diff --git a/rtl837x_common.h b/rtl837x_common.h
index 71b5547e..2ae75fdf 100644
--- a/rtl837x_common.h
+++ b/rtl837x_common.h
@@ -135,9 +135,9 @@ extern __xdata uint8_t uip_buf[UIP_CONF_BUFFER_SIZE+2];
 extern __xdata struct uip_eth_addr uip_ethaddr;
 
 // Headers for calls in the common code area (HOME/BANK0)
-void print_string_no_syslog(__code char *p);
-void print_string_newline_no_syslog(__code char *p);
-void print_string(__code char *p);
+void print_string_no_syslog(__code const char *p);
+void print_string_newline_no_syslog(__code const char *p);
+void print_string(__code const char *p);
 void print_string_x(__xdata char *p);
 void print_long(uint32_t a);
 void print_short(uint16_t a);
diff --git a/rtlplayground.c b/rtlplayground.c
index a2efbf5a..1151d397 100644
--- a/rtlplayground.c
+++ b/rtlplayground.c
@@ -303,19 +303,19 @@ void itoa_short(uint16_t v)
 }
 
 
-void print_string(__code char *p)
+void print_string(__code const char *p)
 {
 	while (*p)
 		write_char(*p++);
 }
 
-void print_string_no_syslog(__code char *p)
+void print_string_no_syslog(__code const char *p)
 {
 	while (*p)
 		write_char_no_syslog(*p++);
 }
 
-void print_string_newline_no_syslog(__code char *p)
+void print_string_newline_no_syslog(__code const char *p)
 {
 	write_char_no_syslog('\n');
 	print_string_no_syslog(p);
diff --git a/test/support.c b/test/support.c
index 58e64ad5..1270554f 100644
--- a/test/support.c
+++ b/test/support.c
@@ -47,7 +47,7 @@ static void out_putc(char c) {
 
 /* --- mocked print helpers (signatures match rtl837x_common.h post-shim) ---- */
 void write_char(char c)            { out_putc(c); }
-void print_string(char *p)         { while (*p) out_putc(*p++); }
+void print_string(const char *p)   { while (*p) out_putc(*p++); }
 void print_string_x(char *p)       { while (*p) out_putc(*p++); }
 void print_cmd_prompt(void)        { print_string((char *)"> "); }
 void itoa(uint8_t v) {                       /* firmware prints a decimal number */