The AVR_ARCH define, provided by the compiler toolchain, has become apparent as a source of particularly relevant information on the so-called "modern AVR" parts, that is parts released since 2016 with the "core independent peripherals" (event system, timers identified by letter and number, not just number, PERIPHERALn.REGISTER instead of PERIPHERAL1REGISTER naming of peripherals, etc. They also run the AVRxt version of the instruction set (which has a few significant timing differences (in brief, SBI/CBI, all CALL variants, ST, and PUSH all lost a cycle = BUT - LDS, inexplicably, gained a cycle. And yes, this does mean you need to re-tune all your hand tuned assembly). But I digress; the topic of this page is the AVR_ARCH define - for these parts, values of 102, 103, and 104 have been seen in the wild. The meaning of these is actually incredibly simple: It tells you whether the entire flash is memorymapped at any given time, and if not, how many sections it is divided across.
__AVR_ARCH__ == 103- All parts where all of the flash is mapped in the data space. This means Dx-series parts with 32k of flash, tinyAVR 0/1/2-series, and megaAVR 0-series.__AVR_ARCH__ == 104- Parts with 128Kb of flash, mapped flash is split into 4 sections (AVR128DA, AVR128DB).__AVR_ARCH__ == 102- Parts with 64Kb of flash, mapped flash is split into 2 sections (AVR64DA, AVR64DB).