diff --git a/cmd_editor.c b/cmd_editor.c
index 866cf40..aebfe7f 100644
--- a/cmd_editor.c
+++ b/cmd_editor.c
@@ -193,7 +193,7 @@ void cmd_edit(void) __banked
 		// Check whether return was pressed:
 		if (sbuf[l] == '\n' || sbuf[l] == '\r') {
 			write_char('\n');
-			cmd_buffer[cmd_line_len] = '\0';
+			cmd_buffer[cmd_line_len] = NUL;
 //			write_char('>'); print_string_x(cmd_buffer); write_char('<');
 			// If there is a command we print the prompt after execution
 			// otherwise immediately because there is nothing to execute
diff --git a/cmd_parser.c b/cmd_parser.c
index 1f4da2d..d75233f 100644
--- a/cmd_parser.c
+++ b/cmd_parser.c
@@ -114,8 +114,8 @@ uint8_t cmd_compare(uint8_t start, __code uint8_t * cmd)
 		uint8_t b = cmd_buffer[i];
 
 		// cmd is garanteerd to be NULL-terminated.
-        if (c == '\0') {
-            if ((b == ' ') || (b == '\0')) {
+        if (c == NUL) {
+            if ((b == ' ') || (b == NUL)) {
 				// Match
                 return 1;
             }
@@ -147,7 +147,7 @@ uint8_t atoi_hex(uint8_t idx)
 	while(1) {
 		c = cmd_buffer[idx];
 
-		if (c == '\0' || c == ' ') {
+		if (c == NUL || c == ' ') {
 			break;
 		}
 
@@ -263,7 +263,7 @@ uint8_t cmd_parse_port_separator(uint8_t idx) {
 		uint8_t c = cmd_buffer[idx];
 		if (c == ' ') {
 			ret++;
-		} else if (c != '\0')
+		} else if (c != NUL)
 			ret = 0;
 	}
 	return ret;
@@ -278,7 +278,7 @@ __bit cmd_is_space(uint8_t idx) {
 // check if the cmd_buffer[idx] is a space or NULL.
 __bit cmd_is_space_or_nul(uint8_t idx) {
 	uint8_t c = cmd_buffer[idx];
-	return c == ' ' || c == '\0';
+	return c == ' ' || c == NUL;
 }
 
 // returns 0 when on parser error or invalid value or no space.
@@ -304,7 +304,7 @@ uint8_t parse_ip(uint8_t idx)
 				idx++;
 				break;
 			}
-			if (ret == '\0')
+			if (ret == NUL)
 				break;
 			goto err;
 		}
@@ -483,7 +483,7 @@ void parse_vlan(void)
 				write_char(cmd_buffer[cmd_words_b[w] + i]);
 				vlan_names[vlan_ptr++] = cmd_buffer[cmd_words_b[w] + i++];
 			}
-			vlan_names[vlan_ptr++] = ' '; vlan_names[vlan_ptr] = '\0';
+			vlan_names[vlan_ptr++] = ' '; vlan_names[vlan_ptr] = NUL;
 			w++;
 			print_string("<\n");
 		}
@@ -738,11 +738,11 @@ void parse_port(void)
 		}
 	} else if (cmd_compare(2, "name")) {
 		uint8_t i = 0;
-		while ( (i < PORT_NAME_SIZE-1) && (cmd_buffer[cmd_words_b[3] + i] != '\0') ) {
+		while ( (i < PORT_NAME_SIZE-1) && (cmd_buffer[cmd_words_b[3] + i] != NUL) ) {
 			port_names[phy_settings.port][i] = cmd_buffer[cmd_words_b[3] + i];
 			i++;
 		}
-		port_names[phy_settings.port][i] = '\0';
+		port_names[phy_settings.port][i] = NUL;
 		print_string("\nName set to: \"");
 		print_string_x(port_names[phy_settings.port]);
 		print_string("\"\n");
@@ -1248,8 +1248,8 @@ void parse_passwd(void)
 		do {
 			c = cmd_buffer[i++];
 			passwd[j++] = c;
-		} while (c != '\0' && j < 20);
-		passwd[j] = '\0';
+		} while (c != NUL && j < 20);
+		passwd[j] = NUL;
 		return;
 	}
 	print_string("Missing password\n");
@@ -1453,7 +1453,7 @@ void cmd_tokenize(void) __banked
 	while(1) {
 		c = cmd_buffer[line_ptr];
 		
