| Offset Hex |
Offset Dec |
Field Size |
Function |
| 00h |
0 |
1 byte |
Current second in binary coded decimal (BCD) |
| 01h |
1 |
1 byte |
Alarm second in BCD |
| 02h |
2 |
1 byte |
Current minute in BCD |
| 03h |
3 |
1 byte |
Alarm minute in BCD |
| 04h |
4 |
1 byte |
Current hour in BCD |
| 05h |
5 |
1 byte |
Alarm hour in BCD |
| 06h |
6 |
1 byte |
Current day of week in BCD |
| 07h |
7 |
1 byte |
Current day in BCD |
| 08h |
8 |
1 byte |
Current month in BCD |
| 09h |
9 |
1 byte |
Current year in BCD |
| 0Ah |
10 |
1 byte |
Status register A |
| |
|
|
|
Bit 7 = Update in progress |
| |
|
|
|
|
0 = Date and time can be read |
| |
|
|
|
|
1 = Time update in progress |
| |
|
|
|
Bits 6-4 = Time frequency divider |
| |
|
|
|
|
010 = 32.768KHz |
| |
|
|
|
Bits 3-0 = Rate
selection frequency |
| |
|
|
|
|
0110 = 1.024KHz
square wave frequency |
| 0Bh |
11 |
1 byte |
Status register
B |
| |
|
|
|
Bit 7 = Clock update
cycle |
| |
|
|
|
|
0 = Update normally |
| |
|
|
|
|
1 = Abort update
in progress |
| |
|
|
|
Bit 6 = Periodic
interrupt |
| |
|
|
|
|
0 = Disable interrupt
(default) |
| |
|
|
|
|
1 = Enable interrupt |
| |
|
|
|
Bit 5 = Alarm interrupt |
| |
|
|
|
|
0 = Disable interrupt
(default) |
| |
|
|
|
|
0 = Disable interrupt
(default) |
| |
|
|
|
|
1 = Enable interrupt |
| |
|
|
|
|
Bit 4 = Update-ended
interrupt |
| |
|
|
|
|
0 = Disable interrupt
(default) |
| |
|
|
|
|
1 = Enable interrupt |
| |
|
|
|
Bit 3 = Status register
A square wave frequency |
| |
|
|
|
|
0 = Disable square
wave (default) |
| |
|
|
|
|
1 = Enable square
wave |
| |
|
|
|
Bit 2 = Date format |
| |
|
|
|
|
0 = Calendar in
BCD format (default) |
| |
|
|
|
|
1 = Calendar in
binary format |
| |
|
|
|
Bit 1 = 24-hour
clock |
| |
|
|
|
|
0 = 24-hour mode
(default) |
| |
|
|
|
|
1 = 12-hour mode |
| |
|
|
|
Bit 0 = Daylight
Savings Time |
| |
|
|
|
|
0 = Disable Daylight
Savings (default) |
| |
|
|
|
|
1 = Enable Daylight
Savings |
| 0Ch |
12 |
1 byte |
Status register
C |
| |
|
|
|
Bit 7 = IRQF flag |
| |
|
|
|
Bit 6 = PF flag |
| |
|
|
|
Bit 5 = AF flag |
| |
|
|
|
Bit 4 = UF flag |
| |
|
|
|
Bits 3-0 = Reserved |
| 0Dh |
13 |
1 byte |
Status register
D |
| |
|
|
|
Bit 7 = Valid CMOS
RAM bit |
| |
|
|
|
|
0 = CMOS battery
dead |
| |
|
|
|
|
1 = CMOS battery
power good |
| |
|
|
|
Bits 6-0 = Reserved |
| 0Eh |
14 |
1 byte |
Diagnostic status |
| |
|
|
|
Bit 7 = Real-time
clock power status |
| |
|
|
|
|
0 = CMOS has not
lost power |
| |
|
|
|
|
1 = CMOS has lost
power |
| |
|
|
|
Bit 6 = CMOS checksum
status |
| |
|
|
|
|
0 = Checksum is
good |
| |
|
|
|
|
1 = Checksum is
bad |
| |
|
|
|
Bit 5 = POST configuration
information status |
| |
|
|
|
|
0 = Configuration
information is valid |
| |
|
|
|
|
1 = Configuration
information is invalid |
| |
|
|
|
Bit 4 = Memory size
