diff --git a/src/device/i2c_gpio.c b/src/device/i2c_gpio.c
index 08096123d..2b8cb9762 100644
--- a/src/device/i2c_gpio.c
+++ b/src/device/i2c_gpio.c
@@ -6,14 +6,12 @@
  *
  *		This file is part of the 86Box distribution.
  *
- *		Emulation of a GPIO-based I2C device.
+ *		Emulation of a GPIO-based I2C host controller.
  *
  *
  *
- * Authors:	Sarah Walker, <http://pcem-emulator.co.uk/>
- *		RichardG, <richardg867@gmail.com>
+ * Authors:	RichardG, <richardg867@gmail.com>
  *
- *		Copyright 2008-2020 Sarah Walker.
  *		Copyright 2020 RichardG.
  */
 #include <stdarg.h>
@@ -27,36 +25,11 @@
 #include <86box/i2c.h>
 
 
-enum {
-    TRANSMITTER_SLAVE = 1,
-    TRANSMITTER_MASTER = 2
-};
-
-enum {
-    I2C_IDLE = 0,
-    I2C_RECEIVE,
-    I2C_RECEIVE_WAIT,
-    I2C_TRANSMIT_START,
-    I2C_TRANSMIT,
-    I2C_ACKNOWLEDGE,
-    I2C_NOTACKNOWLEDGE,
-    I2C_TRANSACKNOWLEDGE,
-    I2C_TRANSMIT_WAIT
-};
-
-enum {
-    SLAVE_IDLE = 0,
-    SLAVE_RECEIVEADDR,
-    SLAVE_RECEIVEDATA,
-    SLAVE_SENDDATA,
-    SLAVE_INVALID
-};
-
 typedef struct {
     char	*bus_name;
     void	*i2c;
-    uint8_t	scl, sda, receive_wait_sda, state, slave_state, slave_addr,
-		slave_read, last_sda, pos, transmit, byte;
+    uint8_t	prev_scl, prev_sda, slave_sda, started,
+		slave_addr_received, slave_addr, slave_read, pos, byte;
 } i2c_gpio_t;
 
 
@@ -90,7 +63,7 @@ i2c_gpio_init(char *bus_name)
 
     dev->bus_name = bus_name;
     dev->i2c = i2c_addbus(dev->bus_name);
-    dev->scl = dev->sda = 1;
+    dev->prev_scl = dev->prev_sda = dev->slave_sda = 1;
     dev->slave_addr = 0xff;
 
