EP_First ($12) - Return first file entry in File Area in slot C

Register parameters

IN:
   A = EP_FIRST
   C = slot number containing File Eprom

OUT:
   Fc = 0, File Eprom available
        Fz = 1, File Entry marked as deleted
        Fz = 0, File Entry is active.
        BHL = pointer to first file entry in slot (B=00h-FFh, HL=0000h-3FFFh).
        (NULL file skipped if found on Intel Flash Card)

   Fc = 1,
        A = RC_Onf
        File Eprom was not found in slot, or File Entry not available

Registers changed after return:
   ...CDE../IXIY same
   AFB...HL/.... different

Notes

Due to a strange side effect with Intel Flash Chips, a special "NULL" file is saved as the first file to the Card. These bytes occupies the first bytes that otherwise could be interpreted as a random boot command for the Intel chip - the behaviour is an Intel chip suddenly gone into command mode for no particular reason. The NULL file prevents this behaviour by saving a file that avoids any kind of boot commands which sends the chip into command mode when the card has been inserted into a Z88 slot.

If the NULL file is identified as the first file, it is skipped and the next file entry pointer is automatically returned.