diff --git a/src/gamegear.c b/src/gamegear.c
index 4d6742e..d6c71d2 100644
--- a/src/gamegear.c
+++ b/src/gamegear.c
@@ -121,6 +121,9 @@ const char* gamegear_get_exception(GameGear *gg)
                 case Z80_EXC_UNIMPLEMENTED_OPCODE:
                     SET_EXC("unimplemented opcode: 0x%02X", gg->cpu.exc_data)
                     break;
+                case Z80_EXC_UNIMPLEMENTED_PORT:
+                    SET_EXC("unimplemented port: 0x%02X", gg->cpu.exc_data)
+                    break;
                 default:
                     SET_EXC("unknown exception")
                     break;
diff --git a/src/z80.c b/src/z80.c
index 5f76e7b..893a4ee 100644
--- a/src/z80.c
+++ b/src/z80.c
@@ -184,10 +184,30 @@ static inline uint16_t stack_pop(Z80 *z80)
 */
 static uint8_t read_port(Z80 *z80, uint8_t port)
 {
-    // TODO
-    (void) z80;
-    (void) port;
-    return 0x00;
+    if (port < 0x06) {
+        // TODO: GG specific registers; initial state: C0 7F FF 00 FF 00 FF
+    } else if (port < 0x3F) {
+        return 0xFF;
+    } else if (port < 0x7F && !(port % 2)) {
+        // TODO: Return the V counter
+    } else if (port < 0x7F) {
+        // TODO: Return the H counter
+    } else if (port < 0xBF && !(port % 2)) {
+        // TODO: Return the VDP data port contents
+    } else if (port < 0xBF) {
+        // TODO: Return the VDP status flags
+    } else if (port == 0xCD || port == 0xDC) {
+        // TODO: Return the I/O port A/B register
+    } else if (port == 0xC1 || port == 0xDD) {
+        // TODO: Return the I/O port B/misc. register
+    } else {
+        return 0xFF;
+    }
+
+    z80->except = true;
+    z80->exc_code = Z80_EXC_UNIMPLEMENTED_PORT;
+    z80->exc_data = port;
+    return 0;
 }
 
 /*
@@ -195,10 +215,25 @@ static uint8_t read_port(Z80 *z80, uint8_t port)
 */
 static void write_port(Z80 *z80, uint8_t port, uint8_t value)
 {
-    // TODO
-    (void) z80;
-    (void) port;
-    (void) value;
+    if (port < 0x06) {
+        // TODO: GG specific registers; initial state: C0 7F FF 00 FF 00 FF
+    } else if (port < 0x3F && !(port % 2)) {
+        // TODO: Write to memory control register
+    } else if (port < 0x3F) {
+        // TODO: Write to I/O control register
+    } else if (port < 0x7F) {
+        // TODO: Write to the SN76489 PSG
+    } else if (port < 0xBF && !(port % 2)) {
+        // TODO: Write to the VDP data port
+    } else if (port < 0xBF) {
+        // TODO: Write to the VDP control port
+    } else {
+        return;
+    }
+
+    z80->except = true;
+    z80->exc_code = Z80_EXC_UNIMPLEMENTED_PORT;
+    z80->exc_data = port;
 }
 
 /*
diff --git a/src/z80.h b/src/z80.h
index 43f895c..4d6bdd5 100644
--- a/src/z80.h
+++ b/src/z80.h
@@ -10,6 +10,7 @@
 
 #define Z80_EXC_NOT_POWERED          0
 #define Z80_EXC_UNIMPLEMENTED_OPCODE 1
+#define Z80_EXC_UNIMPLEMENTED_PORT   2
 
 /* Structs */
 
diff --git a/src/z80_ops.inc.c b/src/z80_ops.inc.c
index 98f3489..40b394c 100644
--- a/src/z80_ops.inc.c
+++ b/src/z80_ops.inc.c
@@ -690,9 +690,28 @@ static uint8_t z80_inst_ret_cc(Z80 *z80, uint8_t opcode)
 
 // RETI
 
-// RETN
+/*
+    RETN (0xED45, 0xED55, 0xED5D, 0xED65, 0xED6D, 0xED75, 0xED7D):
+    Pop PC from the stack, and copy to IFF2 to IFF1.
+*/
+static uint8_t z80_inst_retn(Z80 *z80, uint8_t opcode)
+{
+    (void) opcode;
+    z80->regfile.pc = stack_pop(z80);
+    z80->regfile.iff1 = z80->regfile.iff2;
+    return 14;
+}
 
