512x8存儲(chǔ)器模型。端口信號(hào)為：addr[8:0], wrena, wrdata[7:0], rddata[7:0]. 存儲(chǔ)器用wrena信號(hào)的上升沿將數(shù)據(jù)寫入。再寫一個(gè)測(cè)試臺(tái)， 做以下檢測(cè)：
(1)	將所有地址都寫0；
(2)	檢查每個(gè)地址是否讀0；
(3)	將所有地址都寫8’hff；
(4)	檢查每個(gè)地址是否讀8’hff；
(5)	每個(gè)地址先寫8’h55，讀，再寫8’haa，再讀。重復(fù)所有地址。
    如果有錯(cuò)誤立即停止執(zhí)行，并打印出錯(cuò)信息。
/***********************************************************
 *	File:	m512x8.v
 *	Description: This module is a model of 512x8 memory
 ************************************************************/

module m512x8 (addr, wrena, wrdata, rddata);
    input[8:0]	addr;		// address bus
    input	wrena;		// write enable
    input[7:0]	wrdata;		// data to write into memory
    output[7:0]	rddata;		// data read from memory

    reg[7:0]	mem[0:511];	// memory declaration
    integer	i;		// loop variable

    wire	validaddr = ^addr===1'b1 || ^addr===1'b0;

    always @(posedge wrena) begin
	if(validaddr) mem[addr] = wrdata;
	else begin
	    for(i=0;i<512;i=i+1)
		if((&(addr^i[8:0]))===1'b0 && (|(addr^i[8:0]))!==1'b1)
		    mem[i] = ((mem[i]^wrdata) & 8'hxx) ^ mem[i];
	end
    end

    assign	rddata = (validaddr)? mem[addr]:8'hxx;
endmodule
/*************************************************************
 *	File:	mtest.v
 *	Description: This module is a test bench to test
 *		a memory model.
 *************************************************************/
`timescale	1ns/1ns

module mtest;
    reg		wrena;		// write enable
    reg[8:0]	addr;		// address
    reg[7:0]	wrdata,		// write data
		data;		// temporary data register
    wire[7:0]	rddata;		// read data

    integer	i;		// loop variable

    m512x8	mem(.addr(addr), .wrena(wrena), .wrdata(wrdata),
		    .rddata(rddata));

    initial begin
	$vcdpluson;
	wrena = 1;
	addr = 9'bx;
	wrdata = 8'bx;

	$write($stime,,"Test 1: Write 0 to all addresses........");
	for(i=0;i<512;i=i+1) writem(i,8'd0);
	$display("Done.");

	$write($stime,,"Test 2: Check data for every address....");
	for(i=0;i<512;i=i+1) begin
	    readm(i,data);
	    if(data!==8'd0) begin
		$display("");		// \n
		$display($stime,,"Error: Data read = %X, expected=00",data);
		#20 $finish;
	    end
	end
	$display("Done.");

	$write($stime,,"Test 3: Write FF to all addresses.......");
	for(i=0;i<512;i=i+1) writem(i,8'hff);
	$display("Done.");

	$write($stime,,"Test 4: Check data for every address....");
	for(i=0;i<512;i=i+1) begin
	    readm(i,data);
	    if(data!==8'hff) begin
		$display("");		// \n
		$display($stime,,"Error: Data read = %X, expected=FF",data);
		#20 $finish;
	    end
	end
	$display("Done.");

	$write($stime,,"Test 5: Write and read test.............");
	for(i=0;i<512;i=i+1) begin
	    writem(i,8'h55);		// write 55 first
	    readm(i,data);
	    if(data!==8'h55) begin
		$display("");		// \n
		$display($stime,,"Error: Data read = %X, expected=55",data);
		#20 $finish;
	    end
	    writem(i,8'hAA);		// write AA this time
	    readm(i,data);
	    if(data!==8'haa) begin
		$display("");		// \n
		$display($stime,,"Error: Data read = %X, expected=AA",data);
		#20 $finish;
	    end
	end
	$display("Done.");
	#20 $finish;
    end

    task writem;	// task to write memory
    input[8:0]	ad;		// address to write
    input[7:0]	data;		// data to write
    begin
	addr = ad;
	wrena = 0;
	#10
	wrdata = data;
	#10
	wrena = 1;
	#5
	addr = 9'bx;
	wrdata = 8'bx;
	#5;			// delay to separate operations
    end
    endtask

    task readm;		// task to read memory
    input[8:0]	ad;		// address to read
    output[7:0]	data;		// data read from memory
    begin
	addr = ad;
	#10 data = rddata;
	#5 addr = 8'bx;
	#5;		// wait 5ns before next operation
    end
    endtask
endmodule