( cmd arg1 arg2 -- resp )
( Sends a command to the SD card, along with arguments and
  specified CRC fields. (CRC is only needed in initial commands
  though). This does *not* handle CS. You have to
  select/deselect the card outside this routine. )
: _cmd
    _wait DROP ROT    ( a1 a2 cmd )
    0 _s+crc          ( a1 a2 crc )
    ROT 256 /MOD ROT  ( a2 h l crc )
    _s+crc _s+crc     ( a2 crc )
    SWAP 256 /MOD ROT ( h l crc )
    _s+crc _s+crc     ( crc )
    1 OR              ( ensure stop bit )
    (spix) DROP       ( send CRC )
    _wait  ( wait for a valid response... )
;