Contributor: ALFONS HOOGERVORST
{> Could someone please post what each of the 64 bytes of the CMOS are. I> know how to read/write to the CMOS but do not know whitch byte is what.
I don't think this one is complete, but here you go:
Offset Hex Size (bytes) Description
========== ============ ===========
00h 1 Current second in BCD
01h 1 Alarm second (BCD)
02h 1 Current minute (BCD)
03h 1 Alarm minute (BCD)
04h 1 Current hour (BCD)
05h 1 Alarm hour (BCD)
06h 1 Current day of week (BCD)
07h 1 Current day (BCD)
08h 1 Current month (BCD)
09h 1 Current year (BCD)
0Ah 1 Status register A
Bit 7 = Update in progress
0 = date & time can be read
1 = time update busy
Bit 6-4 = Time frequency divider
010 = 32.768 KHz
Bit 3-0 = Rate selection frequency
0110 = 1.024 KHz sq. wve. freq.
0Bh 1 Status register B
Bit 7 = Clock update cycle
0 = Update normally
1 = Abort update in progress
Bit 6 = Periodic interrupt
0 = disable (default), 1 = enable
Bit 5 = Alarm interrupt
0 = disable (default), 1 = enable
Bit 4 = Update-ended interrupt
0 = disable (default), 1 = enable
Bit 3 = Status register A sq. wve. freq.
0 = disable (default), 1 = enable
Bit 2 = Date format
0 = Calender in BCD format (default)
1 = Calender in binary format
Bit 1 = 24-hour clock
0 = 24-hour, 1 = 12-hour
Bit 0 = Daylight Savings Time
0 = disable (default), 1 = enable
0Ch 1 Status register C
Bit 7 = IRQF flag
Bit 6 = PF Flag
Bit 5 = AF Flag
Bit 4 = UF Flag
Other bits reserved
0Dh 1 Status register D
Bit 7 = Valid CMOS RAM bit
0 = battery dead, 1 = battery OK
Other bits reserved
0Eh 1 Diagnostic status
Bit 7 = Real-time clock power status
0 = OK, 1 = not OK
Bit 6 = CMOS checksum status
0 = good, 1 = bad
Bit 5 = POST config. status
0 = valid, 1 = not valid
Bit 4 = POST Memory size check
0 = OK, 1 = !OK
Bit 3 = Fixd disk/adapter init.
0 = init OK, 1 = init bad
Bit 2 = CMOS time status
0 = OK, 1 = !OK
Other bits reserved
0Fh 1 Shutdown code
00h = Power on or soft reset
04h = POST end; boot system
05h = JMP dword ptr with EOI
06h = Prot. mode tests OK
07h = Prot. mode tests FAILED
08h = Memory size FAILED
09h = int 15h block move
0Ah = JMP dword ptr with EOI
0Bh = Used by 80386
10h 1 Floppy drive types
Bits 7-4= Drive 0 type
Bits 3-0= Drive 1 type
(0000 = none, 0001 = 360K, 0010 = 1.2M, 0011 = 720K,
0100 = 1.44M, 0101 = 2.88K?)
11h 1 Reserved
12h 1 HD types
Bits 7-4= Drive 0 type (0-15)
Bits 3-0= Drive 1 type (0-15)
13h 1 Reserved
14h 1 Installed equipment
Bits 7-6= Number of floppy drives
Bits 5-4= Primary display
00= Adapter BIOS
01= CGA 40 cols
10= CGA 80 cols
11= MDA
Bits 3-2= Reserved
Bit 1 = Math copro. present
Bit 0 = Floppy drive present
15h 1 Base memory low-order byte
16h 1 Base memory high-order byte
17h 1 Ext. mem. low-order byte
18h 1 Ext. mem. high-order byte
19h 1 Hard disk 0 extended type (0-255)
1Ah 1 Hard disk 1 ext. type (0-255)
1Bh 9 Reserved
2Eh 1 CMOS checksum high order byte
2Fh 1 CMOS checksum low order byte
30h 1 Actual extended memory low-order byte
31h 1 Actual extended memory high-order byte
32h 1 Date century in BCD
33h 1 POST information flag
Bit 7 = Top 128K base memory status
0 = not installed
1 = installed
Bit 6 = Setup program flag
0 = Normal (default)
1 = Output user message
Other bits reserved
34h 2 Reserved