Buses and Expansion Slots
September 27 (Day); September 19 (Night)
§ What is an Expansion Bus:
o Expansion buses are special connections on the motherboard that enable you to add hardware to the computer that didn’t come with it (p. 111).
o Buses carry four types of “cargo”:
§ Electrical power
§ Control activity
§ Memory addresses
§ Data
o These expansion devices (sound card, video card, etc.) connect to the expansion buses at connections called expansion slots.
o Expansion slots allow any compatible type of device and brand of device to work. In other words, we can install virtually any device without regard to brand name (p. 111).
o Expansion buses are dependent on the clock chip for their speed. In other words, the same clock chip that controls the speed of the processor also controls the speed of the expansion buses, and thus the devices plugged into the expansion slots (p. 112).
§ However, be aware that the speeds of expansion buses are not the same.
§ An expansion bus connects to the Northbridge before connecting to the processor.
o There are different types of expansion buses. Each type of expansion bus has a different number of wires and run at different speeds (p. 113).
o One measure of quality for a bus is throughput. Throughput is the same amount of data transferred or processed within a specified amount of time. Usually expressed as MB/sec.
§ There are two factors that apply to throughput. One is data width (number of bits transferred at a time) and the other is speed (how fast the data is moving).
§ Types of Expansion Slots:
o 8-bit ISA (p. 121):
§ ISA = Industry Standard Architecture
§ Dates back to the early 1980s
§ Data width = 8 bits
§ Speed = 7 MHz
§ Throughput = 1 MB / sec
8-bit ISA slots
o 16-bit ISA (p. 121-122):
§ Redesign of the 8-bit ISA slot (has a small extension on the end)
§ Data width = 16 bits
§ Speed = 7 MHz
§ Throughput = 8 MB/sec
§ The original 8-bit and 16-bit ISA cards had to be manually configured.
o ISA met its demise when it could no longer take advantage of the faster Pentium processors (p. 125).
16-bit ISA slots
o MCA (pp. 122-123):
§ MCA = Micro Channel Architecture
§ Data width = 32 bits
§ Speed = 12 MHz
§ Throughput = 40 MB/sec
§ Device automatically configured itself.
§ IBM patented MCA bus so that other manufacturers had to pay for it.
o EISA (pp. 123-124):
§ EISA = Extended Industry Standard Architecture
§ Data width = 32 bits
§ Speed = 8.33 MHz
§ Throughput = 32 MB/sec
§ Formed by IBM competitors
§ Device automatically configured itself
§ Maintained backwards compatibility with 8-bit and 16-bit ISA bus
o 32-bit expansion buses did not catch on in the late 1980s because the devices were too slow to take advantage of the extra throughput.
o PCI (pp. 125-126):
§ PCI = Peripheral Component Interconnect
§ Data width = 32 bits
§ Speed = 33 MHz
§ Throughput = 264 MB/sec
§ The PCI bus is actually mezzanine bus. A mezzanine bus is an expansion bus that does not have to be the only expansion bus on the motherboard. In other words, it is coordinated to work with other buses on the motherboard.
· Data traffic on the ISA bus is passed to the PCI bus and “stepped up” to run at the speed of the PCI bus.
§ The PCI bus connects to the Northbridge chip through the PCI expansion bus.
§ PCI cannot handle the greater video requirements of today’s graphically intense games.
PCI slots
o AGP (pp. 126-127):
§ AGP = Accelerated Graphics Port
§ Data width = 32 bits
§ Speed = 66 MHz and above
§ Throughput = 528 MB/sec
§ Designed to provide fast access to video
§ Directly connected to the motherboard.
AGP Slot
o USB (pp. 138-140):
§ USB = Universal Serial Bus
§ Data width = 1 bit
§ Speed = 3 MHz
§ Throughput:
· USB 1.1 = 12 MB/sec
· USB 2.0 = 60 MB/sec
§ USB is hot swappable. Hot swappable means that devices can be added and removed easily while the computer is still running without having to reboot.
§ USB devices may be daisy chained. 127 devices can be connected through a single USB port.
· This is done by plugging a multi-port USB hub into a USB port on the back of the computer. Then plug other USB devices into the hub.
USB Ports
o FireWire (p. 141):
§ FireWire = IEEE 1394
§ Data width = 1 bit
§ Throughput = 50 MB/sec
§ FireWire is hot swappable and daisy-chain 63 devices.
§ FireWire was designed primarily for multimedia devices.