CPU multiplier
Encyclopedia
In computing
, the clock multiplier (or CPU multiplier or bus/core ratio) measures the ratio of an internal CPU clock rate
to the externally supplied clock
. A CPU with a 10x multiplier will thus see 10 internal cycles (produced by PLL-based frequency multiplier
circuitry) for every external clock cycle. For example, a system with an external clock of 133 MHz and a 10x clock multiplier will have an internal CPU clock of 1.33 GHz. The external address and data buses of the CPU (often collectively termed front side bus
or FSB in PC
contexts) also use the external clock as a fundamental timing base, however, they could also employ a (small) multiple of this base frequency (typically two or four) in order to transfer data faster.
". Nearly every desktop CPU produced since the introduction of the 486DX2
in 1992 has employed a clock multiplier to run its internal logic at a higher frequency than its external bus, but still remain synchronous with it. This improves the CPU performance by relying on internal cache memories and/or wide buses (often also capable of more than one transfer per clock cycle) to make up for the frequency difference.
and Opteron
, handle main memory using a separate and dedicated low-level memory bus
. These processors communicate with other devices in the system (including other CPUs) using one or more slightly higher-level HyperTransport
links; like the data and address buses in other designs, these links employ the external clock for data transfer timing (typically 800 MHz or 1 GHz, as of 2007).
menu. Increasing the clock multiplier will increase the CPU clock speed without affecting the clock speed of other components. Increasing the external clock (and bus-speed) will affect the CPU as well as RAM and other components.
These adjustments provide the two common methods of overclocking
and underclocking
a computer, perhaps combined with some adjustment of CPU or memory voltages (changing oscillator crystals occurs only rarely); note that careless overclocking can cause damage to a CPU or other component due to overheating or even voltage break-down. Newer CPUs often have a locked clock multiplier
, meaning that the bus speed or the clock multiplier cannot be changed in the BIOS unless the user hacks the CPU to unlock the multiplier. High End CPUs however normally have an unlocked multiplier.
Examples of clock-doubled CPUs include:
In both these cases the overall speed of the systems increased by about 75%.
By the late 1990s almost all high-performance processors (excluding typical embedded systems) run at higher speeds than their external buses, so the term "clock doubling" has lost much of its impact.
For CPU-bound applications, clock doubling will theoretically improve the overall performance of the machine substantially, provided the fetching of data from memory does not prove a bottleneck. In more modern processors where the multiplier greatly exceeds two, the bandwidth
and latency
of specific memory IC
s (and/or the bus or memory controller) typically become a limiting factor.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
, the clock multiplier (or CPU multiplier or bus/core ratio) measures the ratio of an internal CPU clock rate
Clock rate
The clock rate typically refers to the frequency that a CPU is running at.For example, a crystal oscillator frequency reference typically is synonymous with a fixed sinusoidal waveform, a clock rate is that frequency reference translated by electronic circuitry into a corresponding square wave...
to the externally supplied clock
Clock signal
In electronics and especially synchronous digital circuits, a clock signal is a particular type of signal that oscillates between a high and a low state and is utilized like a metronome to coordinate actions of circuits...
. A CPU with a 10x multiplier will thus see 10 internal cycles (produced by PLL-based frequency multiplier
Frequency multiplier
In electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal...
circuitry) for every external clock cycle. For example, a system with an external clock of 133 MHz and a 10x clock multiplier will have an internal CPU clock of 1.33 GHz. The external address and data buses of the CPU (often collectively termed front side bus
Front side bus
A front-side bus is a computer communication interface often used in computers during the 1990s and 2000s.It typically carries data between the central processing unit and a memory controller hub, known as the northbridge....
or FSB in PC
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
contexts) also use the external clock as a fundamental timing base, however, they could also employ a (small) multiple of this base frequency (typically two or four) in order to transfer data faster.
Basic system structure
, computers have several interconnected devices (CPU, RAM, peripherals, etc. - see diagram) that typically run at different speeds. Thus they use internal buffers and caches when communicating with each other via the shared buses in the system. In PCs, the CPU's external address and data buses connect the CPU to the rest of the system via the "northbridgeNorthbridge (computing)
The northbridge has historically been one of the two chips in the core logic chipset on a PC motherboard, the other being the southbridge. Increasingly these functions have migrated to the CPU chip itself, beginning with memory and graphics controllers. For Intel Sandy Bridge and AMD Fusion...
