The I/O port is a register located within the interface circuit that can be directly accessed by the CPU. This access port is essentially a register found in the interface circuit used for communication. An interface circuit, often associated with peripherals, typically includes multiple ports—such as a command port, a status port, and a data port. These ports allow the CPU to interact with external devices.
There are two main methods of port address coding: unified addressing and independent addressing. The more commonly used approach is independent addressing, where the peripheral's port addresses are mapped separately from memory addresses. This allows the CPU to distinguish between memory and I/O operations. Special I/O instructions, such as IN and OUT, are used for these operations, differing from standard memory move instructions like MOV.
Port address decoding refers to the process of identifying which specific port or device is being addressed based on the given address. For example, consider a design where the port address is set to 218H. When the CPU sends the address 218H to the address bus, it should trigger a valid low-level signal, while other addresses generate an invalid high-level signal to prevent interference.
The decoding circuit works as follows:
[Image: http://i.bosscdn.com/blog/09/44/16/3W2-0.png]
The gate circuit is implemented in the following way:
[Image: http://i.bosscdn.com/blog/09/44/16/2457-1.jpg]
When the address signal corresponds to 218H, the chip select (CS) signal becomes active (low). If the address is different, the CS signal remains inactive (high), ensuring that only the correct peripheral is selected. This mechanism is essential for reliable communication between the CPU and peripheral devices.
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