// This is free and unencumbered software released into the public domain.
//
// Anyone is free to copy, modify, publish, use, compile, sell, or
// distribute this software, either in source code form or as a compiled
// binary, for any purpose, commercial or non-commercial, and by any
// means.
module axi4_memory #(
	parameter AXI_TEST = 0,
	parameter VERBOSE = 0
) (
	input             clk,
	input             mem_axi_awvalid,
	output reg        mem_axi_awready = 0,
	input [31:0]      mem_axi_awaddr,
	input [ 2:0]      mem_axi_awprot,

	input            mem_axi_wvalid,
	output reg       mem_axi_wready = 0,
	input [31:0]     mem_axi_wdata,
	input [ 3:0]     mem_axi_wstrb,

	output reg       mem_axi_bvalid = 0,
	input            mem_axi_bready,

	input            mem_axi_arvalid,
	output reg       mem_axi_arready = 0,
	input [31:0]     mem_axi_araddr,
	input [ 2:0]     mem_axi_arprot,

	output            mem_axi_rvalid = 0,
	input             mem_axi_rready,
	output reg [31:0] mem_axi_rdata
);

	reg [31:0]   memory [0:64*1024/4-1] /* verilator public */;
	reg verbose;
	initial verbose = $test$plusargs("verbose") || VERBOSE;

	reg axi_test;
	initial axi_test = $test$plusargs("axi_test") || AXI_TEST;

	reg [63:0] xorshift64_state = 64'd88172645463325252;

	task xorshift64_next;
		begin
			// see page 4 of Marsaglia, George (July 2003). "Xorshift RNGs". Journal of Statistical Software 8 (14).
			xorshift64_state = xorshift64_state ^ (xorshift64_state << 13);
			xorshift64_state = xorshift64_state ^ (xorshift64_state >>  7);
			xorshift64_state = xorshift64_state ^ (xorshift64_state << 17);
		end
	endtask

	reg [2:0] fast_axi_transaction = ~0;
	reg [4:0] async_axi_transaction = ~0;
	reg [4:0] delay_axi_transaction = 0;

	always @(posedge clk) begin
if (axi_test) begin
		xorshift64_next;
		{fast_axi_transaction, async_axi_transaction, delay_axi_transaction} <= xorshift64_state;
end
	end

	reg latched_raddr_en = 0;
	reg latched_waddr_en = 0;
	reg latched_wdata_en = 0;

	reg fast_raddr = 0;
	reg fast_waddr = 0;
	reg fast_wdata = 0;

	reg [31:0] latched_raddr;
	reg [31:0] latched_waddr;
	reg [31:0] latched_wdata;
	reg [ 3:0] latched_wstrb;
	reg        latched_rinsn;

	task handle_axi_arvalid; begin
		mem_axi_arready <= 1;
		latched_raddr = mem_axi_araddr;
		latched_rinsn = mem_axi_arprot[2];
		latched_raddr_en = 1;
		fast_raddr <= 1;
	end endtask

	task handle_axi_awvalid; begin
		mem_axi_awready <= 1;
		latched_waddr = mem_axi_awaddr;
		latched_waddr_en = 1;
		fast_waddr <= 1;
	end endtask

	task handle_axi_wvalid; begin
		mem_axi_wready <= 1;
		latched_wdata = mem_axi_wdata;
		latched_wstrb = mem_axi_wstrb;
		latched_wdata_en = 1;
		fast_wdata <= 1;
	end endtask

	task handle_axi_rvalid; begin
if(verbose)
		$display("RD: ADDR=%08x DATA=%08x%s", latched_raddr, memory[latched_raddr >> 2], latched_rinsn ? " INSN" : "");
		if (latched_raddr < 64*1024) begin
			mem_axi_rdata <= memory[latched_raddr >> 2];
			mem_axi_rvalid <= 1;
			latched_raddr_en = 0;
		end else begin
			$display("OUT-OF-BOUNDS MEMORY READ FROM %08x", latched_raddr);
			$finish;
		end
	end endtask

