Version: 10 March 1993
By Tom Przeor

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 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.

RTC BIOS interface.

AT BIOS provides a number of basic functions to use RTC. The following short list should provide enough information to use them:

RAM data areas used by RTC:

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

Int 1Ah function 02h – Get RTC time
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)

Int 1Ah function 03h – Set RTC time
entry: AH = 03h CH = hour (BCD) CL = minutes (BCD) DH = seconds (BCD) DL = daylight savings flag (as above) exit: none

Int 1Ah function 04h - Get RTC date entry: AH = 04h exit: CF clear if successful, set on error CH = century (BCD) CL = year (BCD) DH = month (BCD) DL = day (BCD)

Int 1Ah function 05h - Set RTC date entry: AH = 05h CH = century (BCD) CL = year (BCD) DH = month (BCD) DL = day (BCD) exit: none

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.

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.

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

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)

Int 15h function 86h - Wait entry: 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

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.

Direct Access to RTC chip.

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.

Example: read equipment byte from CMOS info

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

NOTE: Original MC146818 has 64 registers (00h to 3Fh). Most of the computers used today have a RTC functional equivalent incorporated in their ‘chipset’ and it can have more registers. Those extra bits are often used by chipset and BIOS designers to store extra information about things like DRAM wait states, refresh, m/b cache or user defined hard drive parameters – don’t fiddle with them or you might end up in trouble. Also leave alone the reserved bytes.

The RTC Registers.

The registers can be divided into three functional groups:

Clock/calendar – updated from on chip clock, on IBM compatibles all quantities are stored in BCD format (ie. 23dec is stored 23h).
Status – they affect working of RTC chip itself.
CMOS configuration data – general purpose memory not affected and not affecting the RTC chip.

Here is detailed list of registers (all byte sized, addr in hex):

Addr Function
==== =========================================

** clock/calendar

00 current second for real-time clock
01 alarm second
02 current minute
03 alarm minute
04 current hour
05 alarm hour
06 current day of week (1=Sunday)
07 current date of month
08 current month
09 current year (final two digits; eg, 93)

** status

0A Status Register A – Read/Write except UIP
== =========================================
��Ŀ
� 7 � 6 � 5 � 4 � 3 � 2 � 1 � 0 �
� UIP � DV2 � DV1 � DV0 � RS3 � RS2 � RS1 � RS0 �
��

    bit 7 - UIP flag, Update In Progress. When set an update
    cycle is in progress and the clock/calendar cannot be
    accessed. When clear, at least 244 microseconds are
    available to access clock/calendar bytes (it's plenty of
    time even on 6MHz AT).

    bits 6-4 - divider bits that define RTC operating frequency.
    ATs have a 32.768 kHz (wrist watch) crystal to operate RTC
    and divider should be set to '010', other values will make a
    time machine from your computer.

    bits 3-0 - Rate Selection bits that define the periodic
    interrupt rate, see another table for details. Default value
    set by BIOS is '0110'.

0B Status Register B – Read/Write
== ==============================
��Ŀ
� 7 � 6 � 5 � 4 � 3 � 2 � 1 � 0 �
� SET � PIE � AIE � UIE � SQWE� DM �24/12� DSE �
��

    bit 7 (SET) - when set to 1, any update in progress is
    aborted and a program may initialize the
    clock/calendar/alarm bytes without an update occurring.
    Setting this bit clears UIE (bit 4). Clearing bit 7 allows
    the update cycle to continue.

    bit 6 (PIE) - Periodic Interrupt Enable, when set the
    periodic interrupt will occur at the frequency specified by
    RS bits in Status Register A.

    bit 5 (AIE) - Alarm Interrupt Enable, when set the alarm
    interrupt will be asserted once for each second that the
    current time matches the alarm time.

    bit 4 (UIE) - Update-ended Interrupt Enable, when set the
    update-ended interrupt will be asserted once each second
    after the end of update cycle. This bit is cleared when SET
    bit goes high but it is not reset when SET is cleared.

    bit 3 (SQWE) - Square Wave Enable, when set, enables the
    square wave output on the SQW pin at the frequency specified
    by the RS bits in the Status Register A. The SQW pin is not
    connected to anything in the AT.

    bit 2 (DM) - Data Mode, indicates mode for clock/calendar
    data: 0=BCD and 1=binary, BIOS setting is 0.

    bit 1 (24/12) - controls hours byte, 0=12-hour and 1=24-hour
    format, BIOS setting is 1.

    bit 0 (DSE) - Daylight Savings Enable, when set two special
    updates will occur: last Sunday in April time will go
    01:59:59 > 03:00:00 and last Sunday in October 01:59:59 >
    01:00:00. BIOS sets it to 0 (ie. no daylight saving).

0C Status Register C – Read-only
== =============================
��Ŀ
� 7 � 6 � 5 � 4 � 3 � 2 � 1 � 0 �
� IRQF� PF � AF � UF � 0 � 0 � 0 � 0 �
��

    bit 7 (IRQF) - Interrupt Request Flag, when set one of the
    interrupts enabled in Status Register B has occurred.

    bit 6 (PF) - Periodic interrupt Flag, when set the periodic
    interrupt has occurred.

    bit 5 (AF) - Alarm interrupt Flag, when set the alarm
    interrupt has occurred.

    bit 4 (UF) - Update-ended interrupt Flag, when set the
    update-ended alarm interrupt has occurred.

