# Pastebin xsrucWFT
liteeth.c writes (e.g. 558 bytes)

s->regs[R_ETHMAC_SRAM_WRITER_LENGTH0] = (tmpsize << 24 ) & 0xff;
    s->regs[R_ETHMAC_SRAM_WRITER_LENGTH1] = (tmpsize << 16 ) & 0xff;
    s->regs[R_ETHMAC_SRAM_WRITER_LENGTH2] = (tmpsize << 8 ) & 0xff;
    s->regs[R_ETHMAC_SRAM_WRITER_LENGTH3] = tmpsize & 0xff;
    s->regs[R_ETHMAC_SRAM_WRITER_EV_PENDING] =  1;

then in the bios, microudp.c gets 46 bytes

if(ethmac_sram_writer_ev_pending_read() & ETHMAC_EV_SRAM_WRITER) {
                rxslot = ethmac_sram_writer_slot_read();
                rxlen = ethmac_sram_writer_length_read();