Hardware Reference
In-Depth Information
In many cases you can fit a memory module rated at the same speed or faster
than that at which a PC's memory system is rated. This means that you should
be able to replace a 70 ns module with one rated at either 70 or 60 ns
but not
one
rated at 80 ns. It is, however, worth noting that some older systems check the
module speed at boot-up and will only accept a module that has the same speed
rating as that of the system to which it is fitted. This explains why some systems
will refuse to accept faster memory modules than those being replaced!
CMOS memory
The PC-AT and later machine's CMOS memory is 64 byte of battery-backed
memory contained within the real-time clock chip (a Motorola MC146818).
Table 1.13
CMOS memory organization
Offset (hex.)
Contents
00
Seconds
01
Seconds alarm
02
Minutes
03
Minutes alarm
04
Hours
05
Hours alarm
06
Day of week
07
Day of the month
08
Month
09
Year
0A
Status Register A
0B
Status Register B
0C
Status Register C
0D
Status Register D
0E
Diagnostic status byte
0F
Shutdown status byte
10
Floppy disk type (drives A and B)
11
Reserved
12
Fixed disk type (drives 0 and 1)
13
Reserved
14
Equipment byte
15
Base memory (low byte)
16
Base memory (high byte)
17
Extended memory (low byte)
18
Extended memory (high byte)
19
Hard disk 0 extended type
1A
Hard disk 1 extended type
1B-2D
Reserved
2E-2F
Check-sum for bytes 10 to 1F
30
Actual extended memory (low byte)
31
Actual extended memory (high byte)
32
Date century byte (in BCD format)
33-3F
Reserved