-		if (c == '\0') {
+		if (c == NUL) {
 			// Store the word count
 			cmd_words_len = word;
 			break;
@@ -1658,13 +1658,13 @@ void cmd_parser(void) __banked
 				__xdata char *dst = hostname;
 				for (uint8_t hn = 0; hn < sizeof(hostname) - 1; hn++) {
 					uint8_t c = *hp++;
-					if (c == '\0' || c == '\r' || c == '\n')
+					if (c == NUL || c == '\r' || c == '\n')
 						break;
 					if (c < 0x20 || c > 0x7e || c == '"' || c == '\\')
 						c = '.';
 					*dst++ = c;
 				}
-				*dst = '\0';
+				*dst = NUL;
 			} else {
 				print_string("Error: hostname [name] - the name must not contain spaces\n");
 			}
@@ -1752,7 +1752,7 @@ void cmd_parser(void) __banked
 			uint8_t i = cmd_words_b[cmd_words_len - 1];
 			do {
 				i++;
-			} while(cmd_buffer[i] != '\0');
+			} while(cmd_buffer[i] != NUL);
 
 			// Copy last cmd-buffer to history.
 			cmd_history_ptr = (cmd_history_ptr + i) & CMD_HISTORY_MASK;
@@ -1800,7 +1800,7 @@ void execute_config(void) __banked
 		uint8_t c = 0;
 		do {
 			if (cmd_idx >= (CMD_BUF_SIZE - 1)) {
-				cmd_buffer[cmd_idx] = '\0';
+				cmd_buffer[cmd_idx] = NUL;
 				print_string("ERROR: Command too long: ");
 				print_string_x(cmd_buffer);
 				write_char('\n');
@@ -1809,7 +1809,7 @@ void execute_config(void) __banked
 			}
 			c = flash_buf[cfg_idx++];
 			if (c == 0 || c == '\n') {
-				cmd_buffer[cmd_idx] = '\0';
+				cmd_buffer[cmd_idx] = NUL;
 				if (cmd_idx) {
 					cmd_tokenize();
 					if (err_status != ERR_OK)
@@ -1845,7 +1845,7 @@ void execute_commands(__xdata uint8_t *p) __banked {
 	while (1) {
 		if (*p == 0 || *p == '\n' || *p == '\r') {
 			if (cmd_idx) {
-				cmd_buffer[cmd_idx] = '\0';
+				cmd_buffer[cmd_idx] = NUL;
 				cmd_tokenize();
 				if (err_status != ERR_OK)
 					return;
@@ -1858,7 +1858,7 @@ void execute_commands(__xdata uint8_t *p) __banked {
 			if (cmd_idx < (CMD_BUF_SIZE - 1)) {
 				cmd_buffer[cmd_idx++] = *p;
 			} else {
-				cmd_buffer[CMD_BUF_SIZE - 1] = '\0';
+				cmd_buffer[CMD_BUF_SIZE - 1] = NUL;
 				print_string("ERROR: Command too long: ");
 				print_string_x(cmd_buffer);
 				write_char('\n');
diff --git a/httpd/httpd.c b/httpd/httpd.c
index ff4eff9..8f52631 100644
--- a/httpd/httpd.c
+++ b/httpd/httpd.c
@@ -119,8 +119,8 @@ bool is_word(__xdata uint8_t *xdata_str_p, __code uint8_t * __xdata code_str_p)
 		u = *xdata_str_p++;
 		c = *code_str_p++;
 
-		if (c == '\0') {
-			if (u != '\0' && u != ' ' && u != '\t' && u != ':' && u != '?' && u != '=' && u != '\n' && u != '\r')
+		if (c == NUL) {
+			if (u != NUL && u != ' ' && u != '\t' && u != ':' && u != '?' && u != '=' && u != '\n' && u != '\r')
 				return false;
 			return true;
 		}
@@ -140,8 +140,8 @@ bool is_url_word_x(__xdata uint8_t *uri_str_p, __xdata uint8_t *src_str_p)
 		u = *uri_str_p++;
 		s = *src_str_p++;
 