	task handle_axi_bvalid; begin
if (verbose)
		$display("WR: ADDR=%08x DATA=%08x STRB=%04b", latched_waddr, latched_wdata, latched_wstrb);
		if (latched_waddr < 64*1024) begin
			if (latched_wstrb[0]) memory[latched_waddr >> 2][ 7: 0] <= latched_wdata[ 7: 0];
			if (latched_wstrb[1]) memory[latched_waddr >> 2][15: 8] <= latched_wdata[15: 8];
			if (latched_wstrb[2]) memory[latched_waddr >> 2][23:16] <= latched_wdata[23:16];
			if (latched_wstrb[3]) memory[latched_waddr >> 2][31:24] <= latched_wdata[31:24];
		end else
		if (latched_waddr == 32'h1000_0000) begin
if (verbose) begin
			if (32 <= latched_wdata && latched_wdata < 128)
				$display("OUT: '%c'", latched_wdata[7:0]);
			else
				$display("OUT: %3d", latched_wdata);
end else begin
			$write("%c", latched_wdata[7:0]);
`ifndef VERILATOR
			$fflush();
`endif
end
		end else begin
			$display("OUT-OF-BOUNDS MEMORY WRITE TO %08x", latched_waddr);
			$finish;
		end
		mem_axi_bvalid <= 1;
		latched_waddr_en = 0;
		latched_wdata_en = 0;
	end endtask

	always @(negedge clk) begin
		if (mem_axi_arvalid && !(latched_raddr_en || fast_raddr) && async_axi_transaction[0]) handle_axi_arvalid;
		if (mem_axi_awvalid && !(latched_waddr_en || fast_waddr) && async_axi_transaction[1]) handle_axi_awvalid;
		if (mem_axi_wvalid  && !(latched_wdata_en || fast_wdata) && async_axi_transaction[2]) handle_axi_wvalid;
		if (!mem_axi_rvalid && latched_raddr_en && async_axi_transaction[3]) handle_axi_rvalid;
		if (!mem_axi_bvalid && latched_waddr_en && latched_wdata_en && async_axi_transaction[4]) handle_axi_bvalid;
	end

	always @(posedge clk) begin
		mem_axi_arready <= 0;
		mem_axi_awready <= 0;
		mem_axi_wready <= 0;

		fast_raddr <= 0;
		fast_waddr <= 0;
		fast_wdata <= 0;

		if (mem_axi_rvalid && mem_axi_rready) begin
			mem_axi_rvalid <= 0;
		end

		if (mem_axi_bvalid && mem_axi_bready) begin
			mem_axi_bvalid <= 0;
		end

		if (mem_axi_arvalid && mem_axi_arready && !fast_raddr) begin
			latched_raddr = mem_axi_araddr;
			latched_rinsn = mem_axi_arprot[2];
			latched_raddr_en = 1;
		end

		if (mem_axi_awvalid && mem_axi_awready && !fast_waddr) begin
			latched_waddr = mem_axi_awaddr;
			latched_waddr_en = 1;
		end

		if (mem_axi_wvalid && mem_axi_wready && !fast_wdata) begin
			latched_wdata = mem_axi_wdata;
			latched_wstrb = mem_axi_wstrb;
			latched_wdata_en = 1;
		end

		if (mem_axi_arvalid && !(latched_raddr_en || fast_raddr) && !delay_axi_transaction[0]) handle_axi_arvalid;
		if (mem_axi_awvalid && !(latched_waddr_en || fast_waddr) && !delay_axi_transaction[1]) handle_axi_awvalid;
		if (mem_axi_wvalid  && !(latched_wdata_en || fast_wdata) && !delay_axi_transaction[2]) handle_axi_wvalid;

		if (!mem_axi_rvalid && latched_raddr_en && !delay_axi_transaction[3]) handle_axi_rvalid;
		if (!mem_axi_bvalid && latched_waddr_en && latched_wdata_en && !delay_axi_transaction[4]) handle_axi_bvalid;
	end
endmodule