Hardware Reference
In-Depth Information
As an example, a ROM checksum can be produced simply by reading each
byte in turn, adding the values returned (ignoring any overflow) and comparing
the result with the known checksum for the ROM. Any difference will indicate
a ROM error and appropriate action can be taken. In the case of the RAM, a
somewhat different technique is employed. Here the process involves writing
and reading each byte of RAM in turn, checking, in each case, that the desired
change has been effected. Where a particular bit refuses to be changed, the
diagnostic procedure is temporarily halted and an appropriate error message is
generated (this may also provide sufficient information for it to be possible to
locate the individual device which has failed).
It is, of course, desirable that RAM diagnostics can also be carried out on
a non-destructive basis. In such cases, the byte read from RAM is replaced
immediately after each byte has been tested. It is thus possible to perform a
major RAM diagnostic routine without destroying data stored in read/write
memory.
The Power On Self Test (POST)
The Power On Self Test (POST) checks the hardware system during initialization
and performs the following checks:
•
System motherboard
•
Memory
•
Keyboard
•
Drives
If the Power On Self Test fails, the normal operating system boot sequence is
halted and an error message is displayed. The error message varies according to
the BIOS type and reference should be made to the BIOS manufacturer's data
in order to determine the appropriate course of action.
Once the system is booted (either into DOS or Windows) it is a relatively
easy matter to determine the hardware configuration using simple diagnostic
software (see Figure 13.1) or using in-built utilities (see page 323). However,
in order to make changes to the low-level system configuration it is necessary
to make use of facilities that are available from within the BIOS (as described
in the next section).
System BIOS
The Basic Input and Output System (BIOS) is a program stored in a read-only
memory (ROM) chip on the motherboard. When a computer is first powered-up
the BIOS program performs a number of functions including performing the
Power On Self Test (POST) and loading the operating system. The BIOS assists
with the management of PC hardware via a set of BIOS run-time service
routines.
In order to configure a PC's settings, a BIOS setup program is provided in
order to optimize and configure the system. Various settings and options are
provided, including:
•
adding additional floppy or hard drives
•
changing a systems boot sequence (e.g., allowing a system to check for a
boot CD before booting from a hard drive)
