This is what the TRS-80 Model 1 does at startup with an Expansion Interface connected and a bootable disk mounted:

read 37ECh
check bit 1: (if 1, E.I. present)
write 37E1h, 1 (select drive 0)
write 37ECh, 3 (select track 0)
check 37ECh, 0 (if true (=1), Disk drive is ready)
write 37EEh, 0 (select sector 0)
write 37ECh, 8Ch (read command)
loop:
  check 37ECh, 1 (if 1, data ready, if not jp loop)
  read 37EFh, write to 4200h++
  if not 256 bytes read, jp loop
jp 4200h