Understand the bootloader limitations

[Yatao Li]

The bootloader for arduino 2560 can be located here: avr/bootloaders/stk500v2/
To compile, it should be set up like this (extracted from the makefile):

-funsigned-char -funsigned-bitfields -fpack-struct -fshort-enums -fno-jump-tables
-Wall -Wstrict-prototypes
-DBOOTLOADER_ADDRESS=3E000
-D_MEGA_BOARD_
-DMCU=atmega2560
-DF_CPU=16000000
-D__AVR_ARCH__=__AVR_ATmega2560__
-D__AVR_ATmega2560__

The bootloader assumes the boot section size:

//#define BOOTSIZE 1024
#if FLASHEND > 0x0F000
	#define BOOTSIZE 8192
#else
	#define BOOTSIZE 2048
#endif

#define APP_END  (FLASHEND -(2*BOOTSIZE) + 1)

Therefore, the application section is [0, 3C000) / [0, 245760) -- that's the limit in the bootloader code.

This contradicts boards.txt in the core: mega.menu.cpu.atmega2560.upload.maximum_size=253952, which assumes that the bootloader takes 4K words, 8KB.

Interestingly, the BOOTSZ fuse bits for arduino specifies:

with 8KB as the maximum, so 16KB is an impossible value.

However, there are some early ATMega2560 batches with the defunct/workaround:

So that may be why the stk500v2 bootloader decided to do that.

If we switch to optiboot, we can reclaim 15872 bytes of PROGMEM.

[jmamma]

Great work.

So we switch to optiboot for development. that will give us extra headroom for debug.

245760 being our max binary size for release.

[Yatao Li]

Let me convert this to a doc.