned XLEN        = config_pkg::XLEN,
+  parameter int unsigned SLICE_WIDTH = config_pkg::SLICE_WIDTH,
+
+  localparam int unsigned SLICE_ADDR_WIDTH = $clog2(XLEN / SLICE_WIDTH)
+) (
+  input logic   clk_i,
+  input logic   rst_ni,
+  input fu_op_t alu_op_i,
+
+  input logic [SLICE_ADDR_WIDTH-1:0] count_i,
+
+  input slice_t a_i,
+  input slice_t b_i,
+
+  output slice_t result_o,
+  output logic   carry_o
+);
+
+  logic negate_b;
+
+  /* We used Karnaugh maps here. If the encoding for fu_op_t changes, this has to be updated. */
+  assign negate_b = alu_op_i[3] || (!alu_op_i[2] && alu_op_i[1]);
+
+  /* Adder width is SLICE_WIDTH + 1 to account for carry-out. */
+  logic [SLICE_WIDTH:0] adder_a, adder_b, adder_result;
+
+  assign adder_a = {1'b0, a_i};
+  assign adder_b = {1'b0, (b_i ^ {SLICE_WIDTH{negate_b}})};
+
+  /* Carry signals. */
+  logic carry, carry_d, carry_q;
+
+  assign carry = (count_i == '0) ? negate_b : carry_q;
+
+  /* Combinational arithmetic/logic core. */
+  always_comb begin : alu_core
+    unique casez (alu_op_i[2:0])
+      /* ADD, SUB, SLT, SLTU. */
+      3'b0??: begin end
+      /* XOR. */
+      3'b100: begin end
+      /* OR. */
+      3'b110: begin end
+      /* AND. */
+      3'b111: begin end
+      default: begin end
+    endcase
+  end : alu_core
+
+  always_ff @(posedge clk_i) begin : carry_ff
+    if (!rst_ni) begin
+      carry_q <= '0;
+    end else begin
+      carry_q <= carry_d;
+    end
+  end : carry_ff
+
+endmodule : alu
diff --git a/rtl/montreal_pkg.sv b/rtl/montreal_pkg.sv
new file mode 100644
index 0000000..741a926
--- /dev/null
+++ b/rtl/montreal_pkg.sv
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: CERN-OHL-P-2.0 */
+
+package montreal_pkg;
+
+  typedef logic unsigned [config_pkg::XLEN-1:0]        word_t;
+  typedef logic unsigned [config_pkg::SLICE_WIDTH-1:0] slice_t;
+
+  typedef enum logic [3:0] {
+    /* Arithmetic operations. */
+    ADD       = 4'b0000,
+    SUB       = 4'b1000,
+    /* Logical operations. */
+    XOR       = 4'b0100,
+    OR        = 4'b0110,
+    AND       = 4'b0111,
+    /* Shift operations. */
+    SLL       = 4'b0001,
+    SRL       = 4'b0101,
+    SRA       = 4'b1101,
+    /* Conditional set operations. */
+    SLT       = 4'b0010,
+    SLTU      = 4'b0011,
+    /* Zicond operations. */
+    CZERO_EQZ = 4'b1001,
+    CZERO_NEZ = 4'b1011
+  } fu_op_t;
+
+endpackage : montreal_pkg