". Nearly every desktop CPU produced since the introduction of the 486DX2
Intel 80486DX2
The Intel 80486DX2 is a CPU produced by Intel that was introduced in 1992. The i486DX2 was nearly identical to the i486DX but for the addition of clock multiplier circuitry. It was the first chip to use clock doubling, whereby the processor runs two internal logic clock cycles per external bus cycle...
in 1992 has employed a clock multiplier to run its internal logic at a higher frequency than its external bus, but still remain synchronous with it. This improves the CPU performance by relying on internal cache memories and/or wide buses (often also capable of more than one transfer per clock cycle) to make up for the frequency difference.
Variants
Some CPUs, such as Athlon 64Athlon 64
The Athlon 64 is an eighth-generation, AMD64-architecture microprocessor produced by AMD, released on September 23, 2003. It is the third processor to bear the name Athlon, and the immediate successor to the Athlon XP...
and Opteron
Opteron
Opteron is AMD's x86 server and workstation processor line, and was the first processor which supported the AMD64 instruction set architecture . It was released on April 22, 2003 with the SledgeHammer core and was intended to compete in the server and workstation markets, particularly in the same...
, handle main memory using a separate and dedicated low-level memory bus
Memory bus
The memory bus is the computer bus which connects the main memory to the memory controller in computer systems. Originally, general-purpose buses like VMEbus and the S-100 bus were used, but to reduce latency, modern memory buses are designed to connect directly to DRAM chips, and thus are...
. These processors communicate with other devices in the system (including other CPUs) using one or more slightly higher-level HyperTransport
HyperTransport
HyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...
links; like the data and address buses in other designs, these links employ the external clock for data transfer timing (typically 800 MHz or 1 GHz, as of 2007).
BIOS settings
Most systems allow owners to change the clock multiplier in the BIOSBIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....
menu. Increasing the clock multiplier will increase the CPU clock speed without affecting the clock speed of other components. Increasing the external clock (and bus-speed) will affect the CPU as well as RAM and other components.
These adjustments provide the two common methods of overclocking
Overclocking
Overclocking is the process of operating a computer component at a higher clock rate than it was designed for or was specified by the manufacturer, but some manufacturers purposely underclock their components to improve battery life. Many people just overclock or 'rightclock' their hardware to...
and underclocking
Underclocking
Underclocking, also known as downclocking, is the practice of modifying a synchronous circuit's timing settings to run at a lower clock rate than it was specified to operate at. It may be said to be the computer equivalent of driving a car below the speed limit...
a computer, perhaps combined with some adjustment of CPU or memory voltages (changing oscillator crystals occurs only rarely); note that careless overclocking can cause damage to a CPU or other component due to overheating or even voltage break-down. Newer CPUs often have a locked clock multiplier
CPU locking
CPU locking is the process of permanently setting a CPU's clock multiplier. AMD CPUs are unlocked in early editions of a model and locked in later editions, but nearly all Intel CPUs are locked and recent models are very resistant to unlocking to prevent overclocking by users...
, meaning that the bus speed or the clock multiplier cannot be changed in the BIOS unless the user hacks the CPU to unlock the multiplier. High End CPUs however normally have an unlocked multiplier.
Clock doubling
The phrase clock doubling implies a clock multiplier of two.Examples of clock-doubled CPUs include:
- the Intel 80486DX2Intel 80486DX2The Intel 80486DX2 is a CPU produced by Intel that was introduced in 1992. The i486DX2 was nearly identical to the i486DX but for the addition of clock multiplier circuitry. It was the first chip to use clock doubling, whereby the processor runs two internal logic clock cycles per external bus cycle...
, which ran at 50 or 66 MHz on a 25 or 33 MHz bus - the WeitekWeitekWeitek Corporation was a chip-design company that originally concentrated on floating point units for a number of commercial CPU designs. During the early to mid-1980s, Weitek designs could be found powering a number of high-end designs and parallel processing supercomputers...
SPARC POWER µP, a clock-doubled 40 MHz version of the SPARCSPARCSPARC is a RISC instruction set architecture developed by Sun Microsystems and introduced in mid-1987....
processor that one could drop into the otherwise 20 MHz SPARCStation 2
In both these cases the overall speed of the systems increased by about 75%.
By the late 1990s almost all high-performance processors (excluding typical embedded systems) run at higher speeds than their external buses, so the term "clock doubling" has lost much of its impact.
For CPU-bound applications, clock doubling will theoretically improve the overall performance of the machine substantially, provided the fetching of data from memory does not prove a bottleneck. In more modern processors where the multiplier greatly exceeds two, the bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
and latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
of specific memory IC
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s (and/or the bus or memory controller) typically become a limiting factor.