Understanding Computer Bus Technology: Data, Address, and Control
A Technology with Many Cables
A Technology with Many Cables
- Works with a binary system, which means using a wire for each bit of data.
- Technology with too many cables.
Current microprocessors, like the Pentium 4 processor, internally use 32-bit data. But when accessing external devices to process more data, they read 64 bits at a time (i.e., to enter information inside the microprocessor, they take two 32-bit data chunks at a time to accelerate the transfer). This implies that if the microprocessor has to receive data from three different devices and dump the results to three other devices, it would require 32 x 2 x 6 = 384 cables!
The Solution to the Problem: Buses
The solution is to replace each set of 64 cables connected to devices individually with a single channel that reaches all devices. Then, we will have a single main channel with 64 cables, ranging from the microprocessor to the vicinity of each device. Because this channel carries a lot of wires, it is called a bus (a set of conducting wires). In our example, we will call it the data bus, as it transports data.
A Data Communication Channel
Now we face another dilemma: we have a communication channel that carries data to all devices, and the information is the same for all. How do the devices know which information is for them? To send information to a particular device, we need some means to identify it.
A Channel for Addresses to the Addressee
The solution is to take advantage of the individual identification that each device possesses. Using the identifications of each device, we can use them as an address. The way to apply this reasoning to our case would be the creation of a new, independent channel to send this notice to a particular device, knowing its address, and notify it of the existence of information on the data bus to be used. Because it is used to locate our devices, this new channel will be called the address bus. As its only function is to locate a device, it is said to be unidirectional.
How do they find the device if the microprocessor does not want to receive information but send it?
A Channel to Indicate Action
The solution to this problem is the addition of a third independent channel to the latter two, by which the operation you want to communicate to the microprocessor is indicated, e.g., read or write. As this channel can control various operations, it will be called the control bus.
Different Types of Buses (Expansion Buses)
These buses can accommodate devices, and for this, they need a connector where the device can be placed. This connector is attached to the bus via the motherboard and is known as a slot. They are called expansion slots, and as we saw in the previous chapter, the goal is to add devices (cards). There are several types of technologies, and they are recognized by the slot type (color and form that these possess), the number of bits of data used, and the speed at which they operate.
Buses
ISA Bus (8 bits): It was the first expansion bus, using 8 bits, and its distinctive color is black. It was adopted by the industry as a de facto standard, and all manufacturers adopted the same technical characteristics, therefore making them compatible.
ISA Bus (16 bits): This is the best-known of the buses, as until today, it can be seen on some motherboards. The difference with its predecessor is that it handles more bits (16), the number of contacts, and the slot size while maintaining its distinctive black color. Because it was chronologically the first, it is also the slowest of all.
New Generation of Bus
PCI: This is Intel’s 32-bit bus version, compatible with its new generation Pentium processor. The characteristic color is white. The connector was also designed to work in the future with 64 bits. This is the technology used today.
Buses in Disuse
VESA Local Bus: It was one of the first offers to work with 32 bits of data. The expansion slot connector used is a 56-pin brown-colored connector that is added below each type of ISA 16.
EISA: Another 32-bit option was a rather special expansion slot because the physical and electronic format is the same as the ISA and has an aggregate of contacts in the deepest part of the connector for 32-bit technology. Thus, the expansion slot maintains compatibility with 16-bit ISA.