Mapper 42 IRQ fixed (#422)

Author: lain-web2000

rtl/mappers/misc.sv

@@ -872,22 +872,17 @@ end else if (ce) begin
 				endcase
 		endcase
 		if (irq_enable)
-			if (mapper == 40)
-				irq_counter <= irq_counter + 13'd1;
-			else // Mapper 42 (broken)
-				irq_counter <= irq_counter + 15'd1;
+			case(mapper)
+				40: irq_counter <= irq_counter + 13'd1;
+				42: irq_counter <= irq_counter + 15'd1;
+			endcase
 		else begin
-			irq <= 1'b0;	// ACK
 			irq_counter <= 0;
 		end
-		if (mapper == 40)
-			if (irq_counter == 13'h1000)
-				irq <= 1'b1;
-			else if (irq_counter == 13'h0000)
-				irq <= 1'b0;	// IRQ will self-acknowledge after 4096 M2 cycles
-		else // Mapper 42 (broken)
-			if (irq_counter == 15'h6000)
-				irq <= 1'b1;
+		case(mapper) //IRQ will self-acknowledge when counter overflows
+			40: irq <= irq_counter[12];
+			42: irq <= &irq_counter[14:13];
+		endcase
 end
 
 always @* begin
@@ -3310,4 +3305,4 @@ assign vram_ce = chr_ain[13];
 assign prg_allow = prg_ain[15] && !prg_write;
 assign prg_aout = {3'b000, outer_prg_bank, prgsel, prg_ain[12:0]};
 
-endmodule
+endmodule