diff --git a/main.c b/main.c
index 9d891ea..2dfda61 100644
--- a/main.c
+++ b/main.c
@@ -11,6 +11,7 @@
 #include "softi2c_master.h"
 
 #define EEPROM_ADDR 0x4000 // EEPROM start address
+static bool eeprom_valid = true; // if the EDID can be read from EEPROM
 // LED pin (sink for on)
 #define LED_PORT GPIO_PA
 #define LED_PIN PA3
@@ -102,7 +103,7 @@ structure shall also meet the requirements of CEA-861-D.
  * EDID 2.0 with its 256 bytes does not seem to be used in HDMI at all
  * DisplayID with its variable-length structure meant to replace E-EDID only seems to be used in DisplayPort
  */
-static uint16_t edid_length(uint8_t* edid)
+static uint16_t edid_length(const uint8_t* edid)
 {
 	puts("EDID check: ");
 	// check EDID 1.3/1.4 fixed pattern header
@@ -144,6 +145,36 @@ static uint16_t edid_length(uint8_t* edid)
 	return 0;
 }
 
+// modify EDID to indicate firewall
+static void edid_modify(uint8_t* edid)
+{
+	// modify EDID to include the character indicating the firewall
+	const char firewall_indicator = '|'; // pipe/wall character to indicate the firewall
+	// ensure descriptor 4 is for Display name
+	if ((0 == edid[108]) && (0 == edid[109]) && (0 == edid[110]) && (0xfc == edid[111]) && (0 == edid[112])) { // ensure descriptor 4 is for Display name
+		uint8_t last_c; // position of last character
+		for (last_c = 113; last_c < 126 && edid[last_c] != '\n'; last_c++); // find position for inserting our character
+		if (firewall_indicator != edid[last_c - 1]) { // the last character is not yet the pipe
+			if (last_c > 125) { // ensure we insert as the last possible character
+				last_c = 125;
+			}
+			edid[last_c++] = firewall_indicator; // insert pipe
+			if (last_c < 126) {
+				edid[last_c++] = '\n'; // insert LF to terminate string
+			}
+			while (last_c < 126) {
+				edid[last_c++] = ' '; // insert padding space
+			}
+			// calculate new checksum
+			uint8_t checksum = 0;
+			for (uint8_t i = 0; i < 127; i++) {
+				checksum += edid[i];
+			}
+			edid[127] = (256 - checksum);
+		}
+	}
+}
+
 void main(void)
 {
 	sim(); // disable interrupts (while we reconfigure them)
@@ -250,23 +281,19 @@ we will only be able to provide 1 extension block since we can only respond to o
 		HPD_PORT->ODR.reg |= HPD_PIN; // pull up HPD line to indicate EDID is ready
 	}
 
-	// check if EDID should be copied
-	EDID_PORT->DDR.reg &= ~EDID_PIN; // switch pin to output
-	EDID_PORT->CR1.reg |= EDID_PIN; // enable pull up
-	if (0 == (EDID_PORT->ODR.reg & EDID_PIN)) { // EDID switched on
-		// TODO copy EDID from sink/monitor
-/*
-An HDMI Sink shall indicate any change to the contents of the E-EDID by driving a low voltage
-level pulse on the Hot Plug Detect pin. This pulse shall be at least 100 msec.
-*/
-	}
-
 	rim(); // re-enable interrupts
 	// even if we don't pull up HPD ourself, we should be able to respond to I²C within 20 ms
 	puts("I²C ready\r\n");
 
