PC Motherboard
The main features of typical motherboards are shown in Figure 1.3. The heart of the system is the microprocessor, a single chip, or central processing unit (CPU). The CPU controls all the other system components, and must have access to a suitable program in memory before it can do anything useful. The blocks of program required at any one time are provided by both the operating system and the application software, which are downloaded to random access memory (RAM) from the hard disk as required. The programs consist of lists of machine code instructions (binary code) that are executed in sequence by the CPU.
The Intel CPU has undergone rapid and continuous development since the introduction of the PC in the early 1980s. Intel processors are classified as complex instruction set computer (CISC) chips, which means they have a relatively large number of instructions that can be used in a number of different ways. This makes them powerful, but relatively slow compared with processors that have fewer instructions; these are classified as reduced instruction set computer (RISC) chips, of which the PIC microcontroller is an example.
The CPU needs memory and input/output devices for getting data in, storing it and sending it out again. The main memory block is made up of RAM chips, which are generally mounted in Dual In-line Memory Modules (DIMMs). As far as possible, input/output (I/O) interfacing hardware is fitted on the motherboard (keyboard, mouse, USB, etc., preferably wireless), but additional peripheral interfacing boards may be fitted in the expansion card slots to connect the main board to extra disk drives and other specialist peripherals, traditionally using the PCI bus, a parallel data highway 32 bits wide.
All these parts are connected together via a pair of bus controller chips, which handle parallel data transfers between the CPU and the system. The ‘northbridge’ provides fast access to RAM and the graphics (screen) interface, while its partner, the ‘southbridge’, handles slower peripherals such as the disk drives, network and PCI bus. The motherboard itself can be represented as a block diagram (Figure 1.4) to show how the components are interconnected.
The block diagram shows that the CPU is connected to the peripheral interfaces by a set of bus lines. These are groups of connections on the motherboard, which work together to transfer the
data from the inputs, such as the keyboard, to the processor, and from the processor to memory. When the data has been processed and stored, it can be sent to an output peripheral, such as the screen.
Buses connect all the main chips in the system together, but, because they mainly operate as shared connections, can only pass data to or from one peripheral interface or memory location at a time. This arrangement is used because separate connections to all the main chips would need an impossible number of tracks on the motherboard. The disadvantage of bus connection is that it slows down the program execution speed, because all data transfers use the same set of lines, and only one data word can be present on the bus at any one time. To help compensate for this, the bus connections are as wide as possible. For example, a 64-bit bus, operating at 100 MHz (108 Hz), can transfer 6.4 gigabits (6.4 x 109 bits) per second. The current generation of Intel® CPUs also use multiple (typically 4) 64-bit cores in one chip to improve performance.