From e29171aa4e057f7c1a2682a0dfd1836beeb71651 Mon Sep 17 00:00:00 2001
From: Joshua Wise <joshua@rebirth.joshuawise.com>
Date: Tue, 6 May 2008 07:43:10 -0400
Subject: [PATCH] Fix some simulator-only bugs involving debugging/illegal
 states.  Make rd and wr be asserted for the right time. (or not?
 http://www.semis.demon.co.uk/Gameboy/newpin2.htm )

---
 GBZ80Core.v | 20 ++++++++++++++------
 System.v    |  7 +++++--
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/GBZ80Core.v b/GBZ80Core.v
index 9db958f..8970e5d 100644
--- a/GBZ80Core.v
+++ b/GBZ80Core.v
@@ -177,17 +177,21 @@ module GBZ80Core(
 	
 	reg bootstrap_enb;
 	
-	wire bus = ((busaddress[15:8] == 8'h00) && bootstrap_enb) || ((busaddress[15:7] == 9'b111111111) && (busaddress != 16'hFFFF));	/* 0 or 1 depending on which bus */
+	wire bus = ((busaddress[15:8] == 8'h00) && bootstrap_enb) || ((busaddress[15:7] == 9'b111111111) && (busaddress != 16'hFFFF))	/* 0 or 1 depending on which bus */
+		`ifdef isim
+			|| (busaddress === 16'hxxxx) /* To avoid simulator glomulation. */
+		`endif
+			;
 		
 	assign bus0address = (bus == 0) ? busaddress : 16'bzzzzzzzzzzzzzzz;
 	assign bus1address = (bus == 1) ? busaddress : 16'bzzzzzzzzzzzzzzz;
 	assign bus0data = ((bus == 0) && buswr) ? buswdata : 8'bzzzzzzzz;
 	assign bus1data = ((bus == 1) && buswr) ? buswdata : 8'bzzzzzzzz;
 	assign busdata = (bus == 0) ? bus0data : bus1data;
-	assign bus0rd = (bus == 0) ? busrd : 1'bz;
-	assign bus1rd = (bus == 1) ? busrd : 1'bz;
-	assign bus0wr = (bus == 0) ? buswr : 1'bz;
-	assign bus1wr = (bus == 1) ? buswr : 1'bz;
+	assign bus0rd = (bus == 0) ? busrd : 1'b0;
+	assign bus1rd = (bus == 1) ? busrd : 1'b0;
+	assign bus0wr = (bus == 0) ? buswr : 1'b0;
+	assign bus1wr = (bus == 1) ? buswr : 1'b0;
 
 	reg ie, iedelay;
 
@@ -296,6 +300,8 @@ module GBZ80Core(
 			end
 		end
 		`STATE_DECODE: begin	/* Make sure this only happens for one clock. */
+			buswr <= 0;
+			busrd <= 0;
 		end
 		endcase
 	
@@ -318,7 +324,7 @@ module GBZ80Core(
 				rdata <= busdata;
 				cycle <= 0;
 			end else begin
-				if (rd) rdata <= busdata;
+				if (rd) rdata <= busdata;	/* Still valid because peripherals are now expected to keep it held valid. */
 				cycle <= cycle + 1;
 			end
 			if (iedelay) begin
@@ -334,6 +340,8 @@ module GBZ80Core(
 			state <= `STATE_EXECUTE;
 		end
 		`STATE_EXECUTE: begin
+			if (opcode[7:0] === 8'bxxxxxxxx)
+				$stop;
 			casex (opcode)
 			`define EXECUTE
 			`include "allinsns.v"
diff --git a/System.v b/System.v
index ebc9be4..95b715c 100644
--- a/System.v
+++ b/System.v
@@ -28,13 +28,16 @@ module BootstrapROM(
 	input wr, rd);
 
 	reg rdlatch = 0;
+	reg [7:0] addrlatch = 0;
 	reg [7:0] brom [255:0];
 	initial $readmemh("bootstrap.hex", brom);
 
 	wire decode = address[15:8] == 0;
-	wire [7:0] odata = brom[address[7:0]];
-	always @(posedge clk)
+	wire [7:0] odata = brom[addrlatch];
+	always @(posedge clk) begin
 		rdlatch <= rd && decode;
+		addrlatch <= address[7:0];
+	end
 	assign data = rdlatch ? odata : 8'bzzzzzzzz;
 endmodule
 
-- 
2.43.0