U2 on the SEthernet board is an ATF16V8/GAL16V8 programmable logic device that provides address decoding and miscellaneous glue logic.
Address decoding is straightforward - when /AS is asserted and address lines
A24..A16 indicate an address in the range 0x80 0000-0x80 FFFF, we assert
the ENC624J600's chip-select pin.
Glue logic is also minimal - the B0SEL and B1SEL byte-select lines on the
ENC624J600 are just active-high equivalents to the active-low /UDS and /LDS
lines on the 68000 bus. The BBU chip on the SE logic board automatically asserts
/DTACK for us on a schedule that we can live with, so there is no need to
generate it ourselves (if we needed additional wait states, we could assert
/EXT.DTK to suppress the BBU's /DTACK signal until we were ready).
Finally, the push-pull interrupt output of the ENC624J600 is buffered and turned into an open-drain output, allowing it to interface with the wired-OR interrupt logic of the SE.