    NOTE: PF, AF, UF are set regardless of corresponding enable
    bits in Status Register B. IRQF will be set only if the
    interrupt flag and its corresponding enable bit are set.
    These four flags are cleared each time Status Register C is
    read.

    bits 3-0 - reserved, always 0.

0D Status Register D – Read-only
== =============================
��Ŀ
� 7 � 6 � 5 � 4 � 3 � 2 � 1 � 0 �
� VRT � 0 � 0 � 0 � 0 � 0 � 0 � 0 �
��

    bit 7 (VRT) - Valid RAM and Time, OK when set, when clear
    indicates power was lost.

    bits 6-0 - reserved.

** configuration

0E POST diagnostics status byte
== ============================
bit 7 = 1 clock lost power
bit 6 = 1 CMOS checksum bad
bit 5 = 1 invalid configuration found at POST
bit 4 = 1 memory size compare error at POST
bit 3 = 1 fixed disk or controller failed
bit 2 = 1 invalid RTC time (eg. 31 Feb)
bits 1-0 – reserved

0F Shutdown Status Byte
== ====================
This byte is read upon startup after CPU reset in order to
determine if the reset cause (to get out of protected mode
etc.)
00 – power on reset
(0 = soft reset (Ctrl-Alt-Del) or unexpected shutdown. Skip
POST) – conflicting info from older reference ??????
01 – memory size pass
02 – memory test pass
03 – memory test fail
04 – POST end, boot system
05 – JMP DWORD PTR 0:[0467h] with EOI (End Of Interrupt)
06 – protected tests pass
07 – protected tests fail
08 – memory size fail
09 – INT 15h block move
0A – JMP DWORD PTR 0:[0467h] without EOI

10 Diskette drive types
== ====================
bits 7-4 – drive 0 type (A:)
bits 3-0 – drive 1 type (B:)
0000b – no drive
0001b – 360k
0010b – 1.2M
0011b – 720k
0100b – 1.44M

11 Reserved
12 Hard disk drive type
== ====================
(for drives C: and D:, when between 1 and 14)
bits 7-4 – fixed disk 0 type (C:)
bits 3-0 – fixed disk 1 type (D:)
0000b = no drive
0001b-1110b = drive type
1111b = drive 0 (1) type stored at addr 19h (1Ah)
13 Reserved
14 Equipment byte
== ==============
bits 7-6 – no. of floppy drives (00=1, 01=2, 10=3, 11=4)
bits 5-4 – primary display 00 = none, EGA, VGA …
01 = 40×25 colour
10 = 80×25 colour
11 = 80×25 monochrome
bits 3-2 – reserved
bit 1 =1 if math copro installed
bit 0 =1 if floppy drive(s) present

15 Base memory (low byte)
16 Base memory (high byte)
== =======================
in kbytes (eg. 0100H=256K, 0200H=512K, 0280H=640K)

17 Extended memory above 1M (low byte)
18 Extended memory (high byte) in kbytes

19 Disk 0 type if (CMOS addr 12H & 0fH) is 0fH
1A Disk 1 type if (CMOS addr 12H & f0H) is f0H

1B-2D Reserved

2E Checksum of CMOS addresses 10H through 20H (high byte)
2F Checksum of CMOS addresses 10H through 20H (low byte)

30 Actual extended memory size (low byte) ???
31 Actual extended memory size (high byte) ???

32 Century in BCD (eg. 19h)

33 Miscellaneous flags
bit 7 – IBM 128K memory option installed
bit 6 – used by “Setup” utility (?)
bits 5-0 – reserved

34-3F Reserved

Using RTC hardware interrupt.

RTC interrupt pin is connected to IRQ8 line in AT bus and generates int 70h when enabled. The chip can generate three different types of interrupts: periodic, alarm and update-ended. To use RTC interrupt
first install interrupt service routine and point int 70h vector to it, then program RTC status registers (details shortly) and ‘unmask’ bit 0 of second PIC’s mask register at port A1h. You can enable more
than one interrupt type at the same time, in that case your interrupt handler should check which type has occurred (by reading Status Register C).

Update-Ended Interrupt

This is the simplest type – interrupt is generated after each clock update exactly every 1 second. To enable set bit 4 (UIE) in Status Register B.

Alarm Interrupt

This is a second type – it generates interrupt at specified time. To use it first set Alarm Seconds (addr 01h), Alarm Minute (addr 03h) and Alarm Hour (addr 05h), then set bit 5 (AIE) in Status Register
B. The special value FFh in one of alarm registers will match any time, eg. FF:FF:00 will generate alarm interrupt every minute, FF:00:FF will generate interrupt every second during first minute of every hour.

Periodic Interrupt

The frequency of this interrupt is programmable from 2 to 8192 per second. To use this type of interrupt first set RS (Rate Select) bits in Status Register A to the required value:

RS Int/sec Period
3210 – –
0000 none none
0001 256 3.90625 ms 0010 128 7.8125 ms 0011 8192 122.070 Micros 0100 4096 244.141 Micros 0101 2048 488.281 Micros 0110 1024 976.562 Micros 0111 512 1.93125 ms 1000 256 3.90625 ms 1001 128 7.8125 ms 1010 64 15.625 ms 1011 32 31.25 ms 1100 16 62.50 ms 1101 8 125.0 ms 1110 4 250.0 ms 1111 2 500.0 ms

Note: usually this is set to 0110 by BIOS at boot up.

After setting RS bits set bit 6 (PIE) in Status Register B.

Tag Archives: cmos

Real Time Clock / CMOS Setup Reference