Question

A microprocessor scans the status of an output I/O device every 20 ms. This is accomplished by means of a timer alerting the processor every 20 ms. The
interface of the device includes two ports: one for status and one for data output.
How long does it take to scan and service the device, given a clocking rate of 8
MHz? Assume for simplicity that all pertinent instruction cycles take 12 clock
cycles. 8.8 In Section 8.3 , one advantage and one disadvantage of memory-
mapped I/O, compared with isolated I/O, were listed. List two more advantages and two more disadvantages.

Answer 1

Answer:

A microprocessor scans the status of an output I/O device every 20 ms.This is accomplished by means of a timer alerting the processor every 20 ms.The interface of the device includes two ports: one for status and one for data output. How long does it take to scan and service the device given a clocking rate of 8 MHz? Assume for simplicity that all pertinent instruction cycles take 12 clock cycles.

Answer 2

Answer:

This would take 12 clock cycles or 12/8 MHz = 1.5 ms to scan and service the device.

Explanation:

The time it takes to scan and service a device depends on how many clock cycles it takes for the CPU to read the device state. Each instruction cycle of the processor consists of 12 clock cycles, so it would take 12 clock cycles  to read the state of the device. Thus, scanning and servicing the device would take 12 clock cycles,

or 12/8 MHz = 1.5 ms.

There are many pros and cons of memory-mapped I/O.

The advantages of memory-mapped I/O are high performance and better compatibility with other devices. It is also easier to implement because it does not require special I/O instructions. Disadvantages include increased complexity, due to the need to separate memory and I/O addresses and also increased competition for memory use.

The debugging memory-mapped I/O devices can be quite difficult because of the memory-mapped I/O being not visible to the debugger.

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