diff --git a/src/gamegear.c b/src/gamegear.c
index e08cc72..41c071e 100644
--- a/src/gamegear.c
+++ b/src/gamegear.c
@@ -207,9 +207,6 @@ 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_IO_ERROR:
-                    SET_EXC("I/O error on port 0x%02X", gg->cpu.exc_data)
-                    break;
                 default:
                     SET_EXC("unknown exception")
                     break;
diff --git a/src/io.c b/src/io.c
index c66172a..70434b6 100644
--- a/src/io.c
+++ b/src/io.c
@@ -2,6 +2,7 @@
    Released under the terms of the MIT License. See LICENSE for details. */
 
 #include "io.h"
+#include "logging.h"
 
 /*
     Initialize an IO object.
@@ -17,7 +18,12 @@ void io_init(IO *io, VDP *vdp, PSG *psg)
 */
 void io_power(IO *io)
 {
-    io->except = false;
+    io->ports[0x00] = 0xC0;  // Overseas mode, NTSC
+    io->ports[0x01] = 0x7F;
+    io->ports[0x02] = 0xFF;
+    io->ports[0x03] = 0x00;
+    io->ports[0x04] = 0xFF;
+    io->ports[0x05] = 0x00;
 }
 
 /*
@@ -29,37 +35,67 @@ bool io_check_irq(IO *io)
 }
 
 /*
+    Read from one of the system ports, which are numbered from 0x00 to 0x06.
+*/
+static uint8_t read_system_port(IO *io, uint8_t port)
+{
+    switch (port) {
+        case 0x00:
+            // TODO: MSB is state of START button
+            return (io->ports[port] & 0x7F) | (0 << 7);
+        case 0x01:
+        case 0x02:
+        case 0x03:
+        case 0x04:
+        case 0x05:
+            return io->ports[port];
+    }
+    return 0xFF;
+}
+
+/*
+    Write to one of the system ports, which are numbered from 0x00 to 0x06.
+*/
+static void write_system_port(IO *io, uint8_t port, uint8_t value)
+{
+    switch (port) {
+        case 0x01:
+        case 0x02:
+        case 0x03:
+            io->ports[port] = value;
+            break;
+        case 0x05:
+            io->ports[port] = value & 0xF8;
+            break;
+        case 0x06:
+            psg_stereo(io->psg, value);
+            break;
+    }
+}
+
+/*
     Read and return a byte from the given port.
 */
 uint8_t io_port_read(IO *io, uint8_t port)
 {
-    if (port <= 0x06) {
-        if (port == 0x00)
-            return 0xC0;
-        // TODO: GG specific registers; initial state: C0 7F FF 00 FF 00 FF
-    } else if (port <= 0x3F) {
+    if (port <= 0x06)
+        return read_system_port(io, port);
+    else if (port <= 0x3F)
         return 0xFF;
-    } else if (port <= 0x7F && !(port % 2)) {
+    else if (port <= 0x7F && !(port % 2))
         return io->vdp->v_counter;
-    } else if (port <= 0x7F) {
+    else if (port <= 0x7F)
         return io->vdp->h_counter;
-    } else if (port <= 0xBF && !(port % 2)) {
+    else if (port <= 0xBF && !(port % 2))
         return vdp_read_data(io->vdp);
-    } else if (port <= 0xBF) {
+    else if (port <= 0xBF)
         return vdp_read_control(io->vdp);
-    } else if (port == 0xCD || port == 0xDC) {
-        // TODO: Return the I/O port A/B register
-        return 0xFF;
-    } else if (port == 0xC1 || port == 0xDD) {
-        // TODO: Return the I/O port B/misc. register
+    else if (port == 0xCD || port == 0xDC)
+        return 0xFF;  // TODO: Return the I/O port A/B register
+    else if (port == 0xC1 || port == 0xDD)
+        return 0xFF;  // TODO: Return the I/O port B/misc. register
+    else
         return 0xFF;
-    } else {
-        return 0xFF;
-    }
-
-    io->except = true;
-    io->exc_port = port;
-    return 0;
 }
 
 /*
@@ -67,28 +103,16 @@ uint8_t io_port_read(IO *io, uint8_t port)
 */
 void io_port_write(IO *io, uint8_t port, uint8_t value)
 {
-    if (port <= 0x06) {
-        // TODO: GG specific registers; initial state: C0 7F FF 00 FF 00 FF
-        goto except;
-    } else if (port <= 0x3F && !(port % 2)) {
-        // TODO: Write to memory control register
-        goto except;
-    } else if (port <= 0x3F) {
-        // TODO: Write to I/O control register
-        goto except;
-    } else if (port <= 0x7F) {
+    if (port <= 0x06)
+        write_system_port(io, port, value);
+    else if (port <= 0x3F && !(port % 2))
+        return;  // TODO: Write to memory control register
+    else if (port <= 0x3F)
+        return;  // TODO: Write to I/O control register
+    else if (port <= 0x7F)
         psg_write(io->psg, value);
-    } else if (port <= 0xBF && !(port % 2)) {
+    else if (port <= 0xBF && !(port % 2))
         vdp_write_data(io->vdp, value);
-    } else if (port <= 0xBF) {
+    else if (port <= 0xBF)
         vdp_write_control(io->vdp, value);
-    } else {
-        return;
-    }
-
-    return;
-
-    except:
-    io->except = true;
-    io->exc_port = port;
 }
diff --git a/src/io.h b/src/io.h
index ec4df5f..578f799 100644
--- a/src/io.h
+++ b/src/io.h
@@ -14,8 +14,7 @@
 typedef struct {
     VDP *vdp;
     PSG *psg;
-    bool except;
-    uint8_t exc_port;
+    uint8_t ports[6];
 } IO;
 
