8086 Processor Quiz: Instruction Execution & Cycles

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Questions and Answers

What is the function of the system clock signal in the CPU?

It drives the operation of the external devices

It synchronizes the execution of instructions (correct)

It provides the addressing mode

It generates the bus cycles

During which phase of instruction execution is the CPU in during T1?

Execution phase

Decode phase

Addressing phase

Fetch phase (correct)

What determines the number of clock cycles required to complete an instruction?

The number of external devices connected

The speed of the system clock

The type of bus cycle used

The complexity of the addressing mode and the complexity of the instruction (correct)

What is the term used to describe the clock cycles needed to execute an instruction?

Time States (T-States) Signup and view all the answers

What occurs during the first clock cycle of the execution of any instruction?

The CPU is in the fetch phase Signup and view all the answers

What happens after the instruction completes?

The CPU starts executing the next instruction from T1 Signup and view all the answers

What is the function of the instruction 'IN AL, 0FEH'?

To store data from I/O port 0FEH into the accumulator Signup and view all the answers

What is the purpose of the 'MOV DX, 0A00H' instruction?

To store the address of an I/O port into the DX register Signup and view all the answers

What is the duration of a bus cycle in a 8 MHz processor?

500 ns Signup and view all the answers

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

To wait for slow devices Signup and view all the answers

How long does the external device have to decode and locate the address?

One clock cycle Signup and view all the answers

How many clock cycles does the external device have to produce and maintain the data?

Two clock cycles Signup and view all the answers

What is the primary advantage of using isolated I/O instructions in the 8086 MPU?

No waste of memory address space. Signup and view all the answers

What is the maximum number of ports that can be addressed using direct I/O instructions?

256 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. Signup and view all the answers

What is the number of clock cycles required to transfer a word using I/O instructions?

2 Signup and view all the answers

What is the register used to store the 16-bit port number in variable I/O instructions?

DX Signup and view all the answers

What is the maximum size of the I/O address space in the 8086 MPU?

64KB Signup and view all the answers

Study Notes

Processors and Assembly Language

The System Clock signal is continuous and drives the operation of the CPU.
Instruction execution is synchronized to the system clock.
An instruction requires a number of clock cycles to complete, which differs depending on the complexity of the instruction and addressing mode.

Time States (T-States)

The number of clock cycles needed to execute an instruction is called Time States (T-States).
The processor does different things during the different T-States of the instruction.
The first clock cycle of the execution of any instruction is known as T1, which is the fetch phase.
Then, T2 is the decode phase, T3, and so on until the instruction completes.
The first cycle of the next instruction goes back to being T1.

Bus Cycle and Time States

A Bus Cycle is the basic operation that the processor performs to communicate with external devices.
There are four types of bus cycles:
- Memory Read
- Memory Write
- Input from external device (I/O Read)
- Output to external device (I/O Write)

I/O Instructions

There are two types of I/O instructions:
- Direct I/O: The port number is provided in the instruction (8-bit port number, 00H – FFH, 256 ports).
- Variable I/O: Uses a 16-bit port number that resides in DX (64KByte I/O address space, up to 65536 byte-sized ports).

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 × 125 ns = 500 ns.
External devices connected to the microprocessor have 1 clock cycle to decode and locate the location, and then produce the data and maintain it on the data bus for 2 clock cycles.

I/O Data Types

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.
Data is transferred using AD0 … AD7 if a byte is transferred, then one clock cycle is needed.
If a word is transferred, then two clock cycles are needed.

Idle and Wait States

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

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Description

Test your knowledge on 8086 microprocessor's instruction execution, clock cycles, and time states. Learn how the system clock signal drives the CPU's operation and how instructions are synchronized to it.