-		if (s == '\0') {
-			if (u != '\0' && u != ' ' && u != '\t' && u != ':' && u != '?' && u != '=' && u != '\n' && u != '\r')
+		if (s == NUL) {
+			if (u != NUL && u != ' ' && u != '\t' && u != ':' && u != '?' && u != '=' && u != '\n' && u != '\r')
 				return false;
 			return true;
 		}
@@ -183,9 +183,9 @@ bool is_word_x(__xdata uint8_t *lhs_str_p, __xdata uint8_t *rhs_str_p)
 		u = *lhs_str_p++;
 		c = *rhs_str_p++;
 
-		if (c == '\0') {
+		if (c == NUL) {
 			/* ';' separates cookies in a Cookie header, so it ends a value too. */
-			if (u != '\0' && u != ' ' && u != '\t' && u != ':' && u != '?' && u != '=' && u != '\n' && u != '\r' && u != ';')
+			if (u != NUL && u != ' ' && u != '\t' && u != ':' && u != '?' && u != '=' && u != '\n' && u != '\r' && u != ';')
 				return false;
 			return true;
 		}
@@ -487,10 +487,10 @@ void handle_post(void)
 		// Find end of request path
 		while (*p && !is_separator(*p))
 			p++;
-		*p++ = '\0';
+		*p++ = NUL;
 
 		// Find end of request header
-		boundary[0] ='\0';
+		boundary[0] =NUL;
 		p = scan_header(p);
 		dbg_string("Boundary: >"); dbg_string_x(boundary); dbg_string("<\n");
 		if (!*p || !content_type) {
@@ -550,7 +550,7 @@ void handle_post(void)
 			dbg_string("Password accepted!\n");
 			read_reg_timer(&last_session_use);
 			gen_random_bytes(session_id, SESSION_ID_LENGTH);
-			session_id[SESSION_ID_LENGTH] = '\0';
+			session_id[SESSION_ID_LENGTH] = NUL;
 			slen = strtox(outbuf, "HTTP/1.1 302 Found\r\nConnection: close\r\nLocation: index.html\r\n" \
 					      "Set-Cookie: session=");
 			for (register uint8_t i = 0; i < SESSION_ID_LENGTH; i++)
@@ -736,7 +736,7 @@ void httpd_appcall(void)
 		__xdata uint8_t *q = p;
 		while (*p && !is_separator(*p))
 			p++;
-		*p = '\0';
+		*p = NUL;
 		dbg_string_x(q);
 		dbg_char('\n');
 
diff --git a/rtl837x_common.h b/rtl837x_common.h
index 27d9449..8f7bf1b 100644
--- a/rtl837x_common.h
+++ b/rtl837x_common.h
@@ -8,6 +8,7 @@
 #define SYS_TICK_HZ 200
 
 #define CPU_PORT        9
+#define NUL			'\0'
 
 // Define Port-masks for 9-port devices and 6-port devices
 #define PMASK_9		0x1ff
diff --git a/rtlplayground.c b/rtlplayground.c
index d94d0fb..a7f1dc5 100644
--- a/rtlplayground.c
+++ b/rtlplayground.c
@@ -1257,11 +1257,11 @@ bool sfp_read_field(__xdata char *dst, uint8_t sfp, uint8_t start, uint8_t lengt
 	if (!sfp_read_block(sfp, start, length))
 		return false;
 
-	dst[length] = '\0';
+	dst[length] = NUL;
 	memcpy(dst, sfp_buf, length);
 
 	while (length > 0 && dst[--length] == ' ')
-		dst[length] = '\0';
+		dst[length] = NUL;
 
 	return true;
 }
@@ -2069,7 +2069,7 @@ void check_and_flash_update_image(void)
  * because itohex() is inline and brings its own frame. */
 void set_hostname_default(void)
 {
-	if (hostname[0] != '\0')
+	if (hostname[0] != NUL)
 		return;
 
 	strcpy((__xdata uint8_t *)hostname, "RTLPlayground-");
@@ -2079,7 +2079,7 @@ void set_hostname_default(void)
 	hostname[17] = hex[uip_ethaddr.addr[4] & 0xf];
 	hostname[18] = hex[uip_ethaddr.addr[5] >> 4];
 	hostname[19] = hex[uip_ethaddr.addr[5] & 0xf];
-	hostname[20] = '\0';
+	hostname[20] = NUL;
 }