On a typical microprocessor,a distinct I/O address is used to refer to the I/O d

Question

On a typical microprocessor,a distinct I/O address is used to refer to the I/O data registers and a distinct address for the control and status registers in an I/O controller for a given device. Such registers are referred to as ports. In the Intel 8088, two I/O instruction formats are used.In one format,the 8-bit opcode specifies an I/O operation; this is followed by an 8-bit port address. Other I/O opcodes imply that the port address is in the 16-bit DX register. How many ports can the 8088 address in each I/O addressing mode?

Explanation / Answer

In the former case, the processor can address 8 ports, one for each bit.
The ports are enabled by setting a bit corresponding to the port.

In the second case, which works much the same, 64 ports can be addressed.
The first byte on the bus selects one of eight groups of eight ports.
The second byte selects the port within the group.

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