{"id":1022,"date":"2022-03-15T17:42:10","date_gmt":"2022-03-15T21:42:10","guid":{"rendered":"https:\/\/www.ecsdump.net\/?page_id=1022"},"modified":"2022-03-15T17:42:10","modified_gmt":"2022-03-15T21:42:10","slug":"real-time-clock-cmos-setup-reference","status":"publish","type":"page","link":"https:\/\/www.ecsdump.net\/?page_id=1022","title":{"rendered":"Real Time Clock \/ CMOS Setup Reference"},"content":{"rendered":"\n
Version: 10 March 1993\nBy Tom Przeor<\/code><\/pre>\n\n\n\n
AT model was the first in IBM PC family to keep track of time while switched off. The designers used Motorola MC146818 Real Time Clock (RTC from now on) chip. This chip provides clock and calendar functions, few registers to program the chip itself and some 50 bytes of general purpose memory. Typically the RTC chip is used only on power on – the BIOS will initialize the DOS<\/a> clock and verify the
configuration. The RTC chip is capable of generating interrupts at specified frequency or time – we’ll get back to it later.<\/p>\n\n\n\n
RTC BIOS interface.<\/h2>\n\n\n\n
AT BIOS provides a number of basic functions to use RTC. The following short list should provide enough information to use them:<\/p>\n\n\n\n
RAM data areas used by RTC:<\/p>\n\n\n\n
Addr hex  Size
0040:0098 4 bytes far pointer to user wait flag
0040:009C 4 bytes wait count
0040:00A0 1 byte wait active flag:
bit 7 - 1 when wait time elapsed
bit 0 - 1 when wait active
bits 6-1 - reserved<\/code><\/p>\n\n\n\n
Int 1Ah function 02h – Get RTC time<\/span>
entry: AH = 02h
exit:  CF clear if successful, set on error
CH = hour (BCD)
CL = minutes (BCD)
DH = seconds (BCD)
DL = daylight savings flag
(00h standard time, 01h daylight time)<\/code><\/p>\n\n\n\n
Int 1Ah function 03h – Set RTC time<\/span>
entry: AH = 03h
CH = hour (BCD)
CL = minutes (BCD)
DH = seconds (BCD)
DL = daylight savings flag (as above)
exit: none<\/code><\/p>\n\n\n\n
Int 1Ah function 04h - Get RTC date<\/span>
entry: AH = 04h
exit: CF clear if successful, set on error
CH = century (BCD)
CL = year (BCD)
DH = month (BCD)
DL = day (BCD)<\/code><\/p>\n\n\n\n
Int 1Ah function 05h - Set RTC date<\/span>
entry: AH = 05h
CH = century (BCD)
CL = year (BCD)
DH = month (BCD)
DL = day (BCD)
exit: none<\/code><\/p>\n\n\n\n
Int 1Ah function 06h – Set RTC alarm
entry: AH = 06h
CH = hour (BCD)
CL = minutes (BCD)
DH = seconds (BCD)
exit: CF clear if successful, set on error
note: place address for alarm routine in interrupt 4Ah
vector before using this service, the alarm occurs
every 24 hours until turned off, invoking int 4Ah
each time.<\/code><\/p>\n\n\n\n
Int 1Ah function 07h - Reset RTC alarm
entry: AH = 07h
exit: none
note: disables alarm set with int1Ah\/fn 06h. Don't forget
to restore old int 4Ah vector.<\/code><\/p>\n\n\n\n
Int 15h function 83h - Set\/Cancel Wait Interval
subfunction 0 - Set Wait Interval
entry: AH = 83h
AL = 00h
CX:DX = microseconds to delay
ES:BX -> byte whose high bit is to be set at end of
interval
exit: CF clear if successful, set on error<\/code><\/p>\n\n\n\n
Int 15h function 83h - Set\/Cancel Wait Interval
subfunction 1 - Cancel Wait Interval
entry: AH = 83h
AL = 01h
exit: CF clear if successful, set on error
error status in AH:
80h invalid command (PC,PCjr)
86h function not supported (XT and later)<\/code><\/p>\n\n\n\n
Int 15h function 86h - Wait
e<\/code>ntry: AH = 86h
CX:DX = interval in microseconds
exit: CF clear if successful (wait interval elapsed)
CF set on error or AH=83h wait already in progress<\/p>\n\n\n\n
There is a difference between functions 83h and 86h. Function 83h sets a wait interval and allows processing to continue. When the wait interval ends the user specified flag (pointed by ES:BX) is set
and it is software responsibility to check that flag. That function can be stopped. Function 86h on the other hand suspends any processing until specified time interval is elapsed (and cannot be stopped). These two functions share the same data areas and cannot be used at the same time. Also note that their resolution is 977 microseconds.<\/p>\n\n\n\n
Direct Access to RTC chip.<\/h2>\n\n\n\n
RTC chip can be accessed through I\/O ports 70h and 71h. To read a byte from the chip, do an OUT 70h,addr; followed by IN al,71H. To write a byte to chip, do an OUT 70h,addr; followed by OUT 71h,value. <\/p>\n\n\n\n
Example: read equipment byte from CMOS info<\/p>\n\n\n\n
mov al,14h   ; register 14h holds equipment byte
out 70h,al   ; select address 14h on RTC chip
jmp $+2      ; a slight delay to settle things
in al,71h    ; AL now has equipment byte<\/code><\/p>\n\n\n\n