From the programmer’s point view, all members of PC family consist of a processor, memory chips, and several smart, or programmable, circuit chips. All the main circuit components that make the computer work are located on the system board; other important parts are located on the expansion boards; which can be plugged into the system board.
The system board consists the microprocessor, which is tied to at least 64 KB of memory, some built-in ROM programs, such as BASIC and the ROM BIOS; and several very important support chips. Some of these chips control external devices, such as the disk drive or the display screen, and others help the microprocessor perform the tasks.
In the section, we discuss the major chip and give a few important technical specifications. These chips are frequently known be more than one name. For example, some peripheral input/output hardware is supervised by a chip known as the 8255. This chip is also referred to as the

8255A and the 8255A-5. The suffixes A and 5 refer to revision numbers and to parts rated for operation at different speeds. For programming purposes, any Intel chip part number starts with 8255 is identical to any other chip whose part number starts with 8255, regardless of the suffix. However, when you replace one of these chips on the circuit board, note the suffix. If the suffixes are different, the part may not operate at the proper speed.

The Microprocessor

In all PCs, the microprocessor is the chips that runs programs. The microprocessor, or central processing unit (CPU), carries out a variety of computations, numeric comparisons, and data transfers in response to programs stored in memory.
The CPU controls the computer’s basic operation by sending and receiving control signals, memory addresses, and data from one part of the computer to another along a group at interconnecting electronic pathways called a bus. Located along the bus are input and output (I/O) ports that connect the various memory and support chips to the bus. Data passes through these I/O ports while it travels to and from the CPU and the parts of the computer.
We’ll point out the similarities and differences between the different microprocessors as we describe them.

The 8088 is the 16-bit microprocessor that controls the standard IBM personal computers, including the original PC, the PC/XT, the Portable PC, and the PCjr. Almost every bit of data that enters or leaves the computer passes through the CPU to be processed.
Inside the 8088, 14 registers provide a working area for data transfer and processing. These internal registers, forming an are 28 bytes in size, are able to temporality store data, memory addresses, instruction pointers, and status and control flags. Through these registers, the 8088 can access 1MB, or more than one million bytes, of memory.
The 8086 is used in the PS/2 models 25 and 30 (and also in many IBM PC clones). The 8086 differs from the 8088 in only the minor respects: it uses a full 16-bit data instead of the 8-bit bus that the 8088 uses. Virtually anything that you read about the 8086 also applies to the 8088, for programming purposes, consider them identical.
The 80286 is used in the PC/AT and in the PS/2 models 50 and 60. Although fully compatible with the 8086, the 80286 supports extra programming features that let it execute programs much more quickly than the 8086. Perhaps the most important enhancement to the 80286 is its support for multitasking.