Section 5 - Bus Structures

Internal bus: Interconnects main memory, the CPU, and all other components on the motherboard.
External bus: (expansion) Connects peripherals to the motherboard.

The width of a bus determines the amount of data, and how large an address it can transmit. The width of the bus is stated in bits. The speed that data moves on a bus is controlled by its clock speed.

The internal bus, also known as the system bus, is the maze of wires you see on the motherboard. It provides the internal components of the computer with four different necessities:

Power: Power comes to the motherboard straight from the power supply. The motherboard uses the system bus to distribute power to its ICs and other components.
Control Signals: The control unit within the CPU sends out control signals to coordinate the activities of the system. These signals are carried on a part of the internal bus called the control bus.
Addresses: PC components pass data and instructions between one another using memory location addresses to reference the location of the data or instruction in memory. These addresses are transmitted on a part of the internal bus called the address bus.
Data: The actual data or instructions being transferred between components are transmitted around the system on the part of the internal bus called the data bus.

The external bus, also known as the expansion bus, allows peripheral devices to communicate with the motherboard and its components. Each connection between the card and the slot form a channel that services a particular need of the expansion card. Some channels carry power, some connect to the address and data buses, and others are used for clock signal, IRQ, and DMA interfaces.

Clock signal: This connection provides the card with the signal of the bus clock so that it can synchronize its communications with the buses of the motherboard.
Interrupt request (IRQ): A request telling the CPU to interrupt what it is doing and take care of the special needs of the device sending the IRQ.
Direct memory access (DMA): DMA channels allow certain devices to bypass the processor and access main memory directly. Some architectures allow more DMA channels than others, but a DMA channel can't be shared by two devices.
Input / Output (I/O) address: Assigned to each device via the device's expansion slot. The I/O address, also called an I/O or hardware port, allows the CPU to send commands directly to the device. The I/O address is a one way only line that works like a reverse IRQ. The CPU uses the I/O address to send a command to the device. If the device responds, it uses the data bus or DMA channel to do so. Only one device can be assigned an I/O address.
Bus mastering: Another feature attached to expansion slots and expansion cards that allows one device to interact directly with another. Usually, the expansion card plugged into a slot has a bus master processor on the card that directs this activity.

Expansion slot architectures that have been used in PCs:

8-bit bus: Characterized by a single slot that supports eight interrupts and four DMA channels, with all of them preassigned. Almost all the architectures that followed are backward compatible with 8-bit cards.
Industry Standard Architecture (ISA): Introduced with the IBM AT and also called AT bus; it provides a 16-bit data bus. The ISA bus is characterized by adding an additional short slot to a slot on the 8-bit bus to create the 16-bit connector. ISA added eight additional IRQs and doubled the number of DMA channels. ISA expansion cards were designated to the appropriate IRQ or DMA numbers through jumpers and DIP switches. The ISA architecture also separated the bus clock from the CPU clock to allow the slower data bus to operate at its own speeds. ISA slots are found on 286, 386, 486, and some Pentium PCs.
Micro-Channel Architecture (MCA): Introduced with the IBM PS/2, MCA was the first 32-bit option, and featured bus mastering and a 10MHz bus clock for expansion cards. The MCA expansion slot is about the same size as the ISA slot, but has about twice as many channels. MCA cards are also configured to their IRQ and DMA assignments by software, an improvement over the jumpers and DIPs of the ISA architecture.
Extended ISA (EISA) This architecture was developed by a group of companies to overcome the limitations of ISA and to compete with MCA. It has a 32-bit data bus, uses software setup, has more I/O addresses available, and ignores IRQs and DMA channels. EISA uses only an 8MHz bus clock to be backward compatible with ISA boards.
Intelligent Drive Electronics (IDE): This architecture is used almost exclusively for disk drives. An IDE adapter card connects up to two different devices on each expansions slot. The Extended IDE (EIDE) architecture allows up to four drives to be connected into a single slot.
VESA Local Bus (VLB or VL-bus) VLB was used first on 486 systems and grew out of the need for the data bus to run at the same clock speed as the CPU. This 32-bit architecture is called Big ISA because beyond the local bus slot, it's basically an ISA architecture - jumpers, DIP switches, and all. VLB slots are mostly proprietary and support expansion cards only form the PCs manufacturer.
Peripheral Component Interconnect (PCI) bus: Introduced with the Pentium PC, PCI is a local bus architecture that supports either a 32-bit or 64-bit bus, which allows it to be used with both 486 and Pentium computers. The PCI bus is also processor independent because of a special bridging circuit contained on PCI boards. Its bus speed is 33MHz, giving it much higher thoughput than earlier cards. The PCI architecture and expansion slot also supports ISA and EISA cards. PCI cards are also Plug-and-Play, which means they automatically configure themselves to the appropriate IRQ, DMA, and I/O port addresses.
Universal Serial Bus (USB): A newly developed architecture that allows for the connection of up to 127 external serial devices. The USB technology is supported only on Pentium motherboards, where it's a built in feature on most. It can also be added to motherboards that don't include it through a PCI slot. USB provides support for both low-volume serial I/O devices, such as a mouse or keyboard, and some higher volume devices, such as a modem, CD-ROM, or scanner. All USB devices are PnP, and you can hot-plug or hot-swap them.