-// RST p
+/*
+    RST p (0xC7, 0xCF, 0xD7, 0xDF, 0xE7, 0xEF, 0xF7, 0xFF):
+    Push PC+1 onto the stack and jump to p (opcode & 0x38).
+*/
+static uint8_t z80_inst_rst_p(Z80 *z80, uint8_t opcode)
+{
+    stack_push(z80, z80->regfile.pc + 1);
+    z80->regfile.pc = opcode & 0x38;
+    return 11;
+}
 
 /*
     IN A, (n) (0xDB):
diff --git a/src/z80_tables.inc.c b/src/z80_tables.inc.c
index 72867a7..ee45802 100644
--- a/src/z80_tables.inc.c
+++ b/src/z80_tables.inc.c
@@ -201,7 +201,7 @@ static DispatchTable instruction_table = {
     [0xC4] = z80_inst_call_cc_nn,
     [0xC5] = z80_inst_push_qq,
     [0xC6] = z80_inst_unimplemented,  // TODO
-    [0xC7] = z80_inst_unimplemented,  // TODO
+    [0xC7] = z80_inst_rst_p,
     [0xC8] = z80_inst_ret_cc,
     [0xC9] = z80_inst_ret,
     [0xCA] = z80_inst_jp_cc_nn,
@@ -209,7 +209,7 @@ static DispatchTable instruction_table = {
     [0xCC] = z80_inst_call_cc_nn,
     [0xCD] = z80_inst_call_nn,
     [0xCE] = z80_inst_unimplemented,  // TODO
-    [0xCF] = z80_inst_unimplemented,  // TODO
+    [0xCF] = z80_inst_rst_p,
     [0xD0] = z80_inst_ret_cc,
     [0xD1] = z80_inst_pop_qq,
     [0xD2] = z80_inst_jp_cc_nn,
@@ -217,7 +217,7 @@ static DispatchTable instruction_table = {
     [0xD4] = z80_inst_call_cc_nn,
     [0xD5] = z80_inst_push_qq,
     [0xD6] = z80_inst_unimplemented,  // TODO
-    [0xD7] = z80_inst_unimplemented,  // TODO
+    [0xD7] = z80_inst_rst_p,
     [0xD8] = z80_inst_ret_cc,
     [0xD9] = z80_inst_exx,
     [0xDA] = z80_inst_jp_cc_nn,
@@ -225,7 +225,7 @@ static DispatchTable instruction_table = {
     [0xDC] = z80_inst_call_cc_nn,
     [0xDD] = z80_prefix_index,
     [0xDE] = z80_inst_unimplemented,  // TODO
-    [0xDF] = z80_inst_unimplemented,  // TODO
+    [0xDF] = z80_inst_rst_p,
     [0xE0] = z80_inst_ret_cc,
     [0xE1] = z80_inst_pop_qq,
     [0xE2] = z80_inst_jp_cc_nn,
@@ -233,7 +233,7 @@ static DispatchTable instruction_table = {
     [0xE4] = z80_inst_call_cc_nn,
     [0xE5] = z80_inst_push_qq,
     [0xE6] = z80_inst_unimplemented,  // TODO
-    [0xE7] = z80_inst_unimplemented,  // TODO
+    [0xE7] = z80_inst_rst_p,
     [0xE8] = z80_inst_ret_cc,
     [0xE9] = z80_inst_unimplemented,  // TODO
     [0xEA] = z80_inst_jp_cc_nn,
@@ -241,7 +241,7 @@ static DispatchTable instruction_table = {
     [0xEC] = z80_inst_call_cc_nn,
     [0xED] = z80_prefix_extended,
     [0xEE] = z80_inst_unimplemented,  // TODO
-    [0xEF] = z80_inst_unimplemented,  // TODO
+    [0xEF] = z80_inst_rst_p,
     [0xF0] = z80_inst_ret_cc,
     [0xF1] = z80_inst_pop_qq,
     [0xF2] = z80_inst_jp_cc_nn,
@@ -249,7 +249,7 @@ static DispatchTable instruction_table = {
     [0xF4] = z80_inst_call_cc_nn,
     [0xF5] = z80_inst_push_qq,
     [0xF6] = z80_inst_unimplemented,  // TODO
-    [0xF7] = z80_inst_unimplemented,  // TODO
+    [0xF7] = z80_inst_rst_p,
     [0xF8] = z80_inst_ret_cc,
     [0xF9] = z80_inst_unimplemented,  // TODO
     [0xFA] = z80_inst_jp_cc_nn,
@@ -257,7 +257,7 @@ static DispatchTable instruction_table = {
     [0xFC] = z80_inst_call_cc_nn,
     [0xFD] = z80_prefix_index,
     [0xFE] = z80_inst_cp_n,
-    [0xFF] = z80_inst_unimplemented   // TODO
+    [0xFF] = z80_inst_rst_p
 };
 
