UPGRADING REPAIRING PCs

42
Chapter 2—System Components and Configuration
Startup and
observe POST.
If errors are
encountered,
go to defect
isolation
procedures.
Defect
Isolation
Procedures.
If count does
not match, go to
defect isolation
procedures.
If problems are
encountered, go
to Defect Isolation
Procedures.
If POST completes
with no errors,
basic functionality
has been tested.
Restart system,
enter Bios (or CMOS)
setup. Verify that
memory count is
equal to installed
amount.
While in the BIOS,
set all cache options
to disabled, save
settings and boot to
a DOS formatted
(floppy) system disk.
Use a diagnostics
program to test
system base and
extended memory.
If no problems are
encountered, memory
has tested OK.
Reboot, enter BIOS
setup re-enable.
Figure 2.11 184-pin DDR (Double Data Rate) SDRAM DIMM.
DDR DIMMs are rated for either PC200 (100MHz x 2) or PC266
(133MHz x 2) operation and normally run on 2.5 volts. They are
basically an extension of the PC100 and PC133 DIMMs redesigned
to support double clocking, where data is sent on each clock transition
(twice per cycle) rather than once per cycle as is standard with
SDRAM.
Parity Versus Non-Parity Memory
Parity-checked RAM uses units of 8 memory bits plus 1 parity bit,
for a total of 9 bits. In addition, parity checking uses both the data
bits and the parity bit to ensure that memory contents are accurate
with each memory access.
Virtually all 386-based and older systems, and most 486-based systems,
require parity-checked memory, which can detect, but not
correct, memory errors. On the other hand, most Pentium-class