     return dev;
@@ -110,206 +83,77 @@ i2c_gpio_close(void *dev_handle)
 }
 
 
-void
-i2c_gpio_next_byte(i2c_gpio_t *dev)
-{
-    dev->byte = i2c_read(dev->i2c, dev->slave_addr);
-    i2c_gpio_log(1, "I2C GPIO %s: Transmitting data %02X\n", dev->bus_name, dev->byte);
-}
-
-
-uint8_t
-i2c_gpio_write(i2c_gpio_t *dev)
-{
-    uint8_t i;
-
-    switch (dev->slave_state) {
-	case SLAVE_IDLE:
-		i = dev->slave_addr;
-		dev->slave_addr = dev->byte >> 1;
-		dev->slave_read = dev->byte & 1;
-
-		i2c_gpio_log(1, "I2C GPIO %s: Initiating %s address %02X\n", dev->bus_name, dev->slave_read ? "read from" : "write to", dev->slave_addr);
-
-		if (!i2c_has_device(dev->i2c, dev->slave_addr) ||
-		    ((i == 0xff) && !i2c_start(dev->i2c, dev->slave_addr, dev->slave_read))) { /* start only once per transfer */
-			dev->slave_state = SLAVE_INVALID;
-			dev->slave_addr = 0xff;
-			return I2C_NOTACKNOWLEDGE;
-		}
-
-		if (dev->slave_read) {
-			dev->slave_state = SLAVE_SENDDATA;
-			dev->transmit = TRANSMITTER_SLAVE;
-			dev->byte = i2c_read(dev->i2c, dev->slave_addr);
-		} else {
-			dev->slave_state = SLAVE_RECEIVEADDR;
-			dev->transmit = TRANSMITTER_MASTER;
-		}
-		break;
-
-	case SLAVE_RECEIVEADDR:
-		i2c_gpio_log(1, "I2C GPIO %s: Receiving address %02X\n", dev->bus_name, dev->byte);
-		dev->slave_state = dev->slave_read ? SLAVE_SENDDATA : SLAVE_RECEIVEDATA;
-		if (!i2c_write(dev->i2c, dev->slave_addr, dev->byte))
-			return I2C_NOTACKNOWLEDGE;
-		break;
-
-	case SLAVE_RECEIVEDATA:
-		i2c_gpio_log(1, "I2C GPIO %s: Receiving data %02X\n", dev->bus_name, dev->byte);
-		if (!i2c_write(dev->i2c, dev->slave_addr, dev->byte))
-			return I2C_NOTACKNOWLEDGE;
-		break;
-
-	case SLAVE_INVALID:
-		return I2C_NOTACKNOWLEDGE;
-    }
-
-    return I2C_ACKNOWLEDGE;
-}
-
-
-void
-i2c_gpio_stop(i2c_gpio_t *dev)
-{
-    i2c_gpio_log(1, "I2C GPIO %s: Stopping transfer\n", dev->bus_name);
-
-    if (dev->slave_addr != 0xff) /* don't stop if no transfer was in progress */
-	i2c_stop(dev->i2c, dev->slave_addr);
-
-    dev->slave_addr = 0xff;
-    dev->slave_state = SLAVE_IDLE;
-    dev->transmit = TRANSMITTER_MASTER;
-}
-
-
 void
 i2c_gpio_set(void *dev_handle, uint8_t scl, uint8_t sda)
 {
     i2c_gpio_t *dev = (i2c_gpio_t *) dev_handle;
 
-    i2c_gpio_log(3, "I2C GPIO %s: scl=%d->%d sda=%d->%d last_valid_sda=%d state=%d\n", dev->bus_name, dev->scl, scl, dev->last_sda, sda, dev->sda, dev->state);
+    i2c_gpio_log(3, "I2C GPIO %s: write scl=%d->%d sda=%d->%d read=%d\n", dev->bus_name, dev->prev_scl, scl, dev->prev_sda, sda, dev->slave_read);
 
-    switch (dev->state) {
-	case I2C_IDLE:
-		if (scl && dev->last_sda && !sda) { /* start condition; dev->scl check breaks NCR SDMS */
-			i2c_gpio_log(2, "I2C GPIO %s: Start condition received (from IDLE)\n", dev->bus_name);
-			dev->state = I2C_RECEIVE;
-			dev->pos = 0;
-		}
-		break;
+    if (dev->prev_scl && scl) {
+    	if (dev->prev_sda && !sda) {
+    		i2c_gpio_log(2, "I2C GPIO %s: Start condition\n", dev->bus_name);
+    		dev->started = 1;
+    		dev->pos = 0;
+    		dev->slave_read = 2; /* start with address transfer */
+    		dev->slave_sda = 1;
+    	} else if (!dev->prev_sda && sda) {
+    		i2c_gpio_log(2, "I2C GPIO %s: Stop condition\n", dev->bus_name);
+    		dev->started = 0;
+    		dev->slave_sda = 1;
+    	}
+    } else if (!dev->prev_scl && scl && dev->started) {
+    	if (dev->pos++ < 8) {
+    		if (dev->slave_read == 1) {
+    			dev->slave_sda = !!(dev->byte & 0x80);
+    			dev->byte <<= 1;
+    		} else {
+    			dev->byte <<= 1;
+    			dev->byte |= sda;
+    		}
 
-	case I2C_RECEIVE_WAIT:
-		if (!dev->scl && scl)
-			dev->state = I2C_RECEIVE;
-		else if (!dev->scl && !scl && dev->last_sda && sda) /* workaround for repeated start condition on Windows XP DDC */
-			dev->receive_wait_sda = 1;
-		/* fall-through */
+    		i2c_gpio_log(2, "I2C GPIO %s: Bit %d = %d\n", dev->bus_name, 8 - dev->pos, (dev->slave_read == 1) ? dev->slave_sda : sda);
+    	}
 
