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Revision as of 20:06, 24 April 2019 view source osdev>Wukl →‎Getting Current Date and Time from RTC: Fix range column lacking leading space for rows below header ← Older edit		Latest revision as of 05:20, 9 June 2024 view source Xtex (talk \| contribs) Interface administrators, Administrators 977 edits m Bot: Replace deprecated source tag with syntaxhighlight
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Line 1: "CMOS" is a tiny bit of very low power static memory that lives on the same chip as the Real-Time Clock (RTC). It It was introduced to IBM PC AT in 1984 which used Motorola MC146818A RTC. ~~is fairly convenient to actually think of the RTC as being "part" of CMOS.~~ CMOS (and the Real-Time Clock) can only be accessed through IO Ports 0x70 and 0x71. The function of the CMOS memory is to store 50 (or 114) bytes of "Setup" information for the BIOS while the computer is turned off -- Line 36 ⟶ 37: as follows: * <tt>outb (0x70, (NMI_disable_bit << 7) \| (selected CMOS register number));</tt> Once a register is selected, you either read the value of that register on Port 0x71 (with inb or an equivalent function), or you write a new value to that register -- also on Port 0x71 (with outb, for example): * <tt>val_8bit = inb (0x71);</tt> Note1: Reading or writing Port 0x71 seems to default the "selected register" back to 0xD. So you need to Line 152 ⟶ 153: === Weekday Register === The RTC chip is able to keep track of the current day of the week. All it does is increment its "Weekday" register at midnight and reset it to ~~zero~~1 if itthe ~~reaches~~incremented value would go above 7[https://groups.google.com/d/msg/alt.os.development/qPxWHKC48rw/yjAt-c8IAQAJ]. Unfortunately there is no guarantee that this register was ever set correctly by anything (including when the user changes the time and date using the BIOS configuration screen). It is entirely unreliable and should not be used. The correct way to determine the current day of the week is to calculate it from the date (see [http://en.wikipedia.org/wiki/Weekday_determination the article on Wikipedia] for details of this calculation). Line 206 ⟶ 207: === Reading from the CMOS === <~~source~~syntaxhighlight lang="c"> ReadFromCMOS (unsigned char array []) { Line 228 ⟶ 229: } } </syntaxhighlight> ~~</source>~~ === Writing to the CMOS === <~~source~~syntaxhighlight lang="c"> WriteTOCMOS(unsigned char array[]) { Line 251 ⟶ 252: } } </syntaxhighlight> ~~</source>~~ === Reading All RTC Time and Date Registers === <~~source~~syntaxhighlight lang="c"> #define CURRENT_YEAR ~~2014~~2023 // Change this each year! int century_register = 0x00; // Set by ACPI table parsing code if possible Line 363 ⟶ 364: } } </syntaxhighlight> ~~</source>~~ == See Also == * [[RTC]] == External Links == * [https://web.archive.org/web/20111209041013/http://www-ivs.cs.uni-magdeburg.de/~zbrog/asm/cmos.html Old CMOS Map] * [http://www.bioscentral.com/misc/cmosmap.htm Better CMOS Map] * [http://bitsavers.trailing-edge.com/pdf/ibm/pc/at/1502494_PC_AT_Technical_Reference_Mar84.pdf IBM PC AT Technical Reference] * [http://web.stanford.edu/class/cs140/projects/pintos/specs/mc146818a.pdf MC146818A REAL-TIME CLOCK PLUS RAM (RTC)] * [http://www.bitsavers.org/pdf/ibm/pc/ps2/PS2_Model_50_Technical_Reference_May88.pdf IBM PS/2 Technical Reference] * [https://www.singlix.com/trdos/archive/pdf_archive/real-time-clock-nmi-enable-paper.pdf Intel Application Note: Accessing the Real Time Clock Registers and NMI Enable Bit] [[Category:X86]]