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

[fpgaboy.git] / GBZ80Core.v

diff --git a/GBZ80Core.v b/GBZ80Core.v
index 75f9722adecc13cec3680858096f4b04a7707a9e..c0c2b26f9712ee16ea2298246e7563343e3ab00a 100644 (file)
--- 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