The main circuit board in a PC is called the motherboard (IBM terminology calls it a planar).
This mainly carries the CPU and RAM, and has a bus with slots for expansion cards.
Also on the motherboard are the ROM subsystem, DMA and IRQ controllers, coprocessor socket, sound (PC speaker, tone generation) circuitry, and keyboard interface.
The original PC also has a cassette interface.

The bus used in the original PC became very popular, and it was later named Industry Standard Architecture (ISA).
It was known originally as the PC-bus or XT-bus; the term ISA was used later when industry leaders chose to continue manufacturing machines based on the IBM PC AT architecture rather than license the PS/2 architecture and its Micro Channel bus from IBM.
The XT-bus was then retroactively named 8-bit ISA or XT ISA, while the unqualified term ISA usually refers to the 16-bit AT-bus (as better defined in the ISA specifications).
The AT-bus is an extension of the PC-/XT-bus and is in use to this day in computers for industrial use, where its relatively low speed, 5-volt signals, and relatively simple, straightforward design (all by year 2011 standards) give it technical advantages (e.g. noise immunity for reliability).

A monitor and any floppy or hard disk drives are connected to the motherboard through cables connected to graphics adapter and disk controller cards, respectively, installed in expansion slots.
Each expansion slot on the motherboard has a corresponding opening in the back of the computer case through which the card can expose connectors; a blank metal cover plate covers this case opening (to prevent dust and debris intrusion and control airflow) when no expansion card is installed.
Memory expansion beyond the amount installable on the motherboard was also done with boards installed in expansion slots, and I/O devices such as parallel, serial, or network ports were likewise installed as individual expansion boards.
For this reason, it was easy to fill the five expansion slots of the PC, or even the eight slots of the XT, even without installing any special hardware.
Companies like Quadram and AST addressed this with their popular multi-I/O cards which combine several peripherals on one adapter card that uses only one slot; Quadram offered the QuadBoard and AST the SixPak.

Intel 8086 and 8088-based PCs require expanded memory (EMS) boards to work with more than 640 kB of memory. (
Though the 8088 can address one megabyte of memory, the last 384 kB of that is used or reserved for the BIOS ROM, BASIC ROM, extension ROMs installed on adapter cards, and memory address space used by devices including display adapter RAM and even the 64 kB EMS page frame itself.)
The original IBM PC AT used an Intel 80286 processor which can access up to 16 MB of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs).
Intel 80286-based computers running under OS/2 can work with the maximum memory.
The main circuit board in a PC is called the motherboard (IBM terminology calls it a planar). This mainly carries the CPU and RAM, and has a bus with slots for expansion cards. Also on the motherboard are the ROM subsystem, DMA and IRQ controllers, coprocessor socket, sound (PC speaker, tone generation) circuitry, and keyboard interface. The original PC also has a cassette interface.
The bus used in the original PC became very popular, and it was later named Industry Standard Architecture (ISA). It was known originally as the PC-bus or XT-bus; the term ISA was used later when industry leaders chose to continue manufacturing machines based on the IBM PC AT architecture rather than license the PS/2 architecture and its Micro Channel bus from IBM. The XT-bus was then retroactively named 8-bit ISA or XT ISA, while the unqualified term ISA usually refers to the 16-bit AT-bus (as better defined in the ISA specifications). The AT-bus is an extension of the PC-/XT-bus and is in use to this day in computers for industrial use, where its relatively low speed, 5-volt signals, and relatively simple, straightforward design (all by year 2011 standards) give it technical advantages (e.g. noise immunity for reliability).
A monitor and any floppy or hard disk drives are connected to the motherboard through cables connected to graphics adapter and disk controller cards, respectively, installed in expansion slots. Each expansion slot on the motherboard has a corresponding opening in the back of the computer case through which the card can expose connectors; a blank metal cover plate covers this case opening (to prevent dust and debris intrusion and control airflow) when no expansion card is installed. Memory expansion beyond the amount installable on the motherboard was also done with boards installed in expansion slots, and I/O devices such as parallel, serial, or network ports were likewise installed as individual expansion boards. For this reason, it was easy to fill the five expansion slots of the PC, or even the eight slots of the XT, even without installing any special hardware. Companies like Quadram and AST addressed this with their popular multi-I/O cards which combine several peripherals on one adapter card that uses only one slot; Quadram offered the QuadBoard and AST the SixPak.
Intel 8086 and 8088-based PCs require expanded memory (EMS) boards to work with more than 640 kB of memory. (Though the 8088 can address one megabyte of memory, the last 384 kB of that is used or reserved for the BIOS ROM, BASIC ROM, extension ROMs installed on adapter cards, and memory address space used by devices including display adapter RAM and even the 64 kB EMS page frame itself.) The original IBM PC AT used an Intel 80286 processor which can access up to 16 MB of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs). Intel 80286-based computers running under OS/2 can work with the maximum memory.
IBM Personal Computer