Datasheet

Identifying Motherboards
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External Speed (Clock Speed)
The clock speed, or external speed, is the speed at which the motherboard communicates
with the CPU. Its determined by the motherboard, and its cadence is set by a quartz crystal
(the system crystal) that generates regular electrical pulses.
Internal Speed
The internal speed is the maximum speed at which the CPU can perform its internal opera-
tions. This may be the same as the motherboard’s speed (the external speed), but it’s more
likely to be a multiple of it. For example, a CPU may have an internal speed of 1.3GHz but
an external speed of 133MHz. That means for every tick of the system crystal’s clock, the
CPU has 10 internal ticks of its own clock.
Cache Memory
A cache is an area of extremely fast memory used to store data that is waiting to enter
or exit the CPU. The Level 1 cache, also known as the L1 or front-side cache, holds data
that is waiting to enter the CPU. On modern systems, the L1 cache is built into the CPU.
The Level 2 cache, also known as the L2 or back-side cache, holds data that is exiting the
CPU and is waiting to return to RAM. On modern systems, the L2 cache is in the same
packaging as the CPU but on a separate chip. On older systems, the L2 cache was on a
separate circuit board installed in the motherboard, and was sometimes called cache on
a stick (COAST).
On some CPUs, the L2 cache operates at the same speed as the CPU; on others, the cache
speed is only half the CPU speed. Chips with full-speed L2 caches have better performance.
Some newer systems also have an L3 cache, which is external to the CPU. It sits between
the CPU and RAM to optimize data transfer between them.
The Bus
The processor’s ability to communicate with the rest of the system’s components relies on
the supporting circuitry. The system board’s underlying circuitry is called the bus. The
computer’s bus moves information into and out of the processor and other devices. A bus
allows all devices to communicate with one another. The motherboard has several buses.
The external data bus carries information to and from the CPU and is the fastest bus on
the system. The address bus typically runs at the same speed as the external data bus and
carries data to and from RAM. The PCI, AGP, and ISA interfaces also have their own
buses with their own widths and speeds. With newer architectures, the System or Front
Side Bus (FSB) connects the CPU to northbridge (or memory) hub. The back side bus con-
nects CPU with Level 2 (L2) cache, aka secondary or external cache and the memory bus
connects northbridge (or memory) hub to RAM.
The CPU must be compatible with the motherboard in the following ways:
Physical Connectivity The CPU must be in the right kind of package to fit into the
motherboard.
Speed The motherboard’s chipset dictates its external data bus speed; the CPU must be
capable of operating at that external speed.
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