Sometimes an Arduino sketch needs to reset the MCU, but still wants to have some variables to be able to survive.

ksmith3036 · ksmith3036 · commit aafa688a3f33 · 2020-12-23T12:39:08.000+01:00
The bootloader itself uses very little RAM and excludes by default the last 4 bytes of RAM from use by the stack.
To allow sketches using a modified linker script to take the same approach, the changed bootloader linker script excludes the last 1 KByte of RAM from stack.
diff --git a/bootloaders/zero/bootloader_samd21x18.ld b/bootloaders/zero/bootloader_samd21x18.ld
@@ -27,7 +27,7 @@
 MEMORY
 {
   FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 0x2000 /* First 8KB used by bootloader */
-  RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000-0x0004 /* 4 bytes used by bootloader to keep data between resets */
+  RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000-0x0400 /* last 4 bytes used by bootloader to keep data between resets, but reserves 1024 bytes for sketches to have same possibility */
 }
 
 /* Linker script to place sections and symbol values. Should be used together

Original file line number	Diff line number	Diff line change
`@@ -27,7 +27,7 @@`
`27`	`27`	`MEMORY`
`28`	`28`	`{`
`29`	`29`	`FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 0x2000 /* First 8KB used by bootloader */`
`30`		`- RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000-0x0004 /* 4 bytes used by bootloader to keep data between resets */`
	`30`	`+ RAM (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000-0x0400 /* last 4 bytes used by bootloader to keep data between resets, but reserves 1024 bytes for sketches to have same possibility */`
`31`	`31`	`}`
`32`	`32`
`33`	`33`	`/* Linker script to place sections and symbol values. Should be used together`