diff --git a/cores/esp8266/Esp.cpp b/cores/esp8266/Esp.cpp
index 42715605c1..b18574dad2 100644
--- a/cores/esp8266/Esp.cpp
+++ b/cores/esp8266/Esp.cpp
@@ -132,9 +132,43 @@ uint64_t EspClass::deepSleepMax()
 
 }
 
+/* 
+Layout of RTC Memory is as follows:
+Ref: Espressif doc 2C-ESP8266_Non_OS_SDK_API_Reference, section 3.3.23 (system_rtc_mem_write)
+
+|<------system data (256 bytes)------->|<-----------------user data (512 bytes)--------------->|
+
+SDK function signature:   
+bool	system_rtc_mem_read	(
+				uint32	des_addr,	
+				void	*	src_addr,	
+				uint32	save_size
+)   
+
+The system data section can't be used by the user, so:
+des_addr must be >=64 (i.e.: 256/4) and <192 (i.e.: 768/4)
+src_addr is a pointer to data
+save_size is the number of bytes to write
+
+For the method interface:
+offset is the user block number (block size is 4 bytes) must be >= 0 and <128
+data is a pointer to data, 4-byte aligned
+size is number of bytes in the block pointed to by data
+
+Same for write.
+
+Note: If the Updater class is in play, e.g.: the application uses OTA, the eboot
+command will be stored into the first 128 bytes of user data, then it will be
+retrieved by eboot on boot. That means that user data present there will be lost.
+Ref: 
+- discussion in PR #5330.
+- https://github.com/esp8266/esp8266-wiki/wiki/Memory-Map#memmory-mapped-io-registers
+- Arduino/bootloaders/eboot/eboot_command.h RTC_MEM definition
+*/
+
 bool EspClass::rtcUserMemoryRead(uint32_t offset, uint32_t *data, size_t size)
 {
-    if (size + offset > 512) {
+    if (offset*4 + size > 512 || size == 0) {
         return false;
     } else {
         return system_rtc_mem_read(64 + offset, data, size);
@@ -143,13 +177,15 @@ bool EspClass::rtcUserMemoryRead(uint32_t offset, uint32_t *data, size_t size)
 
 bool EspClass::rtcUserMemoryWrite(uint32_t offset, uint32_t *data, size_t size)
 {
-    if (size + offset > 512) {
+    if (offset*4 + size > 512 || size == 0) {
         return false;
     } else {
         return system_rtc_mem_write(64 + offset, data, size);
     }
 }
 
+
+
 extern "C" void __real_system_restart_local();
 void EspClass::reset(void)
 {
diff --git a/cores/esp8266/flash_utils.h b/cores/esp8266/flash_utils.h
index 67dc6ebadd..932a2c7c54 100644
--- a/cores/esp8266/flash_utils.h
+++ b/cores/esp8266/flash_utils.h
@@ -21,6 +21,8 @@
 #ifndef FLASH_UTILS_H
 #define FLASH_UTILS_H
 
+#include "spi_flash_sec_size.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -31,7 +33,7 @@ int SPIRead(uint32_t addr, void *dest, size_t size);
 int SPIWrite(uint32_t addr, void *src, size_t size);
 int SPIEraseAreaEx(const uint32_t start, const uint32_t size);
 
-#define FLASH_SECTOR_SIZE 0x1000
+#define FLASH_SECTOR_SIZE SPI_FLASH_SEC_SIZE
 #define FLASH_BLOCK_SIZE 0x10000
 #define APP_START_OFFSET 0x1000
 
diff --git a/tests/host/common/spi_flash_sec_size.h b/tests/host/common/spi_flash_sec_size.h
new file mode 100644
index 0000000000..a4096f7a65
--- /dev/null
+++ b/tests/host/common/spi_flash_sec_size.h
@@ -0,0 +1,8 @@
+#ifndef SPI_FLASH_SEC_SIZE_H
+#define SPI_FLASH_SEC_SIZE_H
+
+//pulled this define from tools/sdk/include/spi_flash.h for reuse in the Arduino core without pulling in a bunch of other stuff
+#define SPI_FLASH_SEC_SIZE      4096
+
+
+#endif
diff --git a/tools/sdk/include/spi_flash.h b/tools/sdk/include/spi_flash.h
index 00f8a08b79..d5c7e9a348 100644
--- a/tools/sdk/include/spi_flash.h
+++ b/tools/sdk/include/spi_flash.h
@@ -25,6 +25,8 @@
 #ifndef SPI_FLASH_H
 #define SPI_FLASH_H
 
+#include "spi_flash_sec_size.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -44,8 +46,6 @@ typedef struct{
 	uint32  status_mask;
 } SpiFlashChip;
 
-#define SPI_FLASH_SEC_SIZE      4096
-
 extern SpiFlashChip * flashchip; // in ram ROM-BIOS
 
 uint32 spi_flash_get_id(void);
diff --git a/tools/sdk/include/spi_flash_sec_size.h b/tools/sdk/include/spi_flash_sec_size.h
new file mode 100644
index 0000000000..48724d0df2
--- /dev/null
+++ b/tools/sdk/include/spi_flash_sec_size.h
@@ -0,0 +1,8 @@
+#ifndef SPI_FLASH_SEC_SIZE_H
+#define SPI_FLASH_SEC_SIZE_H
+
+//pulled this define from spi_flash.h for reuse in the Arduino core without pulling in a bunch of other stuff
+#define SPI_FLASH_SEC_SIZE      4096
+
+
+#endif