Abit IT5A User Manual page 23

Installing the Mainboard
AT Bus clock
Also referred to as ISA SPEED, or AT CLOCK, or even ISA Bus clock. Ten
years ago, the original specification of AT Bus clock installed in the first
generation PC/AT computers was 8MHz, this means that there are some
interface cards which can only work at 8MHz. In order to guarantee
compatibility with older hardware, we still support 8MHz AT Bus clock, but
if your interface card is newer or faster, you can choose a higher speed for
the AT Bus clock, in order to increase the transmission rate of the interface
cards. But we recommend you not be too ambitious. An 8MHz setup ensures
maximum compatibility.
CPU Voltage - Vcore and Vio
From the voltage point of view, 586 series CPU can be divided into two
categories: single voltage CPU and dual voltage CPUs. Single voltage CPUs
include: Intel Pentium P54C series, AMD-K5 and Cyrix 6x86, etc.
Dual voltage CPUs include: Intel P55C with MMX, future AMD-K6 CPUs,
Cyrix 6x86L and 6x86MX.
The voltage of dual voltage CPUs has two components: Vcore and Vio.
Vcore provides all the power for internal processing, and its power
consumption is quite important. Vio provides the power necessary for the
external interface of the CPU.
P-Rating
The P-Rating was defined by some manufacturers other than Intel to rate the
performance of their CPU in comparison with Intel Pentium CPUs. For
instance, Cyrix 6x86 P166+ has a higher actual speed compared with Intel
Pentium P166, its internal clock needs only 133MHz versus 166MHz for
Intel's product. The main reason is that Cyrix has improved the internal
architecture of the 6x86 CPU, which means that with the same internal clock
speed, the actual clock speed will be different.
Voltage and Frequency Setup
IT5A uses DIP switches to setup the CPU Frequency and voltage. Please
make sure which kind of CPU and its frequency and voltage before you setup.
There are two DIP switches on the mainboard, one is use to setup the CPU
frequency (including external clock and rating parameter) which is called
SW2, another one is SW1 which is used to setup the CPU voltage.
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