A-code exec codes

### A-code exec codes supported by the A-code kernel 12.73 The A-code minor directive EXEC currently accepts the following codes:
    exec  1   # Save game
    exec  2   # Attempt to restore game
    exec  3   # Delete saved game
    exec  4   # (Adv550 legacy -- "flush cache")
    exec  5   # (Adv550 legacy -- check for "prime time")
    exec  6   # Save value over restore attempt
                (Adv550 legacy -- list opening hours)
    exec  7   # Restore saved value after a restore attempt
                (Adv550 legacy -- save value over restore attempt)
NB: saving/restoring a value over a restore attempt is obsolete and depracated. Local variables are not affected by save/restore and so should be used in preference to this mechanism.
    exec  8   # Get time (in minutes) since game dumped
                (Adv550 legacy -- restore saved value afetr a restore attempt)
    exec  9   # Fudge value into ARG1
    exec 10   # Fudge value into ARG2
    exec 11   # Copy the 1st word of player command to the 2nd word
Codes 12 and up are not used in in STYLE 1 (Adv550 legacy code). Values without descriptions are not in use.
    exec 12   # Check for end of player input
    exec 13
    exec 14   # Get persistent data item
    exec 15   # Store persistent data item
    exec 16   # Clear persistent data item
    exec 17   #
    exec 18   #
    exec 19   # Toggle text justification
    exec 20   # Set line length
    exec 21   # Set margin
    exec 22   # Set lines per screen
    exec 23   # Preserve player input (for CGI operation)
    exec 24   # Restore player input (for CGI operation)
    exec 25   #
    exec 26   #
    exec 27   # Pre-say!
    exec 28   # Recover from failed restore
    exec 29   # Swap ARG1 and ARG2
    exec 30   #
    exec 31   # Replace ARG2 word with what player typed
    exec 32   # Check exceptions list (for EXCEPT processing)
    exec 33   # Check a memory save exists
    exec 34   # List available saved games
Finally, there are three exec codes internal to the A-code kernel.
    exec 997 |
    exec 998 |- Internal to A-code kernel
    exec 999 |