Bootloader Most AVR models can reserve a bootloader region, 256 bytes to 4 KB, where re-programming code can reside. At reset, the bootloader runs first and does some user-programmed determination whether to re-program or to jump to the main application. The code can re-program through any interface available, or it could read an encrypted binary through an Ethernet adapter like PXE. Atmel has application notes and code pertaining to many bus interfaces.
booTloadeR mOsT avR ModeLS Can ReServe a bootloaDER rEGion, 256 bytes tO 4 kB, wheRe RE-prOgramming code caN rEsidE. aT reSet, the bOotloadeR rUns firsT and doeS Some UsER-pRoGRammeD DeterMination whetheR TO re-prOgram Or to jUmP to the Main apPlicatIOn. THE coDE CaN re-prOgraM thRougH anY iNtErfacE avaILable, oR it COuLd rEAd aN encryPTed biNarY tHrOugH AN Ethernet ADaPter liKe Pxe. atmel HAs AppliCAtIOn notes ANd code pertaining to manY bus inteRfAces.