From: Joshua Wise <joshua@rebirth.joshuawise.com>
Date: Mon, 31 Mar 2008 03:34:09 +0000 (-0400)
Subject: Add RET/IRET. Fix a bug in RST where the PC pushed to the stack was incorrect.
X-Git-Url: http://git.joshuawise.com/fpgaboy.git/commitdiff_plain/abae5818eeec14c040361b5181ff1a31cad79868?ds=sidebyside

Add RET/IRET. Fix a bug in RST where the PC pushed to the stack was incorrect.
---

diff --git a/FPGABoy.ise b/FPGABoy.ise
index 8260443..491bbb3 100644
Binary files a/FPGABoy.ise and b/FPGABoy.ise differ
diff --git a/GBZ80Core.v b/GBZ80Core.v
index 75f9722..c0c2b26 100644
--- a/GBZ80Core.v
+++ b/GBZ80Core.v
@@ -35,6 +35,7 @@
 `define INSN_ALU8				8'b10xxxxxx	// 10 xxx yyy
 `define INSN_NOP				8'b00000000
 `define INSN_RST				8'b11xxx111
+`define INSN_RET				8'b110x1001	// 1 = RETI, 0 = RET
 
 `define INSN_reg_A		3'b111
 `define INSN_reg_B		3'b000
@@ -84,6 +85,8 @@ module GBZ80Core(
 	reg [7:0] buswdata;
 	assign busdata = buswr ? buswdata : 8'bzzzzzzzz;
 	
+	reg ie = 0;
+	
 	initial begin
 		registers[ 0] <= 0;
 		registers[ 1] <= 0;
@@ -341,17 +344,19 @@ module GBZ80Core(
 			end
 			`INSN_RST: begin
 				case (cycle)
-				0: begin
+				0:	begin
+						`EXEC_INC_PC;		// This goes FIRST
+					end
+				1: begin
 						wr <= 1;
 						address <= {registers[`REG_SPH],registers[`REG_SPL]}-1;
 						wdata <= registers[`REG_PCH];
 					end
-				1: begin
+				2: begin
 						wr <= 1;
 						address <= {registers[`REG_SPH],registers[`REG_SPL]}-2;
 						wdata <= registers[`REG_PCL];
 					end
-				2:	begin /* wee */ end
 				3: begin
 						`EXEC_NEWCYCLE;
 						{registers[`REG_PCH],registers[`REG_PCL]} <=
@@ -359,6 +364,23 @@ module GBZ80Core(
 					end
 				endcase
 			end
+			`INSN_RET: begin
+				case (cycle)
+				0:	begin
+						rd <= 1;
+						address <= {registers[`REG_SPH],registers[`REG_SPL]};
+					end
+				1:	begin
+						rd <= 1;
+						address <= {registers[`REG_SPH],registers[`REG_SPL]} + 1;
+					end
+				2:	begin /* twiddle thumbs */ end
+				3:	begin
+						`EXEC_NEWCYCLE;
+						// do NOT increment PC!
+					end
+				endcase
+			end
 			default:
 				$stop;
 			endcase
@@ -596,6 +618,26 @@ module GBZ80Core(
 					end
 				endcase
 			end
+			`INSN_RET: begin
+				case (cycle)
+				0:	cycle <= 1;
+				1:	begin
+						cycle <= 2;
+						registers[`REG_PCL] <= rdata;
+					end
+				2:	begin
+						cycle <= 3;
+						registers[`REG_PCH] <= rdata;
+					end
+				3:	begin
+						cycle <= 0;
+						{registers[`REG_SPH],registers[`REG_SPL]} <=
+							{registers[`REG_SPH],registers[`REG_SPL]} + 2;
+						if (opcode[4])	/* RETI */
+							ie <= 1;
+					end
+				endcase
+			end
 			endcase
 			state <= `STATE_FETCH;
 		end
diff --git a/rom.hex b/rom.hex
index b54409e..0cef4b0 100644
--- a/rom.hex
+++ b/rom.hex
@@ -4,25 +4,13 @@
 01
 // LD SP, HL
 F9
-// POP BC
-C1
-// LD A, 10h
-3E
-10
-// ADD C
-81
-// LD H, A
-67
-// LD L, 34h
-2E
-34
-// XOR B
-A8
-// LD (HL), A
-77
+// RET
+C9
+
+@80
 // RST 00h
 C7
 
 @100
-02
-44
\ No newline at end of file
+80
+00
\ No newline at end of file