Real Time Clock - Revision history

Heth: added Category:Time using HotCat

2016-10-06T09:16:16Z

2013-07-24T11:04:45Z

‎Motorola MC146818

2013-07-24T11:04:15Z

‎Motorola MC146818

2013-07-22T13:42:35Z

‎Problem with MC146818

2013-07-22T13:39:26Z

‎Still a problem

2013-07-22T12:20:26Z

‎Problem with MC146818

2013-07-22T10:58:57Z

‎MC146818 memory map

2013-07-22T10:36:46Z

‎MC146818 memory map

2013-07-22T10:36:38Z

‎MC146818 memory map

2013-07-22T10:35:46Z

‎MC146818 memory map

@@ Line 20: / Line 20: @@
   || [[Image:MC146818 memory map.png|500px|right|thumb|MC146818 memory map, showing clock time registers]]
 |-
 <source lang=asm>
   ; Assembly source code example to PC compatible MC146818 RTC

@@ Line 17: / Line 17: @@
 == MC146818 memory map ==
 The MC146818 content are read- and writeable by the CPU using the assembly IN and OUT instructions. The RTC is located at port number 0x70 and 0x71. Port 0x70 is used to transfer the address of the intended register using the OUT instruction. After transferring the register address to port 0x70, it is possible to read the requested register using the IN instruction or write to the register using the OUT instruction.
 <source lang=asm>
   ; Assembly source code example to PC compatible MC146818 RTC
@@ Line 26: / Line 29: @@
                  ;AL register now contains the day of the month.
 </source>
 |-
 |}

@@ Line 30: / Line 30: @@
 |}
 ==Problem with MC146818==
-The MC146818 keeps track of time, date and year, but does not have a century register. This lead to big problems called the year 2000 or Y2K problem<ref>[[w:Year 2000 problem|Year 2000 problem]]</ref> when date shifted from 31<sup>st</sup> December 1999 to 1<sup>st</sup> January 2000. Some operating reported the date as 1<sup>st</sup> January 1900.
+The MC146818 keeps track of time, date and year, but does not have a century register. This lead to big problems called the year 2000 or Y2K problem<ref>[[w:Year 2000 problem|Year 2000 problem]]</ref> when date shifted from 31<sup>st</sup> December 1999 to 1<sup>st</sup> January 2000. Some operating systems reported the date as 1<sup>st</sup> January 1900.
 ===Century problem workaround===
 To resolve the problem with shift of century, it was decided to use a byte of RAM in the MC146818 for the century. Register address 0x32 holds the century byte. The century byte do not count automatically, but should be incremented manually by the operating system after century rollover.

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 ==Still a problem==
-In modern day RTC's there are still no century register, but a Century Rollover bit<ref>[http://www.intel.com/content/dam/doc/datasheet/5-chipset-3400-chipset-datasheet.pdf Intel P55 chipset Century Rollover bit. (See section 5.11.4)]</ref> is set if the year register changes from 99 to 00.
+In modern day RTC's there are still no automatic century register, but a Century Rollover bit<ref>[http://www.intel.com/content/dam/doc/datasheet/5-chipset-3400-chipset-datasheet.pdf Intel P55 chipset Century Rollover bit. (See section 5.11.4)]</ref> is set if the year register changes from 99 to 00. The operating system must increment the MC146818 register at 0x32 manually.
 ==Modern day PC mainboards==

@@ Line 20: / Line 20: @@
   ; Assembly source code example to PC compatible MC146818 RTC
   ; Reading the day of the month to x86 AL register.
-  MOV   AL,7   ; move 7 to the AL register
+  MOV   AL,7    ; move 7 to the AL register
                  ;(Day of month address in MC1468818 is 7)
   OUTB  70h      ; Transfer content of AL register to port number 0x70