-	case I2C_RECEIVE:
-		if (!dev->scl && scl) {
-			dev->byte <<= 1;
-			if (sda)
-				dev->byte |= 1;
-			else
-				dev->byte &= 0xfe;
-			if (++dev->pos == 8)
-				dev->state = i2c_gpio_write(dev);
-		} else if (dev->scl && scl) {
-			if (sda && !dev->last_sda) { /* stop condition */
-				i2c_gpio_log(2, "I2C GPIO %s: Stop condition received (from RECEIVE)\n", dev->bus_name);
-				dev->state = I2C_IDLE;
-				i2c_gpio_stop(dev);
-			} else if (!sda && dev->last_sda) { /* start condition */
-				i2c_gpio_log(2, "I2C GPIO %s: Start condition received (from RECEIVE)\n", dev->bus_name);
-				dev->pos = 0;
-				dev->slave_state = SLAVE_IDLE;
-			}
-		}
-		break;
+    	if (dev->pos == 8) {
+    		i2c_gpio_log(2, "I2C GPIO %s: Byte = %02X\n", dev->bus_name, dev->byte);
 
-	case I2C_ACKNOWLEDGE:
-		if (!dev->scl && scl) {
-			i2c_gpio_log(2, "I2C GPIO %s: Acknowledging transfer to %02X\n", dev->bus_name, dev->slave_addr);
-			sda = 0;
-			dev->receive_wait_sda = 0; /* ack */
-			dev->pos = 0;
-			dev->state = (dev->transmit == TRANSMITTER_MASTER) ? I2C_RECEIVE_WAIT : I2C_TRANSMIT;
-		}
-		break;
+    		/* (N)ACKing here instead of at the 9th bit may sound odd, but is required by the Matrox Mystique Windows drivers. */
+    		switch (dev->slave_read) {
+    			case 2: /* address transfer */
+    				dev->slave_addr = dev->byte >> 1;
+	    			dev->slave_read = (dev->byte & 1);
 
-	case I2C_NOTACKNOWLEDGE:
-		if (!dev->scl && scl) {
-			i2c_gpio_log(2, "I2C GPIO %s: Not acknowledging transfer\n", dev->bus_name);
-			sda = 1;
-			dev->pos = 0;
-			dev->state = I2C_IDLE;
-			dev->slave_state = SLAVE_IDLE;
-		}
-		break;
+	    			/* slave ACKs? */
+	    			dev->slave_sda = !(i2c_has_device(dev->i2c, dev->slave_addr) && i2c_start(dev->i2c, dev->slave_addr, dev->slave_read));
+				i2c_gpio_log(2, "I2C GPIO %s: Slave %02X %s %sACK\n", dev->bus_name, dev->slave_addr, dev->slave_read ? "read" : "write", dev->slave_sda ? "N" : "");
 
-	case I2C_TRANSACKNOWLEDGE:
-		if (!dev->scl && scl) {
-			if (sda) { /* not acknowledged; must be end of transfer */
-				i2c_gpio_log(2, "I2C GPIO %s: End of transfer\n", dev->bus_name);
-				dev->state = I2C_IDLE;
-				i2c_gpio_stop(dev);
-			} else { /* next byte to transfer */
-				dev->state = I2C_TRANSMIT_START;
-				i2c_gpio_next_byte(dev);
-				dev->pos = 0;
-				i2c_gpio_log(2, "I2C GPIO %s: Next byte = %02X\n", dev->bus_name, dev->byte);
-			}
-		}
-		break;
+				if (!dev->slave_sda && dev->slave_read) /* read first byte on an ACKed read transfer */
+					dev->byte = i2c_read(dev->i2c, dev->slave_addr);
 
-	case I2C_TRANSMIT_WAIT:
-		if (dev->scl && scl) {
-			if (dev->last_sda && !sda) { /* start condition */
-				i2c_gpio_next_byte(dev);
-				dev->pos = 0;
-				i2c_gpio_log(2, "I2C GPIO %s: Next byte = %02X\n", dev->bus_name, dev->byte);
-			}
-			if (!dev->last_sda && sda) { /* stop condition */
-				i2c_gpio_log(2, "I2C GPIO %s: Stop condition received (from TRANSMIT_WAIT)\n", dev->bus_name);
-				dev->state = I2C_IDLE;
-				i2c_gpio_stop(dev);
-			}
-		}
-		break;
+				dev->slave_read |= 0x80; /* slave_read was overwritten; stop the master ACK read logic from running at the 9th bit if we're reading */
+				break;
 
