1.4 ARCHITECTURE AND BUSES
• All of the components are tied together by computer buses.
• There are a number of separate buses in a computer,
• Address Bus,
- carries locations from the CPU, so that the proper RAM, ROM, and I/O locations may be addressed
- This is often 16 bits, but newer computers have moved to 32 an 64 bits.
- The size of this bus limits immediately available memory.
• Data Bus,
- Carries data to and from the CPU and RAM, ROM, and I/O devices.
- The size of this bus makes the difference when specifying whether the CPU is 8 bit, 16 bit, 32 bit, or 64 bit.
- A wider data bus allows the CPU to run faster, but the hardware is more complex
• Control Bus,
- Runs between all devices, and is used to direct reads, writes, data ready, address ready, interrupts, and a number of other signals.
• The bus structure of a computer may be as below,
1.4.1 Clock Speed and the Buses
• Clock speed determined how often the CPU will process instructions
• Each instruction will use a variable number of clock cycles. For example a get from memory will take longer than a binary or operation.
• Different CPU’s may use anywhere from 5 to a fraction of a clock cycle to process an instruction
• The bus speeds are limited by
- the physical layout of the board,
- memory speed
- CPU speed (which is related to clock speed)
• Each instruction for the CPU is made up of a variable number of bytes. These are loaded from memory during execution. In an 8 bit bus these are loaded 1 at a time. To speed this up wider buses are used. For example a 64 bit bus could probably load up to 8 instructions at one. This allows instructions to be stored and processed faster, and overcome the bus speed limitations of computers.
• Other Computers use a trick called caching to speed the computer. This uses a shorter bus to local memory which may be run at higher speeds, thus reducing the slower access to distant memory.