 /* Functions */
diff --git a/src/psg.c b/src/psg.c
index 786502e..c0f792a 100644
--- a/src/psg.c
+++ b/src/psg.c
@@ -41,3 +41,13 @@ void psg_write(PSG *psg, uint8_t byte)
     (void) psg;
     TRACE("PSG ignoring write: 0x%02X", byte)
 }
+
+/*
+    Send a byte to the PSG's stereo control.
+*/
+void psg_stereo(PSG *psg, uint8_t byte)
+{
+    // TODO
+    (void) psg;
+    TRACE("PSG ignoring stereo: 0x%02X", byte)
+}
diff --git a/src/psg.h b/src/psg.h
index 1fe7246..deb3502 100644
--- a/src/psg.h
+++ b/src/psg.h
@@ -19,3 +19,4 @@ void psg_init(PSG*);
 void psg_free(PSG*);
 void psg_power(PSG*);
 void psg_write(PSG*, uint8_t);
+void psg_stereo(PSG*, uint8_t);
diff --git a/src/vdp.c b/src/vdp.c
index 37d41a9..75fe332 100644
--- a/src/vdp.c
+++ b/src/vdp.c
@@ -433,43 +433,4 @@ void vdp_dump_registers(const VDP *vdp)
     DEBUG("- $08:  0x%02X (HS)", regs[0x08])
     DEBUG("- $09:  0x%02X (VS)", regs[0x09])
     DEBUG("- $0A:  0x%02X (LC)", regs[0x0A])
-
-    // TODO: remove me!
-    DEBUG("Dumping CRAM:")
-    for (uint8_t i = 0; i < 32; i += 8) {
-        uint16_t w[8];
-        for (uint8_t j = 0; j < 8; j++)
-            w[j] = get_color(vdp, (i + j) % 16, i & 16);
-
-        DEBUG("- %04X %04X %04X %04X %04X %04X %04X %04X",
-            w[0], w[1], w[2], w[3], w[4], w[5], w[6], w[7])
-    }
-
-    return;
-
-    DEBUG("Dumping PNT:")
-    for (uint16_t i = 0; i < 18; i++) {
-        uint16_t w[20];
-        for (uint8_t j = 0; j < 20; j++)
-            w[j] = get_background_tile(vdp, i + 3, j + 6);
-
-        DEBUG("- %03X %03X %03X %03X %03X %03X %03X %03X %03X %03X"
-               " %03X %03X %03X %03X %03X %03X %03X %03X %03X %03X",
-            w[0x00], w[0x01], w[0x02], w[0x03], w[0x04], w[0x05], w[0x06],
-            w[0x07], w[0x08], w[0x09], w[0x0A], w[0x0B], w[0x0C], w[0x0D],
-            w[0x0E], w[0x0F], w[0x10], w[0x11], w[0x12], w[0x13])
-    }
-
-    DEBUG("Dumping PGT:")
-    for (uint16_t i = 0; i < /* 512 */ 32; i++) {
-        uint32_t w[8];
-        for (uint8_t j = 0; j < 8; j++)
-            w[j] = vdp->vram[32 * i + 4 * j] +
-                  (vdp->vram[32 * i + 4 * j + 1] <<  8) +
-                  (vdp->vram[32 * i + 4 * j + 2] << 16) +
-                  (vdp->vram[32 * i + 4 * j + 3] << 24);
-
-        DEBUG("- 0x%04X: %08X %08X %08X %08X %08X %08X %08X %08X", i,
-            w[0], w[1], w[2], w[3], w[4], w[5], w[6], w[7])
-    }
 }
diff --git a/src/z80.c b/src/z80.c
index 0cbbe24..7c4943d 100644
--- a/src/z80.c
+++ b/src/z80.c
@@ -130,37 +130,6 @@ static inline uint16_t stack_pop(Z80 *z80)
 }
 
 /*
-    Check for errors after an I/O operation.
-*/
-static void handle_io_errors(Z80 *z80)
-{
-    if (z80->io->except) {
-        z80->except = true;
-        z80->exc_code = Z80_EXC_IO_ERROR;
-        z80->exc_data = z80->io->exc_port;
-    }
-}
-
-/*
-    Read and return a byte from the given port, and check for errors.
-*/
-static inline uint8_t read_port(Z80 *z80, uint8_t port)
-{
-    uint8_t value = io_port_read(z80->io, port);
-    handle_io_errors(z80);
-    return value;
-}
-
-/*
-    Write a byte to the given port, and check for errors.
-*/
-static inline void write_port(Z80 *z80, uint8_t port, uint8_t value)
-{
-    io_port_write(z80->io, port, value);
-    handle_io_errors(z80);
-}
-
-/*
     Extract an 8-bit register from the given opcode and return a pointer to it.
 */
 static inline uint8_t* extract_reg(Z80 *z80, uint8_t opcode)
diff --git a/src/z80.h b/src/z80.h
index 4b7ee56..631ca78 100644
--- a/src/z80.h
+++ b/src/z80.h
@@ -11,7 +11,6 @@
 