-	case I2C_TRANSMIT_START:
-		if (!dev->scl && scl)
-			dev->state = I2C_TRANSMIT;
-		if (dev->scl && scl && !dev->last_sda && sda) { /* stop condition */
-			i2c_gpio_log(2, "I2C GPIO %s: Stop condition received (from TRANSMIT_START)\n", dev->bus_name);
-			dev->state = I2C_IDLE;
-			i2c_gpio_stop(dev);
-		}
-		/* fall-through */
-
-	case I2C_TRANSMIT:
-		if (!dev->scl && scl) {
-			dev->scl = scl;
-			if (!dev->pos)
-				i2c_gpio_log(2, "I2C GPIO %s: Transmit byte %02X\n", dev->bus_name, dev->byte);
-			dev->sda = sda = dev->byte & 0x80;
-			i2c_gpio_log(2, "I2C GPIO %s: Transmit bit %02X %d\n", dev->bus_name, dev->byte, dev->pos);
-			dev->byte <<= 1;
-			dev->pos++;
-			return;
-		}
-		if (dev->scl && !scl && (dev->pos == 8)) {
-			dev->state = I2C_TRANSACKNOWLEDGE;
-			i2c_gpio_log(2, "I2C GPIO %s: Acknowledge mode\n", dev->bus_name);
-		}
-		break;
+    			case 0: /* write transfer */
+    				dev->slave_sda = !i2c_write(dev->i2c, dev->slave_addr, dev->byte);
+    				i2c_gpio_log(2, "I2C GPIO %s: Write %02X %sACK\n", dev->bus_name, dev->byte, dev->slave_sda ? "N" : "");
+    				break;
+    		}
+    	} else if (dev->pos == 9) {
+    		if (dev->slave_read == 1) { /* read transfer (unless we're in an address transfer) */
+    			if (!sda) /* master ACKs? */
+    				dev->byte = i2c_read(dev->i2c, dev->slave_addr);
+    			i2c_gpio_log(2, "I2C GPIO %s: Read %02X %sACK\n", dev->bus_name, dev->byte, sda ? "N" : "");
+    		} else
+    			dev->slave_read &= 1; /* if we're in an address transfer, clear it */
+    		dev->pos = 0; /* start over */
+    	}
+    } else if (dev->prev_scl && !scl && (dev->pos != 8)) { /* keep (N)ACK computed at the 8th bit when transitioning to the 9th bit */
+    	dev->slave_sda = 1;
     }
 
-    if (!dev->scl && scl)
-	dev->sda = sda;
-    dev->last_sda = sda;
-    dev->scl = scl;
+    dev->prev_scl = scl;
+    dev->prev_sda = sda;
 }
 
 
@@ -317,7 +161,7 @@ uint8_t
 i2c_gpio_get_scl(void *dev_handle)
 {
     i2c_gpio_t *dev = (i2c_gpio_t *) dev_handle;
-    return dev->scl;
+    return dev->prev_scl;
 }
 
 
@@ -325,17 +169,8 @@ uint8_t
 i2c_gpio_get_sda(void *dev_handle)
 {
     i2c_gpio_t *dev = (i2c_gpio_t *) dev_handle;
-    switch (dev->state) {
-	case I2C_TRANSMIT:
-	case I2C_ACKNOWLEDGE:
-		return dev->sda;
-
-	case I2C_RECEIVE_WAIT:
-		return dev->receive_wait_sda;
-
-	default:
-		return 1;
-    }
+    i2c_gpio_log(3, "I2C GPIO %s: read myscl=%d mysda=%d slavesda=%d\n", dev->bus_name, dev->prev_scl, dev->prev_sda, dev->slave_sda);
+    return dev->prev_sda && dev->slave_sda;
 }