( Initiate SPI exchange with the SD card. n is the data to
  send. )
CODE _sdcSR  ( n -- n )
    HL POP,
    chkPS,
    A L LDrr,
    SDC_SPI OUTiA,
    NOP, NOP,
    SDC_SPI INAi,
    L A LDrr,
    HL PUSH,
;CODE