The CPU, memory, and registers work together to process an instruction. Whether the instruction is in the form of a command that you type or an icon that you point to with a mouse, these instructions that you issue to the computer is broken down by your computer system into several smaller machine-level instructions called microcode. Each piece of microcode corresponds directly to a set of the computerís circuits.
? microcode: instructions that are built into
the CPU to control the
operation of its circuitry
The computer system has a built-in system clock that synchronizes its operations. During each clock tick, a single piece of microcode can be executed, a single piece of data can be moved from one part of the computer system to another. Microcode instructions are coded in machine language.
? system clock: the timing mechanism within
the computer system that
governs the transmission of instructions and data through the circuitry
The processing of a single, machine-level instruction is called a machine cycle. It has two parts: an instruction cycle ( I- cycle ) and the execution cycle ( E-cycle ). During the I-cycle, the control unit fetches a program instruction from memory and prepares for subsequent processing. During the E-cycle, the data are located and the instruction is executed.
? machine cycle: the series of operations
involved in the execution of a
single machine-level instruction
? I-cycle: the part of the machine cycle in
which the control unit fetches an
instruction from memory and prepares it for subsequent processing
? E-cycle: the part of the machine cycle in
which data are located, an
instruction is executed, and results are stored
Letís take a look on how the machine cycle works in detail.
1. The control unit fetches from memory the next instructionto be executed.
2. The control unit decodes the instruction.
3. The control unit puts the part of the instruction that shows what to do into the
4. The control unit puts the part of the instruction that shows where the
associated data are located into the address register.
5. Using the information in the address register, the control unit retrieves data
from memory and places them into the storage register.
6. Using the information in the instruction register, the control unit commands
the ALU to perform the required operation.
7. The ALU performs the specified operation, adding together the values found
in the storage register and in the accumulator.
8. The result of the operation is placed back into the accumulator, destroying the
value that was there previously.
The Machine Cycle
Place result in Memory
Although this seems to be an extremely tedious process for the machine cycle to process a single program fully, computers are very fast. In the slowest of them, cycle times are measured in milliseconds ( one thousandth of a second ). In other, they are measured in microseconds ( one millionth of a second ). In the fastest computers, they are measured in nanoseconds ( one billionth of a second ) or in picoseconds ( one trillionth of a second ).
A processor is described in terms of its word
length, speed, and RAM capacity.
Processor speed is how fast the computer finishes a machine cycle. For microcomputers, speed are rated in megahertz ( MHz ). Each megahertz represents a million clock ticks per second. Speeds are rated in millions of instruction per second ( MIPS ) for workstations and microcomputers; in the supercomputer world, itís million floating-point operations per second ( MFLOPS ).