8086 MPU I/O Interfacing

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Questions and Answers

What is the advantage of using I/O instructions over memory read/write instructions?

They are slower

They are faster (correct)

They can only transfer between Accumulator and memory

They use more memory address space

What is the width of I/O port numbers in the 8086 MPU?

64-bits

8-bits

32-bits

16-bits (correct)

How many clock cycles are needed to transfer a word to an I/O port?

Two clock cycles (correct)

Eight clock cycles

One clock cycle

Four clock cycles

What is the range of port numbers that can be used in Direct I/O?

00H to FFH Signup and view all the answers

What is the purpose of the IO/M signal in the 8086 MPU?

To indicate that the processor is accessing I/O, not memory Signup and view all the answers

What is the maximum number of byte-sized I/O ports that can be addressed in the 8086 MPU?

65536 Signup and view all the answers

What is the purpose of the instruction IN AL, 0FEH?

To transfer data from I/O port 0FEH to AL register Signup and view all the answers

What type of I/O is used in the instruction MOV DX, 0A00H; IN AL, DX?

Indirect I/O Signup and view all the answers

What is the length of a normal bus cycle when the CPU is operating at 8 MHz?

500 ns Signup and view all the answers

What is the purpose of the wait state in a bus cycle?

To wait for very slow devices Signup and view all the answers

How many clock cycles does an external device have to produce the data and maintain it on the data bus?

2 clock cycles Signup and view all the answers

What is the purpose of the idle state in a bus cycle?

No bus cycles are required Signup and view all the answers

What is the size of the I/O address space?

64Kbytes Signup and view all the answers

How many bits can a port address have?

8 bits or 16 bits Signup and view all the answers

What is the purpose of isolated I/O?

To use special I/O instructions for I/O Signup and view all the answers

What is memory-mapped I/O?

Using memory addresses for I/O Signup and view all the answers

What is the range of addresses assigned for I/O devices in memory-mapped I/O?

E0000H through E0FFFH Signup and view all the answers

How are I/O devices treated in memory-mapped I/O?

As if they are memory locations Signup and view all the answers

Study Notes

I/O Interfacing

I/O data transfer in the 8088/8086 MPU can be byte-wide or word-wide.
I/O port numbers are 16-bits in length.
Output over AD0 … AD15, A16, A19 will hold Zero.
Data is transferred using AD0 … AD7; if a byte is transferred, one clock cycle is needed; if a word is transferred, two clock cycles are needed.
The processor sets IO/M to 0 to indicate that it is accessing I/O not memory.

I/O Instructions

Two types of I/O instructions: Direct I/O and Variable I/O.
Direct I/O: The port number is provided in the instruction; port numbers are 8-bits only (00H to FFH); only 256 ports are available.
Variable I/O: Uses 16-bit port number that resides in DX; DX is not an offset, but the actual port number; 64KByte I/O address space is available; up to 65536 byte-sized ports.

Examples

Example of Direct I/O: IN AL, 0FEH (data is transferred and takes one cycle).
Example of Variable I/O: MOV DX, 0A00H; IN AL, DX (address of I/O).

Bus Cycle Timing

Each bus cycle is 4 clock cycles (4 T-States).
If the CPU is operating at 8 MHz, then each clock cycle is 125 ns long; length of a normal bus cycle is 4 x 125 ns = 500 ns.
External device connected to the microprocessor can expect the following timing requirements for a READ cycle:
- CPU produces the address on the address bus and maintains it for 1 clock cycle.
- External device has 1 clock cycle to decode and locate the location.
- External device produces the data and maintains it on the data bus for 2 clock cycles.

Bus Cycle States

Idle state: No bus cycles required; the bus remains idle.
Wait state: States inserted between T3 and T4 of the current bus cycle; used to wait for very slow devices.

I/O Address Space

The I/O address space is 64Kbytes.
Ports can have an address of 8 bits or 16 bits.

Memory-Mapped I/O

Place I/O devices within the memory address space of the processor.
Some of the memory space is dedicated to I/Os.
Reading from and writing to memory-mapped I/O devices uses the same exact instructions as reading from and writing to memory locations.