The PC’s Real-Time Clock
An introduction to the capabilities
and programming of the RealTime Clock and CMOS memory
Non-Volatile Memory
• The original IBM-PC had no internal clock
• Users had to run a utility program to reset
the date and time after any system reboot
• This defect was eliminated in the IBM-AT
• A special battery-powered peripheral was
added to keep track of the time and date
• It also provided a small amount of memory
which could retain configuration settings
Motorola’s MC146818A
• PC-AT’s Real-Time Clock plus RAM was
manufactured by Motorola Corporation
• Other companies have ‘cloned’ this chip
• Its capabilities are described online in an
official datasheet by Dallas Semiconductor
(see ‘Maxim’ integrated circuit: DS12887)
• You can also get the Motorola datasheet
(by writing to its corporate headquarters)
Features of DS12887
• Can operate over ten years without power
• Counts seconds, minutes, hours, days,
day-of-the-week, date, month, and year
(with leap-year compensation), valid up
until the year 2100 AD, with options for
12/24-hour clock and Daylight Savings
• Can use binary or BCD representation
• Provides 114 bytes of nonvolative storage
Programming Interface
• The RTC interfaces with system software
as an array of 128 bytes, accessed via i/o
ports 0x70 and 0x71 using a multiplexing
scheme:
port 0x70: address-port
port 0x71: data-port
• A system quirk: The most significant bit at
port 0x70 is used to control a gate that can
‘mask’ the Non-Maskable Interrupt circuitry
Ten clock/calendar bytes
0x0
Current seconds
Range is 0..59
0x1
Alarm seconds
Range is 0..59
0x2
Current minutes
Range is 0..59
0x3
Alarm minutes
Range is 0..59
0x4
Current hours
Range is 0..23 or 1..12
0x5
Alarm hours
Range is 0..23 or 1..12
0x6
0x7
0x8
0x9
Day-of-the-Week
Date of the Month
Current Month
Current Year
Range is 1..7 (Sunday=1)
Range is 1..31
Range is 1..12 (January=1)
Range is 0..99
Operating Capabilities
• The RTC can be programmed to generate
an interrupt under any combination of the
following three conditions:
1) time/date counters were updated
2) current time equals the alarm time
3) periodic frequency interval restarts
• The frequency of the periodic interrupt is a
selectable rate (e.g., from 122 to 500ms)
Four Status/Control bytes
0xA
UIP
0xB
SET
PIE
AIE
UIE SQWE DM 24/12 DSE
0xC
IRQF
PF
AF
UF
0
0
0
0
0xD
VRT
0
0
0
0
0
0
0
Divider bits
Rate-Select
Other NVRAM locations
• Besides these 14 dedicated RTC bytes,
there are 114 byte locations which can
serve as nonvolatile storage in whatever
manner the system designer decides
• IBM has established some ‘standard’ uses
for many (but not all) of these locations
• A fairly complete CMOS Memory Map is
accessible online (see course website)
Example: Diagnostic Status
0xE
Power Check POST
RAM
Status Sum Config Size
failure
bad
invalid wrong
Fixed
Disk
bad
CMOS
Time reserved reserved
invalid
During the Power-On Self-Test, the ROM-BIOS routines
perform tests of the memory and peripheral devices, and
record any failures/errors in this Diagnostic Status byte
Note on the NMI circuitry
• The CPU has a special input-signal that is
‘non-maskable’ (i.e., CLI / STI instructions
have no effect on it), intended to be used
for signaling urgent or catastrophic events
(such as loss of power or memory failures)
• But sometimes, during a “critical section”
of system code, it is necessary to prohibit
even these urgent interrupts (e.g., when
stack or Interrupt Descriptors are invalid)
Non-Maskable Interrupt gate
Error-signals
AND
CPU
Logic GATE
(port 0x70, bit 7)
PIC
PIC
IRQ 8-15
IRQ 0-7
NMI
INTR
IF
Example: setting alarm time
; inhibit clock updates while reprogramming
mov al, #0x8B
; access register B
out
#0x70, al
; and disable NMI
in
al, #0x71
; read register B
or
al, #0x80
; set the SET bit
out
#0x71, al
; write register B
Set the alarm for 6:30:00 am
mov
out
mov
out
mov
out
ax, #0x0685
#0x70, ax
ax, #0x3083
#0x70, ax
ax, #0x0081
#0x70, ax
; hours
; minutes
; secons
Finish SET (and reenable NMI)
; clear the SET bit in register B
mov al, #0x8B
; select register B
out
#0x70, al
; for access w/o NMI
in
al, #71
; read register B
and
al, #0x7F
; clear its SET bit
out
#71, al
; write register B
; reenable the Non-Maskable Interrupts
mov
al, #0x0D
; select register D
out
#0x70, al
; and reenable NMI
Our ‘rtcdemo.s’ program
• We illustrate the Real-Time Clock chip’s
‘update’ interrupt by redisplaying the time
whenever the clock-counters are updated
• Register B controls which interrupts occur
• But don’t change bits 0, 1, 2 (they control
the data-format (binary or BCD), select 12
or 24 hour clock, and activate the Daylight
Savings Time counter-compensation
Event-Queue paradigm
HEAD
free
base
free
Event
record
Event
record
Event
record
free
free
free
edge
TAIL
The Interrupt Service Routine(s) insert new event-records in the queue,
while the main application-loop removes event-records from the queue.
An important ‘design pattern’
• Many modern user-centered applications use
the ‘event-queue’ paradigm:
initialize_the_system();
while ( !done )
{
dequeue_next_event( &event_record);
process_that_event( &event_record );
}
cleanup_the_system();
exit(0);
In-class exercise
• Modify our ‘rtcdemo.s’ program so that an
‘alarm’ interrupt will get triggered after an
agreed amount of time has elapsed (for
example, 10 seconds) and use that event
to exit the program’s main loop (instead of
using the built-in loop-counter variable)
Suggested implementation
• Easiest and most elegant way to achieve the
requested program modification is to adjust the
Interrupt-Service Routine so it will save a 4-part
event-record (instead of a 3-part event-record),
the new item to be included is the RTC statusregister value
• Then the main loop can test the status it finds
within an event-record, and can either loop or
exit, depending on which event-type it found