-	// read sink EDID
-	if (!i2c_fwd) { // ensure we are the only master on the I²C sink lines
+	// check if EDID should be copied
+	EDID_PORT->DDR.reg &= ~EDID_PIN; // switch pin to output
+	EDID_PORT->CR1.reg |= EDID_PIN; // enable pull up
+	if (EDID_PORT->IDR.reg & EDID_PIN) { // EDID switched off
+		puts("EDID protected\r\n");
+	} else if (i2c_fwd) { // we are not the only master on the I²C sink lines
+		puts("can't read EDID: I²C lines forwarded\r\n");
+		LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate error
+	} else {
 		IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
 		puts("reading sink EDID: ");
 		softi2c_master_setup(100); // start the I²C master to talk to sink
@@ -297,13 +324,81 @@ i2c_end:
 		if (i2c_success) {
 			puts("success\r\n");
 			const uint16_t edid_len = edid_length(edid_sink); // get length
+			if (edid_len) { // EDID is valid
+				edid_modify(edid_sink); // modify EDID to include firewall indication
+				// compare saved/source and sink EDID
+				bool edid_equal = true;
+				for (uint16_t i = 0; i < edid_len && edid_equal; i++) {
+					if (*(uint8_t*)(EEPROM_ADDR + i) != edid_sink[i]) {
+						edid_equal = false;
+					}
+				}
+				if (edid_equal) {
+					puts("EDID not changed\r\n");
+				} else {
+					LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate we are saving EDID
+					puts("saving EDID: ");
+					// note: the STM8S103 does not support RWW
+					// try to make fast small operations to not stall I²C communication
+					eeprom_valid = false; // invalidate saved EDID while re-programming it
+					// disable DATA (e.g. EEPROM) write protection
+					if (0 == (FLASH_IAPSR & FLASH_IAPSR_DUL)) {
+						FLASH_DUKR = FLASH_DUKR_KEY1;
+						FLASH_DUKR = FLASH_DUKR_KEY2;
+					}
+					// erase EEPROM (we don't do faster block erase since it needs to be done from RAM)
+					for (uint16_t i = 0; i < 256; i += 4U) { // go through word
+						IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
+						FLASH_CR2 |= FLASH_CR2_WPRG; // set word programming
+						FLASH_NCR2 &= ~FLASH_NCR2_NWPRG; // set word programming
+						*(uint32_t*)(EEPROM_ADDR + i) = 0; // erase word
+						while (FLASH_CR2 & FLASH_CR2_WPRG); // wait for erase to complete
+						// check if programming failed
+						// we don't check for WR_PG_DIS (while checking EOP) because EEPROM isn't (and can't be) write protected
+						if (!(FLASH_IAPSR & FLASH_IAPSR_EOP)) {
+							FLASH_IAPSR &= ~FLASH_IAPSR_DUL; // re-enable write protection
+							puts("failed\r\n");
+							break;
+						}
+					}
+					// save need EDID
+					for (uint16_t i = 0; i < edid_len; i += 4U) { // go through word
+						IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
+						FLASH_CR2 |= FLASH_CR2_WPRG; // set word programming
+						FLASH_NCR2 &= ~FLASH_NCR2_NWPRG; // set word programming
+						*(uint32_t*)(EEPROM_ADDR + i) = *(uint32_t*)(&edid_sink[i]); // write word
+						while (FLASH_CR2 & FLASH_CR2_WPRG); // wait for write to complete
+						// check if programming failed
+						// we don't check for WR_PG_DIS (while checking EOP) because EEPROM isn't (and can't be) write protected
+						if (!(FLASH_IAPSR & FLASH_IAPSR_EOP)) {
+							FLASH_IAPSR &= ~FLASH_IAPSR_DUL; // re-enable write protection
+							puts("failed\r\n");
+							break;
+						}
+					}
+					FLASH_IAPSR &= ~FLASH_IAPSR_DUL; // re-enable write protection
+					eeprom_valid = true; // re-enable EDID reading
+					LED_PORT->ODR.reg |= LED_PIN; // switch LED off to indicate we are completed saving EDID
+					puts("done\r\n");
+					// indicate there is a new EDID
+					if (!hpd_fwd) { // we have to pull up
+/*
+HDMI v1.3 spec:
+An HDMI Sink shall indicate any change to the contents of the E-EDID by driving a low voltage
+level pulse on the Hot Plug Detect pin. This pulse shall be at least 100 msec.
+*/
+						HPD_PORT->ODR.reg &= ~HPD_PIN; // pull down HPD line to indicate EDID change
+						wait_10us(110 * 100); // wait over 100 ms
+						HPD_PORT->ODR.reg |= HPD_PIN; // pull up HPD line to indicate EDID is ready
+					}
+				}
+			}
 		} else {
 			puts("fail\r\n");
 		}
 	}
 
 	bool action = false; // if an action has been performed
-	LED_PORT->ODR.reg &= ~LED_PIN; // switch LED on to indicate we are ready
 	puts("loop\r\n");
 	while (true) {
 		IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
@@ -337,7 +432,12 @@ void i2c(void) __interrupt(IRQ_I2C) // auto wakeup
 	const uint8_t sr2 = I2C_SR2;
 	const uint8_t sr3 = I2C_SR3; // clears ADDR after reading SR1
 	if (sr1 & I2C_SR1_TXE) { // transmission buffer is empty
-		I2C_DR = *(uint8_t*)(EEPROM_ADDR + addr++); // transmit next byte
+		// transmit next byte
+		if (eeprom_valid) {
+			I2C_DR = *(uint8_t*)(EEPROM_ADDR + addr++);
+		} else {
+			I2C_DR = 0xff;
+		}
 	}
 	if (sr1 & I2C_SR1_RXNE) { // receive buffer is full
 		const uint8_t data = I2C_DR; // read data (also clears flag)
@@ -352,7 +452,11 @@ void i2c(void) __interrupt(IRQ_I2C) // auto wakeup
 	if (sr1 & I2C_SR1_ADDR) { // our slave address has been selected
 		i2c_transaction_new = true; // notify main loop transaction started
 		if (sr3 & I2C_SR3_TRA) { // start data transmission
-			I2C_DR = *(uint8_t*)(EEPROM_ADDR + addr++); // transmit selected byte
+			if (eeprom_valid) {
+				I2C_DR = *(uint8_t*)(EEPROM_ADDR + addr++); // transmit selected byte
+			} else {
+				I2C_DR = 0xff;
+			}
 			i2c_input_new = false; // notify we send data
 		} else { // we will receive data
 			I2C_CR2 |= I2C_CR2_ACK; // ACK next received byte