verilog/subleq.v
/**
* @param write If true, data will be written to memory.
* Else, data will be read.
* @param halt If true, the device has entered an single instruction infinite loop.
* No further changes will be made to memory until reset.
* */
module subleq #(
parameter BITS = 1
) (
input wire clock,
input wire reset,
output wire halt,
output wire write,
output wire [BITS-1:0] address,
inout wire [BITS-1:0] data
);
enum reg [1:0] {
STAGE_READ_A,
STAGE_READ_B,
STAGE_READ_C,
STAGE_WRITE
} stage;
reg [BITS-1:0] pc, a, b, c;
reg halt_reg;
wire signed [BITS-1:0] b_next;
assign address =
(stage == STAGE_READ_A) ? pc + BITS'(0) :
(stage == STAGE_READ_B) ? pc + BITS'(1) :
(stage == STAGE_READ_C) ? pc + BITS'(2) :
b
;
assign b_next = b - a;
assign data = write ? b_next : {BITS{1'bz}};
assign halt = halt_reg;
assign write = stage == STAGE_WRITE;
always @(posedge clock) begin
if (reset == 1'b1) begin
halt_reg <= 0;
pc <= 0;
stage <= STAGE_READ_A;
end else begin
case(stage)
STAGE_READ_A: a <= data;
STAGE_READ_B: b <= data;
STAGE_READ_C: c <= data;
STAGE_WRITE: pc <= (b_next <= 0) ? c : pc + BITS'(3);
endcase
if (stage == STAGE_WRITE && a == 0 && b <= 0 && c == pc) begin
halt_reg <= 1;
end
stage <= stage + 1;
end
end
endmodule