There are several attributes of a microprocessor which determine its power and capabilities:
* Width of external data bus in bits
* Width of address bus in bits
* Width of internal data bus in bits
* Cache Memory
* Clock Speed in Megahertz (MHz)
* MIPS (Million Instructions per Second)
* Power Consumption (Watts)
External Data Bus
The earliest microprocessors could handle data only in bytes (8 bits). As the width of the data bus increases, the width of the data bus determines how much information can be moved in or out of the processor in one operation. It also determines the number and length of instructions which can be used.
The width of the address bus determines how much memory can be addressed. The Intel 8086 had a 20 bit address bus. Since a 20 bit binary number can represent 2 different numbers, the 8086 could address 1MB of memory. The 386 and 486 could address 4GB of memory using a 32-bit address bus, and the Pentium class processors have a 36-bit address bus capable of connecting 64GB of memory.
Internal data bus
The width of the internal data bus and the storage registers may differ from the external data bus. The 386SX processor, for example had the same internal 32-bit registers as other 386s, but only a 16-bit external bus. The Pentium class processors, have an external 64-bit data bus, but the internal registers are only 32-bit. For this reason the Pentium processors are referred to as 32-bit. Workstation processors like the SPARC and Alpha are 64-bit, as is the next generation of Intel chips, the Itanium.
As processor speeds increased, the speed of main memory (RAM) could not keep up. To minimize size and cost, RAM memory uses Dynamic RAM (DRAM). Static RAM (SRAM) is much faster, but also more expensive, so it is used in small quantities as a temporary storage location for data on the microprocessor or closely connected to it. This high-speed memory is known as cache memory. It is operated by a cache controller which attempts to identify which data or instructions will be needed next and load them into the cache so that the processor will not have to stand idle while waiting for data to be retrieved from RAM. Today's microprocessors have cache memory in two levels, referred to as Level 1 (L1)and Level 2 (L2). The L2 cache was originally mounted on separate chips outside the CPU, and operated at a lower speed than the processor, but improvements in manufacturing technology have permitted the L2 cache to be moved onto the processor chip where it operates at the same speed as the processor. In processors like the Pentium III, an additional external bus operating at processor speed connects the L2 cache; this is known as the backside bus in contrast to the frontside bus which connects to main memory.
An oscillator mounted on the motherboard generates a series of electrical pulses which the computer uses to synchronize the operations of its many components. Each complete change in the signal, from positive to negative and back again is known as a cycle, and the number of cycles per second, or frequency, is measured in Hertz.
1 000 Hz = 1 kilohertz = 1KHz
1 000 000 Hz = 1 Megahertz = 1 MHz
1 000 000 000 Hz = 1 Gigahertz = 1 GHz.
The speed of the processor is often a multiple of the external bus speed: for example a 500 MHz chip installed on a 100MHz mainboard will operate at 5x the bus speed.
The clock speed does not relate directly to the speed at which the CPU processes instructions. Early microprocessors required as many as 10 clock cycles to complete a single instruction. Modern microprocessors with what is called 'superscalar" architecture have dual or multiple 'pipelines' so that more than one instruction can be executed at once. Therefore, a more accurate measure of processor speed is MIPS (Millions of Instructions per Second), although the number of actual instructions processed rarely reaches the theoretical maximum.
Power consumption is an often overlooked measure of microprocessor performance. Much of the power consumed is given off as heat, which must be dissipated, or it will cause malfunctions. Low power consumption is also a critical factor in extending the life of batteries in notebook computers.
The formula for power consumption is Volts x Amps = Watts. However, a decrease in operating voltage also produces a drop in amperage (Ohm's Law). Older chips functioned at 5 volts, while Pentium class chips operate in the range of 2 volts. This results in a power saving of 84% without any other improvements in the circuitry.
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