Bus Architectures
BUS	WIDTH (bits)	Speed (MHz)	Configured
8-bit	8	8	Jumpers and DIP switches
ISA	16	8	Jumpers and DIP switches
MCA	32	10	Software
EISA	32	8	Software
VL-Bus	32	Processor Speed	Jumpers and DIP switches
PCI	32/64	Processor Speed	PnP

The PC Card, or its original name PCMCIA bus, is used to add external devices to a notebook or hand-held computer. PC cards are used to add memory, modems, network interface cards, and even hard drives to portable computers. PCMCIA cards are hot-swappable. Three standards exist:

Type I: Cards that are 3.3mm thick, are used for memory additions, and have a single row of connectors.
Type II: Cards that are 5mm thick and used primarily to add modems or network interface cards (NICs). Type II cards have two rows of connectors.
Type III: Cards up to 10.5mm thick, which are often used to add an external hard disk to a notebook computer. Type III cards have four rows of connectors.

SCSI

     SCSI is an interface, and not an architecture. It is used to connect a wide variety of both internal and external devices. SCSI is not actually a bus architecture itself, but is actually a technology usd for interfacing multiple devices through a single connection on the motherboard. SCSI adapter cards can be PCI, VL-Bus, EISA, or ISA.

IRQs

     Interrupt requests are sent to a special system component, called an interrupt conroller, which either is a separate chip on the motherboard or is incorporated into the chipset. The interrupt controller receives and verifies requests and passes them on to the processor.

     Two interrupt controllers have been on PCs since the 286, each managing eight IRQ lines with each IRQ tied directly to a particular device. The two interrupt controllers are linked, or cascaded, through IRQ 2, which is set aside for this purpose. PC/XT systems that used IRQ 2 now use IRQ 9 instead.

I/O Addresses

     Every device in the PC uses input / output addresses, also called port addresses.The address in the I/O address points to the location in memory that's assigned to a specific device to use for exchanging information between itself and the rest of the PC. All devices have an I/O address assigned to them along with a segment of memory to hold messages and data. The size of the memory segement varies with the amount of data a device needs to pass on to other devices, but in general, the memory segment assigned to a device ranges from 1 to 32 bytes, with 4, 8, or 16 bytes being common. These areas of memory allow devices to do its work without worrying about what other devices or processor may be doing.

     The process of using I/O addresses to complete input / output operations is called memory-mapped I/O. I/O addresses are expressed in hex.

Common I/O Address Assignments
I/O Address Range	Common I/O Address Assignments
000-00Fh	DMA channels 0-3 controller
020-021h	IRQ 0-7 interrupt controller
060h, 061h	Keyboard
0F8-0FFh	Math coprocessor
130-14Fh	SCSI host adapter
170-177h	Secondary hard disk controller
1F0-1F7h	Primary hard disk controller
200-207h	Game port
220-22Fh	Sound cards
278-27Fh	LPT2 or LPT3
2E8-2EFh	COM4
2F8-2FFh	COM2
300-30Fh	Network cards
3B0-3BBh	VGA video adapter
3C0-3DFh	VGA video adapter
378-37Fh	LPT1 or LPT2
3E8-3EFh	COM3
3F0-3F7h	Floppy disk
3F8-3FFh	COM1

DMA Addressing

A direct memory access (DMA) channel allows a device to bypass the processor to directly access memory. Those devices with a DMA channel assignment gain the advantage of faster data transfers that do not have to pass through the CPU.

DMA Channel Assignments
0	DRAM refresh
1	Available
2	Floppy disk controller
3	Available
4	Link to second DMA controller
5	Available
6	Available
7	Available

Channels 0 through 3 are generally available in all PCs, but channels 4 through 7 are available only in 286 and later systems. Each group of four channels has its own DMA controller chip that services its requests. DMA channel assignments are made through the BIOS setup utility, with the dedicated configuration software of a device, or with DIP switches or jumpers on the device adapter card.

Logical Device Naming

Many devices are assigned both a physical address and a logical name. Logical device names are assigned to serial ports and parallel ports. Logical names eliminate the need for software to use what could be the moving target of I/O addresses. Logical device names are assigned during the POST process by the system BIOS.

Logical Device Name Assignments
PORT	I/O ADDRESS	IRQ
COM1	3F8-3FFh	4
COM2	2F8-2FFh	3
COM3	3E8-3EFh	4
COM4	2E8-2EFh	3
LPT1	378-37Fh	7
LPT2	278-27Fh	5