I/O Mapping in 8085 Microprocessor

I/O interfacing

There are two methods of interfacing the Input / Output devices with the microprocessor. They are,

1) Memory mapped I/O and

2) I/O mapped I/O.

Memory mapped I/O

In this method the I/O devices are treated like the memory. A part of the memory address space is used for the I/O devices. The memory mapped I/O scheme is shown in figure.

Figure: Memory mapped I/O scheme

• In memory mapped I/O scheme, the same address space is used for both memory and I/O devices.

• The microprocessor uses the sixteen address line A₀ – A₇ and A₈ – A₁₅ for the memory as well as for the I/O devices.

• The I/O devices share the address space with the memory. All the memory related instructions are used for addressing I/O devices also.

• No separate IN and OUT instructions are required in memory mapped I/O scheme.

• IO/𝑀’ pin is not required.

Steps for memory operations (memory read and memory write) :

1. When the memory related instructions like LDA and STA are used, the microprocessor places the 16-bit address on the address bus.

2. 𝑅𝐷’ is activated for read operation and 𝑊𝑅’ is activated for write operation.

Steps for I/O operations (I/O read and I/O write) :

The same steps used for memory operations are used for I/O operations also.

I/O mapped I/O

In this method, I/O devices are treated as I/O devices and memory is treated as memory. Separate address space is used for memory and I/O. The I/O mapped I/O scheme is shown in figure.

Figure: I/O mapped I/O scheme

• In I/O mapped I/O scheme, the microprocessor uses the sixteen address lines A₀ – A₇ and A₈ – A₁₅ for the memory and eight address lines A₀ to A₇ to identify an input / output device.

• Here, the full address space 0000 – FFFF is used for the memory and a separate address space 00 – FF is used for the I/O devices.

• Hence, the microprocessor can address 65536 (2¹⁶) memory locations 256 (2⁸) input devices and 256 (2⁸) output devices separately.

• IN and OUT instructions are used to activate the IO/𝑀’ signal.

• When IO/𝑀’ is low, the memory is selected for reading and writing operations.

• When IO/𝑀’ is high, the I/O port is selected for reading and writing operations.

Steps for memory operations (memory read and memory write) :

1. When the memory related instructions like LDA and STA are used, the microprocessor places the 16-bit address on the address bus.

2. The microprocessor makes the IO/𝑀’ linelow.

3. The microprocessor makes the 𝑅𝐷’ low for read operation and 𝑊𝑅’ low for write operation.

Steps for I/O operations (I/O read and I/O write) :

1. When the I/O related instructions like IN and OUT are used, the microprocessor places the 8-bit address on the address bus A₀ – A₇ as well as A₈ – A₁₅.

2. IO/𝑀’ line is made high.

3. The microprocessor makes the 𝑅𝐷’ low for read operation and 𝑊𝑅’ low for write operation.