Types of Input/Output

• The I/O system allows peripherals to provide data or receive results of processing the data
• Using I/O ports
• The 8088/8086 MPU can employ two types of I/O
• Isolated I/O
• Memory-mapped I/O
• They differ in how I/O ports are mapped into the address space

Types of Input/Output (cont.)

• Isolated input/output
• When using isolated I/O in a microcomputer system, the I/O device are treated separate from memory
• The memory address space contains 1 M consecutive byte address in the range 00000₁₆ through FFFFF₁₆
• The I/O address space contains 64K consecutive byte addresses in the range 0000₁₆ through FFFF₁₆
• The bytes in two consecutive I/O addresses can be accessed as word-wide data
• Page 0: 0000₁₆ ~ 00FF₁₆
• Certain I/O instructions can only perform in this range

Types of Input/Output (cont.)

• Isolated input/output

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8088/8086 memory and I/O address spaces

Types of Input/Output (cont.)

• Isolated input/output

Isolated I/O ports

Types of Input/Output (cont.)

• Isolated input/output
• Advantages:
• The complete 1Mbyte memory address space is available for use with memory
• Special instructions have been provided to perform I/O operations with maximized performance
• The bytes in two consecutive I/O addresses can be accessed as word-wide data
• Disadvantages:
• All input and output data transfers must take place between the AL or AX register and I/O port

Types of Input/Output (cont.)

• Memory-mapped input/output
• MPU looks at the I/O port as though it is a storage location in memory
• Some of the memory address space is dedicated to I/O ports
• Instructions that affect data in memory are used instead of the special I/O instructions
• More instructions and addressing modes are available to perform I/O operations
• I/O transfers can take place between I/O port and other internal registers
• The memory instructions tend to execute slower than those specifically designed for isolated I/O
• Part of the memory address space is lost

Types of Input/Output(cont.)

• Memory-mapped input/output

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Memory mapped I/O ports

Isolated Input/Output Interface

• I/O devices:
• Keyboard
• Printer
• Mouse
• 82C55A, etc
• Functions of interface circuit:
• Select the I/O port
• Latch output data
• Sample input data
• Synchronize data transfer
• Translate between TTL voltage levels and those required to operate the I/O devices

Isolated Input/Output Interface (cont.)

• Minimum-mode interface

Minimum-mode 8088 system I/O interface

Isolated Input/Output Interface (cont.)

• Minimum-mode interface

Minimum-mode 8086 system I/O interface

Isolated Input/Output Interface (cont.)

• Maximum-mode interface

Maximum-mode 8088 system I/O interface

Isolated Input/Output Interface (cont.)

• Maximum-mode interface

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Maximum-mode 8086 system I/O interface

Isolated Input/Output Interface (cont.)

• Maximum-mode interface

I/O bus cycle status codes

Input/Output Data Transfer

• Input/output data transfers in the 8088 and 8086 microcomputers can be either byte-wide or word-wide
• I/O addresses are 16 bits in length and are output by the 8088 to the I/O interface over bus lines AD0 through AD7 and A8 through A15
• In 8088, the word transfers is performed as two consecutive byte-wide data transfer and takes two bus cycle
• In 8086, the word transfers can takes either one or two bus cycle
• Word-wide I/O ports should be aligned at even-address boundaries

Input/Output Data Instructions

Input/Output Data Instructions�(cont.)

EXAMPLE:

• Write a sequence of instructions that will output the data FF₁₆ to a byte-wide output port at address AB₁₆ of the I/O address space.

Solution:

• First, the AL register is loaded with FF16 as an immediate operand in the instruction

MOV AL, 0FFH

• Now the data in AL can be output to the byte-wide output port with the instruction

OUT 0ABH, AL

Input/Output Data Instructions�(cont.)

EXAMPLE:

• Write a series of instructions that will output FF₁₆ to an output port located at address B000₁₆ of the I/O address space

Solution:

• The DX register must first be loaded with the address of the output port. This is done with the instruction

MOV DX, 0B000H

• Next, the data that are to be output must be loaded into AL with the instruction

MOV AL, 0FFH

• Finally, the data are output with the instruction

OUT DX, AL

Input/Output Data Instructions�(cont.)

EXAMPLE:

• Data are to be read in from two byte-wide input ports at addresses AA₁₆ and A9₁₆ and then output as a word-wide output port at address B000₁₆. Write a sequence of instructions to perform this input/output operation.

Input/Output Data Instructions�(cont.)

Solution:

• First read in the byte at address AA₁₆ into AL and move it into AH.

IN AL, 0AAH

MOV AH, AL

• Now the other byte can be read into AL by the instruction

IN AL, 09AH

• And to write out the word of data

MOV DX, 0B000H

OUT DX, AX

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Input/Output Bus Cycles

• Input bus cycle of the 8088

Input/Output Bus Cycles (cont.)

• Output bus cycle of the 8088

Input/Output Bus Cycles (cont.)

• Input bus cycle of the 8086

Input/Output Bus Cycles (cont.)

• Output bus cycle of the 8086