Merge pull request #1 from ianrrees/fixed-dbl-tap-addr

Make the magic "double tap" address constant

README.md

@@ -48,12 +48,12 @@ The USBCRM mode should be universal, as it doesn't depend on optional external c
 
 Pre-compiled images are already available via this project's Releases tab.
 
-[Rowley Crossworks for ARM](http://www.rowley.co.uk/arm/) is presently suggested to compile this code, as it includes support for Clang; at this time, I am not aware of any other ready-to-use (and multi-OS to boot) Clang ARM cross-compiler package that I can readily point users to.  With Crossworks for ARM v4.6.1, compiling v1.05 using the Clang 9.0.1 compiler produces a 1003 byte image.  The more mainstream GCC lags behind Clang, although more recent GCC versions produce code that is less overweight than in years past.
+[Rowley Crossworks for ARM](http://www.rowley.co.uk/arm/) is presently suggested to compile this code, as it includes support for Clang; at this time, I am not aware of any other ready-to-use (and multi-OS to boot) Clang ARM cross-compiler package that I can readily point users to.  With Crossworks for ARM v4.8.1, compiling v1.06 using the Clang 11.1.0 compiler produces a 995 byte image.  The more mainstream GCC lags behind Clang, although more recent GCC versions produce code that is less overweight than in years past.
 
-|bootloader variant|Clang 9.0.1 (-O1) |GNU Arm 2018-q3 (-Os) |GNU Arm 2019-q4 (-Os) |
-|------------------|------------------|----------------------|----------------------|
-| USE_DBL_TAP      | 1003 bytes       | 1044 bytes (too big!)| 1041 bytes (too big!)|
-| GPIO input       | 979 bytes        | 1008 bytes           | 1006 bytes           |
+|bootloader variant|Clang 9.0.1 (-O1) |Clang 11.1.0 (-O1) |GNU Arm 2018-q3 (-Os) |GNU Arm 2019-q4 (-Os) |
+|------------------|------------------|-------------------|----------------------|----------------------|
+| USE_DBL_TAP      | 1003 bytes       | 995 bytes         | 1044 bytes (too big!)| 1041 bytes (too big!)|
+| GPIO input       | 979 bytes        | 975 bytes         | 1008 bytes           | 1006 bytes           |
 
 A Makefile supporting GCC is provided, but even if you have the latest GCC, it may not be able to output a version within the 1024 bytes available.  The latest GCC for ARM can be here: [GNU Arm Embedded Toolchain](https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm).  Note that if you are adapting the Makefile for use with clang, try replacing the "-Os" argument in CFLAGS with something like "-O1" if the output size is larger than expected.
 

bootloader.c

@@ -226,9 +226,8 @@ static void USB_Service(void)
 }
 
 #ifdef USE_DBL_TAP
-  extern int __RAM_segment_used_end__;
-  static volatile uint32_t *DBL_TAP_PTR = (volatile uint32_t *)(&__RAM_segment_used_end__);
   #define DBL_TAP_MAGIC 0xf02669ef
+  static volatile uint32_t __attribute__((section(".vectors_ram"))) double_tap;
 #endif
 
 void bootloader(void)
@@ -259,20 +258,20 @@ void bootloader(void)
   return; /* we've checked everything and there is no reason to run the bootloader */
 #else
   if (PM->RCAUSE.reg & PM_RCAUSE_POR)
-    *DBL_TAP_PTR = 0; /* a power up event should never be considered a 'double tap' */
+    double_tap = 0; /* a power up event should never be considered a 'double tap' */
 
-  if (*DBL_TAP_PTR == DBL_TAP_MAGIC)
+  if (double_tap == DBL_TAP_MAGIC)
   {
     /* a 'double tap' has happened, so run bootloader */
-    *DBL_TAP_PTR = 0;
+    double_tap = 0;
     goto run_bootloader;
   }
 
   /* postpone boot for a short period of time; if a second reset happens during this window, the "magic" value will remain */
-  *DBL_TAP_PTR = DBL_TAP_MAGIC;
+  double_tap = DBL_TAP_MAGIC;
   volatile int wait = 65536; while (wait--);
   /* however, if execution reaches this point, the window of opportunity has closed and the "magic" disappears  */
-  *DBL_TAP_PTR = 0;
+  double_tap = 0;
   return;
 #endif
 

linker/samd11d14.ld

@@ -67,6 +67,11 @@ SECTIONS
     KEEP(*(.bss.$RESERVED*))
   } > ram
 
+  .dont_move_block (NOLOAD): ALIGN(4)
+  {
+    *(.vectors_ram)
+  }
+
   .data : ALIGN(4)
   {
     FILL(0xff)