1. Buses Warning: some of the terminology is used inconsistently within the field

2. What is a bus? A bus is a basically just wires through which data travels from one part of a computer to another. Usually it’s implied the path is shared by a number of parts There is more than one bus and more than one kind of bus Data, address, control System, expansion, local, external USB, AGP, ISA, EISA, MCA, PCI, VESA (Parallel, serial)

3. What does it carry? In the simple architecture we’ve been considering, data and addresses travel on the same bus, while control information traveled along individual wires (not shared) Saves on pins In more complicated architectures, they are separate

4. Data, address and control buses The data bus carries data and instructions The address bus carries information about where the data should go. The control bus carries information from the CPU to other parts of the computer, telling what they should be doing Some use control bus as a synonym for system bus

5. Bus characteristics The highway analogy: moving data along the buses is like moving cars on the highway. Bus width (number of lanes) How many bits are moving around in parallel Bus speed (speed limit) How fast those bits are moving

6. Memory size The width of the system’s address bus puts an upper limit on the amount of memory locations For example, if the address bus width is 32, then there are 2 32 (4,294,967,296) addresses Note that instead of addressing individual words, computers usually address individual bytes, so that would mean 4 GB Of course, most computers have a lot less than 4 GB of memory, it’s just an upper limit

7. Bus speeds Measured in MHz (millions of cycles per second) It doesn’t make much sense to have a very fast processor speed and a slow bus speed; they should be compatible The bus speed is slower than the processor speed and often limits the speed of the computer

8. Multiple buses A bus should not be too long; its speed is determined in part by its length Also slower devices do not need faster (and more expensive) buses. The computer should not be held back by the slowest device Solution: More than one bus

9. System bus The system bus connects the CPU, memory and other motherboard parts This bus should be well coordinated with the processor and memory access speeds Other buses must interface with the system bus if they want to interact with the processor

10. Frontside and backside buses The bus within a processor that connects the CPU with main memory. It's used to communicate between the motherboard and other components in a computer system. In contrast, a backside bus connects the CPU to a Level 2 cache.

11. Expansion bus The expansion bus connects the system bus to the expansion slots (where cards are inserted to expand the computer’s capabilities) This bus usually works at slower speeds Early PCs used an expansion bus called the ISA bus. Most PCs today have a much faster PCI bus but usually have an ISA bus for backward compatibility.

12. Local bus If a device or devices require a great deal of speed (e.g. video), then one solution is for the device to have its own high-speed, direct (or nearly direct) connection to the processor. Such a connection is called a local bus Can only support a few devices

13. ISA Industry Standard Architecture (ISA) is the bus used in early IBM PC and their clones. The AT version of the bus is called the “AT” bus and became an industry standard. Worked at 8.33 MHz

14. Plug and Play In 1993, Intel and Microsoft introduced a version of the ISA called Plug and Play ISA. Plug and Play ISA enables the operating system to do the configuring, instead of the user setting switches and jumpers

15. PCI Peripheral Component Interconnect, a local bus standard introduced by Intel. PCI is a 64-bit bus, though it is usually implemented as a 32-bit bus. It can run at clock speeds of 33 or 66 MHz. At 32 bits and 33 MHz, it yields a throughput rate of 133 MBps (Mega bits per second).

16. EISA and MCA Between ISA and PCI were some short-lived bus architectures Extended Industry Standard Architecture (EISA) Micro Channel Architecture (MCA) The principal difference between EISA and MCA is that EISA is backward compatible with the ISA bus, while MCA is not.

17. VLB Short for VESA Local-Bus, a local bus created by the Video Electronics Standard Association (VESA). 33 MHz Although it was used a lot in PCs made in 1993 and 1994, PCI has become more popular

18. external bus A bus that connects a computer to peripheral devices. Two examples are the Universal Serial Bus (USB) and IEEE 1394.

19. USB Universal Serial Bus, a new external bus standard that supports data transfer rates of 12 Mbps (12 million bits per second). A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, and keyboards. USB also supports Plug-and-Play installation and hot plugging.

20. bus mastering Refers to a feature supported by some bus architectures that enables a controller connected to the bus to communicate directly with other devices on the bus without going through the CPU. Most modern bus architectures, including PCI, support bus mastering because it improves performance.

21. DMA Direct Memory Access Gives a peripheral device access to the memory without going through the CPU Speeds up data transfer

22. Three State Logic AEOutput 00Z (High impedance) 010 10 111

23. Tri-state buffer

24. In the high impedance state

26. In the “enabled” state