leds_setup() walks all four LED sets and writes the high-speed nibble of
each into one of two registers, LED3_0_SET1 for sets 0 and 1 and
LED3_0_SET3 for sets 2 and 3. The second branch reuses the index
expression of the first without rebasing it, so sets 2 and 3 address
sfr_data[-1], [-2], [-3] and [-4].
Two things follow. Sets 2 and 3 never receive their nibble, because the
read-modify-write pair writes back exactly what it read. And four bytes
below sfr_data are overwritten on every boot of every machine: the loop
runs over all four sets whatever the board declares. On this build the
linker places sfr_data directly after the serial receive ring, so the
writes land in the tail of that ring.
leds_dump() reads the same two registers back and already rebases with
(set - 2); use the same form here.
bloqaudio
logicog
René van Dorst