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back2root:ibm-pc-ms-dos:hardware:informations:cmos_ram
Table des matières
CMOS RTC - Real Time Clock and Memory (ports 70h & 71h)
The information here is only applicable to AT and PS/2 systems
| Reg# | Description |
|---|---|
| 00 | RTC seconds |
| 01 | RTC seconds alarm |
| 02 | RTC minutes |
| 03 | RTC minutes alarm |
| 04 | RTC hours |
| 05 | RTC hours alarm |
| 06 | RTC day of week |
| 07 | RTC day of month |
| 08 | RTC month |
| 09 | RTC year |
| 0A | RTC Status register A |
| 0B | RTC Status register B |
| 0C | RTC Status register C (read only) |
| 0D | RTC Status register D (read only) |
| 0E | Diagnostic status byte |
| 0F | Shutdown status byte |
| 10 | Diskette drive type for A: and B |
| 11 | Reserved |
| 12 | Fixed disk drive type for drive 0 and drive 1 |
| 13 | Reserved |
| 14 | Equipment byte |
| 15 | LSB of system base memory in 1k blocks |
| 16 | MSB of system base memory in 1k blocks |
| 17 | LSB of total extended memory in 1k blocks |
| 18 | MSB of total extended memory in 1k blocks |
| 19 | Drive C extension byte (reserved AT) |
| 1A | Drive D extension byte (reserved AT) |
| 1B | 13 bytes reserved |
| 2E | CMOS checksum of bytes 10h-20h (MSB) |
| 2F | CMOS checksum of bytes 10h-20h (LSB) |
| 30 | LSB of extended memory size found above 1 megabyte during POST |
| 31 | MSB of extended memory size found above 1 megabyte during POST |
| 32 | Date century byte in BCD ( BIOS interface to read and set) |
| 33 | Information flags (set during power-on) |
| 34 | 12 bytes reserved |
RTC Status register A
| bit | descr. |
|---|---|
| 0-3 | rate selection Bits for divider output frequency (set to 0110 = 1.024kHz, 976.562 µs) |
| 4-6 | 22 stage divider, time base being used; (initialized to 010 = 32.768kHz) |
| 7 | 1=time update in progress, 0=time/date available |
RTC Status register B
| bit | Description |
|---|---|
| 0 | 0=disable (default) 1=enable daylight savings |
| 1 | 0=12 hour mode (24 default) 1=24 hour mode |
| 2 | 0=BCD (BCD default) 1=time/date in binary |
| 3 | 0=disable 1=enable square wave frequency |
| 4 | 0=disable 1=enable update ended interrupt |
| 5 | 0=disable 1=enable alarm interrupt |
| 6 | 0=disable 1=enable periodic interrupt |
| 7 | 0=update count normally 1=disable clock update |
RTC Status register C (read only)
| bit | Description |
|---|---|
| 0-3 | reserved (set to 0) |
| 4 | update ended interrupt enabled |
| 5 | alarm interrupt enabled |
| 6 | periodic interrupt enabled |
| 7 | IRQF flag |
RTC Status register D (read only)
Diagnostic status byte
| bit | Description |
|---|---|
| 0-1 | reserved |
| 2 | 0=ok (POST validity check) 1=time is invalid |
| 3 | 0=ok 1=fixed disk 0 failed initialization |
| 4 | 0=ok 1=memory size doesn't match config info |
| 5 | 0=ok (see below) 1=invalid config info found |
| 6 | 0=ok 1=config record checksum is bad |
| 7 | 0=power state stable 1=RTC lost power |
Shutdown status byte
| value | Description |
|---|---|
| 0 | soft reset or unexpected shutdown |
| 1 | shut down after memory size determination |
| 2 | shut down after memory test |
| 3 | shut down with memory error |
| 4 | shut down with boot loader request |
| 5 | JMP DWORD request with INT init |
| 6 | protected mode test 7 passed |
| 7 | protected mode test 7 failed |
| 8 | protected mode test1 failed |
| 9 | block move shutdown request |
| A | JMP DWORD request without INT init |
Diskette drive type for A: and B:
| bit | Description |
|---|---|
| 0-3 | second diskette type |
| 4-7 | first diskette type |
| values | description |
|---|---|
| 0000 | no drive installed |
| 0001 | DSDD 48 TPI drive (Double Side Double Density) |
| 0010 | DSQD 96 TPI drive (Double Side Quad Density) |
other values are reserved
Fixed disk drive type for drive 0 and drive 1
Equipment byte
| bit | Description |
|---|---|
| 0 | 0=none 1=diskette drives installed |
| 1 | 0=none 1=math coprocessor installed |
| 2-3 | unused |
| 4-5 | primary display |
| 6-7 | number of diskette drives installed |
| Bits 54 | Primary Display |
|---|---|
| 00 | reserved |
| 01 | 40 column color |
| 10 | 80 column color |
| 11 | monochrome |
| Bits 76 | Number of Drives |
|---|---|
| 00 | 1 diskette drive |
| 01 | 2 diskette drives |
| 10 | reserved |
| 11 | reserved |
Information Flags
Programming Considerations
- Write CMOS address to read or write to port 70h
- Read/write port 71h to get/set data
Notes
- INT 1A is used to read/set the Time of Day and Alarm. To use the alarm, INT 4A must be a valid interrupt service routine.
- configuration settings are maintained using the Motorola MC146818 Real Time Clock. Each of this chips 64 memory registers is used for storage (0-3F).
- Bit 5 of the diagnostic (0Eh) status byte is set during a power on test. This Bit is set if no floppy disks are found or the display doesn't match the system display switch setting.
- all addresses sent to port 70h have Bits 7&6 clear since Bit 7 of port 70h is used to enable/disable NMI. Setting this Bit 7 enables NMI, clearing this Bit disables NMI.
- when masking the NMI through using port 70H, port 71H should be read immediately after or the RTC may be left in an unknown state.
This wont affect the PS/2 watchdog timer or system channel timeout.
see INT 1A
back2root/ibm-pc-ms-dos/hardware/informations/cmos_ram.txt · Dernière modification : 2023/01/16 17:54 de frater
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