OS_Ht, Hardware time manipulation

IN:  A = HT_RES ($01), reset hardware timer

OUT: -

Registers changed after return:
     A.BCDEHL/IXIY same
     .F....../.... different

IN:  A = HT_RD ($02), read hardware timer
         HL = pointer to a 5 bytes buffer
OUT: -

Registers changed after return:
     A.BCDEHL/IXIY same
     .F....../.... different

IN:  A = HT_MDT ($03), read monthy/date/time address
OUT: Fc = 0
     BHL = month, date, time address

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

IN:	 A = HT_UPT ($04), read uptime
OUT: Fc = 0
	 BCD = minutes, E = seconds since latest hard reset

Registers changed after return:
     AF....HL/IXIY same
     ..BCDE../.... different