 static DispatchTable instruction_table_extended = {
@@ -330,7 +330,7 @@ static DispatchTable instruction_table_extended = {
     [0x42] = z80_inst_unimplemented,  // TODO
     [0x43] = z80_inst_unimplemented,  // TODO
     [0x44] = z80_inst_unimplemented,  // TODO
-    [0x45] = z80_inst_unimplemented,  // TODO
+    [0x45] = z80_inst_retn,
     [0x46] = z80_inst_im,
     [0x47] = z80_inst_unimplemented,  // TODO
     [0x48] = z80_inst_in_r_c,
@@ -346,7 +346,7 @@ static DispatchTable instruction_table_extended = {
     [0x52] = z80_inst_unimplemented,  // TODO
     [0x53] = z80_inst_unimplemented,  // TODO
     [0x54] = z80_inst_unimplemented,  // TODO
-    [0x55] = z80_inst_unimplemented,  // TODO
+    [0x55] = z80_inst_retn,
     [0x56] = z80_inst_im,
     [0x57] = z80_inst_unimplemented,  // TODO
     [0x58] = z80_inst_in_r_c,
@@ -354,7 +354,7 @@ static DispatchTable instruction_table_extended = {
     [0x5A] = z80_inst_unimplemented,  // TODO
     [0x5B] = z80_inst_unimplemented,  // TODO
     [0x5C] = z80_inst_unimplemented,  // TODO
-    [0x5D] = z80_inst_unimplemented,  // TODO
+    [0x5D] = z80_inst_retn,
     [0x5E] = z80_inst_im,
     [0x5F] = z80_inst_unimplemented,  // TODO
     [0x60] = z80_inst_in_r_c,
@@ -362,7 +362,7 @@ static DispatchTable instruction_table_extended = {
     [0x62] = z80_inst_unimplemented,  // TODO
     [0x63] = z80_inst_unimplemented,  // TODO
     [0x64] = z80_inst_unimplemented,  // TODO
-    [0x65] = z80_inst_unimplemented,  // TODO
+    [0x65] = z80_inst_retn,
     [0x66] = z80_inst_im,
     [0x67] = z80_inst_unimplemented,  // TODO
     [0x68] = z80_inst_in_r_c,
@@ -370,7 +370,7 @@ static DispatchTable instruction_table_extended = {
     [0x6A] = z80_inst_unimplemented,  // TODO
     [0x6B] = z80_inst_unimplemented,  // TODO
     [0x6C] = z80_inst_unimplemented,  // TODO
-    [0x6D] = z80_inst_unimplemented,  // TODO
+    [0x6D] = z80_inst_retn,
     [0x6E] = z80_inst_im,
     [0x6F] = z80_inst_unimplemented,  // TODO
     [0x70] = z80_inst_in_r_c,
@@ -378,7 +378,7 @@ static DispatchTable instruction_table_extended = {
     [0x72] = z80_inst_unimplemented,  // TODO
     [0x73] = z80_inst_unimplemented,  // TODO
     [0x74] = z80_inst_unimplemented,  // TODO
-    [0x75] = z80_inst_unimplemented,  // TODO
+    [0x75] = z80_inst_retn,
     [0x76] = z80_inst_im,
     [0x77] = z80_inst_nop2,
     [0x78] = z80_inst_in_r_c,
@@ -386,7 +386,7 @@ static DispatchTable instruction_table_extended = {
     [0x7A] = z80_inst_unimplemented,  // TODO
     [0x7B] = z80_inst_unimplemented,  // TODO
     [0x7C] = z80_inst_unimplemented,  // TODO
-    [0x7D] = z80_inst_unimplemented,  // TODO
+    [0x7D] = z80_inst_retn,
     [0x7E] = z80_inst_im,
     [0x7F] = z80_inst_nop2,
     [0x80] = z80_inst_unimplemented,  // TODO