InputOutput Addresses (IOs)

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Input/Output Addresses (I/Os)

In order to be recognized by an operating system and programs, input and output devices attached to a computer system require unique identifications in memory. These unique identifications are called memory addresses. They are also referred to as I/O addresses and I/O port addresses.

It is likely that you will encounter memory address questions on the exam. In a question such as 'Which address does a particular device use?', the address in question is the memory address. Table 5.5 identifies memory address assignments. (Memorize this table.)

Table 5.5: Base Memory Address Assignments

Memory Address	Device
00F0	Math coprocessor
060h	Keyboard controller
170h	Secondary IDE hard drive controller
1F0h	Primary IDE hard drive controller
220h	Sound card
300h	NIC
330h	SCSI adapter
3F2h	Floppy drive controller
3F8h	COM1
2F8h	COM2
3E8h	COM3
2E8h	COM4
378h	LPT1
278h	LPT2
C000-C7FFF	Video adapter (memory address)

Devices attached to the motherboard perform I/O operations by accepting instructions sent to them by operating systems and programs. The instruction sets are first sent to preassigned unique memory addresses or memory address ranges that represent each attached device. Devices require different amounts of memory for processing. The size of memory allocated to a device's memory address can vary. The average device in a system uses 4, 8, or 32 bytes of memory address space. A video card, which requires significant amounts of memory to process output, may need more memory space assigned to its I/O address than a mouse would. The bus architecture of a device is the determining factor for the amount of memory space allocated to it.

Many memory addresses are available in a typical computer system. However, an I/O memory address conflict may occur if a device attempts to use the same memory address space already assigned to another device. It is possible to change a device's I/O address by a process called memory-mapped I/O. Once a device is mapped to a legitimate I/O address, the CPU can recognize it.

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