compare during POST |
| |
|
|
|
|
0 = POST memory
equals |
| |
|
|
|
|
configuration |
| |
|
|
|
|
1 = POST memory
not equal to |
| |
|
|
|
|
configuration |
| |
|
|
|
Bit 3 = Fixed disk/adapter
initialization |
| |
|
|
|
|
0 = Initialization
good |
| |
|
|
|
|
1 = Initialization
failed |
| |
|
|
|
Bit 2 = CMOS time
status indicator |
| |
|
|
|
|
0 = Time is valid |
| |
|
|
|
|
1 = Time is Invalid |
| |
|
|
|
Bits 1-0 = Reserved |
| 0Fh |
15 |
1 byte |
Shutdown code |
| |
|
|
|
00h = Power on or
soft reset |
| |
|
|
|
01h = Memory size
pass |
| |
|
|
|
02h = Memory test
pass |
| |
|
|
|
03h = Memory test
fail |
| |
|
|
|
04h = POST end;
boot system |
| |
|
|
|
05h = JMP double
word pointer with EOI |
| |
|
|
|
06h = Protected
mode tests pass |
| |
|
|
|
07h = Protected
mode tests fail |
| |
|
|
|
07h = Protected
mode tests fail |
| |
|
|
|
08h = Memory size
fail |
| |
|
|
|
09h = Int 15h block
move |
| |
|
|
|
0Ah = JMP double
word pointer without |
| |
|
|
|
EOI |
| |
|
|
|
0Bh = used by 80386 |
| 10h |
16 |
1 byte |
Floppy disk 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 |
| 11h |
17 |
1 byte |
Reserved |
| 12h |
18 |
1 byte |
Hard disk types |
| |
|
|
|
Bits 7-4 = Hard
disk 0 type (0-15) |
| |
|
|
|
Bits 3-0 = Hard
disk 1 type (0-15) |
| 13h |
19 |
1 byte |
Reserved |
| 14h |
20 |
1 byte |
Installed equipment |
| |
|
|
|
Bits 7-6 = Number
of floppy disk drives |
| |
|
|
|
|
00 = 1 floppy disk
drive |
| |
|
|
|
|
01 = 2 floppy disk
drives |
| |
|
|
|
Bits 5-4 = Primary
display |
| |
|
|
|
|
00 = Use display
adapter BIOS |
| |
|
|
|
|
01 = CGA 40-column |
| |
|
|
|
|
10 = CGA 80-column |
| |
|
|
|
|
11 = Monochrome
Display Adapter |
| |
|
|
|
Bits 3-2 = Reserved |
| |
|
|
|
Bit 1 = Math coprocessor
present |
| |
|
|
|
Bit 0 = Floppy disk
drive present |
| 15h |
21 |
1 byte |
Base memory low-order
byte |
| 16h |
22 |
1 byte |
Base memory high-order
byte |
| 17h |
23 |
1 byte |
Extended memory
low-order byte |
| 18h |
24 |
1 byte |
Extended memory
high-order byte |
| 19h |
25 |
1 byte |
Hard Disk 0 Extended
Type (0-255) |
| 1Ah |
26 |
1 byte |
Hard Disk 1 Extended
Type (0-255) |
| 1Bh |
27 |
9 bytes |
Reserved |
| 2Eh |
46 |
1 byte |
CMOS checksum high-order
byte |
| 2Fh |
47 |
1 byte |
CMOS checksum low-order
byte |
| 30h |
48 |
1 byte |
Actual extended
memory low-order byte |
| 31h |
49 |
1 byte |
Actual extended
memory high-order byte |
| 32h |
50 |
1 byte |
Date century in
BCD |
| 33h |
51 |
1 byte |
POST information
flag |
| |
|
|
|
Bit 7 = Top 128K
base memory status |
| |
|
|
|
|
0 = Top 128K base
memory not installed |
| |
|
|
|
|
1 = Top 128K base memory installed |
| |
|
|
|
Bit 6 = Setup program flag |
| |
|
|
|
|
0 = Normal (default) |
| |
|
|
|
|
1 = Put out first user message |
| |
|
|
|
Bits 5-0 = Reserved |
| 34h |
52 |
2 bytes |
Reserved |