 #define Z80_EXC_NOT_POWERED          0
 #define Z80_EXC_UNIMPLEMENTED_OPCODE 1
-#define Z80_EXC_IO_ERROR             2
 
 /* Structs */
 
diff --git a/src/z80_ops.inc.c b/src/z80_ops.inc.c
index 23c838c..e363247 100644
--- a/src/z80_ops.inc.c
+++ b/src/z80_ops.inc.c
@@ -1969,7 +1969,7 @@ static uint8_t z80_inst_in_a_n(Z80 *z80, uint8_t opcode)
 {
     (void) opcode;
     uint8_t port = mmu_read_byte(z80->mmu, ++z80->regs.pc);
-    z80->regs.a = read_port(z80, port);
+    z80->regs.a = io_port_read(z80->io, port);
     z80->regs.pc++;
     return 11;
 }
@@ -1980,7 +1980,7 @@ static uint8_t z80_inst_in_a_n(Z80 *z80, uint8_t opcode)
 */
 static uint8_t z80_inst_in_r_c(Z80 *z80, uint8_t opcode)
 {
-    uint8_t data = read_port(z80, z80->regs.c);
+    uint8_t data = io_port_read(z80->io, z80->regs.c);
     if (opcode != 0x70)
         *extract_reg(z80, opcode) = data;
 
@@ -1996,7 +1996,8 @@ static uint8_t z80_inst_in_r_c(Z80 *z80, uint8_t opcode)
 static uint8_t z80_inst_ini(Z80 *z80, uint8_t opcode)
 {
     (void) opcode;
-    mmu_write_byte(z80->mmu, z80->regs.hl, read_port(z80, z80->regs.c));
+    uint8_t data = io_port_read(z80->io, z80->regs.c);
+    mmu_write_byte(z80->mmu, z80->regs.hl, data);
     set_flags_blockio(z80);
 
     z80->regs.hl++;
@@ -2025,7 +2026,8 @@ static uint8_t z80_inst_inir(Z80 *z80, uint8_t opcode)
 static uint8_t z80_inst_ind(Z80 *z80, uint8_t opcode)
 {
     (void) opcode;
-    mmu_write_byte(z80->mmu, z80->regs.hl, read_port(z80, z80->regs.c));
+    uint8_t data = io_port_read(z80->io, z80->regs.c);
+    mmu_write_byte(z80->mmu, z80->regs.hl, data);
     set_flags_blockio(z80);
 
     z80->regs.hl--;
@@ -2055,7 +2057,7 @@ static uint8_t z80_inst_out_n_a(Z80 *z80, uint8_t opcode)
 {
     (void) opcode;
     uint8_t port = mmu_read_byte(z80->mmu, ++z80->regs.pc);
-    write_port(z80, port, z80->regs.a);
+    io_port_write(z80->io, port, z80->regs.a);
     z80->regs.pc++;
     return 11;
 }
@@ -2068,7 +2070,7 @@ static uint8_t z80_inst_out_n_a(Z80 *z80, uint8_t opcode)
 static uint8_t z80_inst_out_c_r(Z80 *z80, uint8_t opcode)
 {
     uint8_t value = opcode != 0x71 ? *extract_reg(z80, opcode) : 0;
-    write_port(z80, z80->regs.c, value);
+    io_port_write(z80->io, z80->regs.c, value);
     z80->regs.pc++;
     return 12;
 }
@@ -2080,7 +2082,8 @@ static uint8_t z80_inst_out_c_r(Z80 *z80, uint8_t opcode)
 static uint8_t z80_inst_outi(Z80 *z80, uint8_t opcode)
 {
     (void) opcode;
-    write_port(z80, z80->regs.c, mmu_read_byte(z80->mmu, z80->regs.hl));
+    uint8_t data = mmu_read_byte(z80->mmu, z80->regs.hl);
+    io_port_write(z80->io, z80->regs.c, data);
     set_flags_blockio(z80);
 
     z80->regs.hl++;
@@ -2109,7 +2112,8 @@ static uint8_t z80_inst_otir(Z80 *z80, uint8_t opcode)
 static uint8_t z80_inst_outd(Z80 *z80, uint8_t opcode)
 {
     (void) opcode;
-    write_port(z80, z80->regs.c, mmu_read_byte(z80->mmu, z80->regs.hl));
+    uint8_t data = mmu_read_byte(z80->mmu, z80->regs.hl);
+    io_port_write(z80->io, z80->regs.c, data);
     set_flags_blockio(z80);
 
     z80->regs.hl--;