MPU communicates with the outside world with the help of some external devices which are known as Input/Output devices. The MPU accepts the binary data from input devices such as keyboard and analog/digital converters and sends data to output devices such as printers and LEDs. For performing this task, MPU first need to identify the input/output devices.
There are two different methods by which I/O devices can be identified: Using 8-bit address, and Using 16-bit address. These methods are described briefly in the following sections:
- I/Os with 8-bit addresses –
This is also known as peripheral-mapped I/O or I/O-mapped-I/O. In this type of I/O, MPU uses eight address lines to identify an input or an output devices. This is an 8-bit numbering system for I/Os used in conjunction with the input and output instructions. This is also known as I/O space that is separate from the memory space which is 16-bit numbering system. The eight address lines have 2^8 combinations which is total 256 addresses; therefore MPU can identify 256 input devices and 256 output devices with addresses ranging from 00H to FFH.The input and output devices can be differentiated by using the control lines I/O Read and I/O Write. MPU uses I/O Read control signal for input devices and I/O Write control signal for output devices. The individual addresses of I/O Map is known as I/O port numbers. These I/O devices cannot be connected directly to the data bus or the address bus; all connections must be made through tri-state interfacing devices so they will enabled and connected to the buses only when the MPU chooses to communicate with them.
- I/Os with 16-bit addresses –
This is also known as Memory-mapped I/O. In this type of I/O, MPU uses sixteen address lines to identify an input or an output devices; an I/O is connected as if it is a memory register. The MPU uses same control signal (Memory Read and Memory Write) and instructions as those of memory. In some microprocessors, such as the Motorola 6800, all I/Os have 16-bit addresses; I/Os and memory share the same memory map (64K). The steps in communicating with an I/O device are similar for both 8-bit and 16-bit addresses. The steps are summarized below:- The MPU places an 8-bit address ( or 16-bit address) on the address bus, which is decoded by external decode logic.
- The MPU sends a control signal (I/O Read or I/O Write) and enables the I/O device.
- Data are transferred using the data bus.
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