diff --git a/lib/README b/lib/README
index 38eaadb2..5976b7cc 100644
--- a/lib/README
+++ b/lib/README
@@ -21,7 +21,9 @@ compile with gcc's LTO feature. See sam3x.patch for the modifications.
 The samd21 directory contains code from the
 Atmel.SAMD21_DFP.1.3.304.atpack zip file found at:
   http://packs.download.atmel.com/
-version 1.3.304 (extracted on 20180725).
+version 1.3.304 (extracted on 20180725). It has been modified to
+compile with gcc's LTO feature and to work with chips that have a
+bootloader. See samd21.patch for the modifications.
 
 The lpc176x directory contains code from the mbed project:
   https://github.com/ARMmbed/mbed-os
diff --git a/lib/samd21/samd21.patch b/lib/samd21/samd21.patch
new file mode 100644
index 00000000..ae7d7670
--- /dev/null
+++ b/lib/samd21/samd21.patch
@@ -0,0 +1,26 @@
+diff --git a/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld b/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld
+index c6f289e7..34dfb6ba 100644
+--- a/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld
++++ b/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld
+@@ -35,7 +35,7 @@ SEARCH_DIR(.)
+ /* Memory Spaces Definitions */
+ MEMORY
+ {
+-  rom      (rx)  : ORIGIN = 0x00000000, LENGTH = 0x00040000
++  rom      (rx)  : ORIGIN = 0x00000000 + FLASH_START , LENGTH = 0x00040000 - FLASH_START
+   ram      (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000
+ }
+ 
+diff --git a/lib/samd21/samd21a/gcc/gcc/startup_samd21.c b/lib/samd21/samd21a/gcc/gcc/startup_samd21.c
+index 3dce0edd..a480077f 100644
+--- a/lib/samd21/samd21a/gcc/gcc/startup_samd21.c
++++ b/lib/samd21/samd21a/gcc/gcc/startup_samd21.c
+@@ -105,7 +105,7 @@ void PTC_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler
+ void I2S_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
+ 
+ /* Exception Table */
+-__attribute__ ((section(".vectors")))
++__attribute__ ((section(".vectors"))) __attribute__((externally_visible))
+ const DeviceVectors exception_table = {
+ 
+         /* Configure Initial Stack Pointer, using linker-generated symbols */
diff --git a/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld b/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld
index c6f289e7..34dfb6ba 100644
--- a/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld
+++ b/lib/samd21/samd21a/gcc/gcc/samd21g18a_flash.ld
@@ -35,7 +35,7 @@ SEARCH_DIR(.)
 /* Memory Spaces Definitions */
 MEMORY
 {
-  rom      (rx)  : ORIGIN = 0x00000000, LENGTH = 0x00040000
+  rom      (rx)  : ORIGIN = 0x00000000 + FLASH_START , LENGTH = 0x00040000 - FLASH_START
   ram      (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000
 }
 
diff --git a/lib/samd21/samd21a/gcc/gcc/startup_samd21.c b/lib/samd21/samd21a/gcc/gcc/startup_samd21.c
index 3dce0edd..a480077f 100644
--- a/lib/samd21/samd21a/gcc/gcc/startup_samd21.c
+++ b/lib/samd21/samd21a/gcc/gcc/startup_samd21.c
@@ -105,7 +105,7 @@ void PTC_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler
 void I2S_Handler             ( void ) __attribute__ ((weak, alias("Dummy_Handler")));
 
 /* Exception Table */
-__attribute__ ((section(".vectors")))
+__attribute__ ((section(".vectors"))) __attribute__((externally_visible))
 const DeviceVectors exception_table = {
 
         /* Configure Initial Stack Pointer, using linker-generated symbols */