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Chapter: Microprocessor and Microcontroller : 8086 System Bus Structure

Important Short Questions and Answers: Microprocessor and Microcontroller - 8086 System Bus Structure

Microprocessor and Microcontroller - 8086 System Bus Structure - Important Short Questions and Answers: Microprocessor and Microcontroller - 8086 System Bus Structure


1. Differentiate between minimum and maximum mode


2. Give any four pin definitions for the minimum mode.


3.  What are the pins that are used to indicate the type of transfer in minimum mode?

The M/IO, RD, WR lines specify the type of transfer. It is indicated in the following table:


4. What are the functional parts of 8086 CPU?


The two independent functional parts of the 8086 CPU are: i. Bus Interface Unit (BIU):


BIU sends out addresses, fetches instruction from memory, reads data from ports and memory and writes data to ports and memory.


ii. Execution Unit (EU):


EU tells the BIU where to fetch instructions or data, decodes instructions and executes instructions.


5. What is the operation of S0, S1 and S2 pins in maximum mode?

S2, S1, S0 indicates the type of transfer to take place during the current bus cycle.


6. Give any four pin definitions for maximum mode.


7. Draw the bus request and bus grant timings in minimum mode system.

8. What is the purpose of a decoder in EU?


The decoder in EU translates instructions fetched from memory into a series of actions, which the EU carries out.


9. Give the register classification of 8086. The 8086 contains:


i. General purpose registers: They are used for holding data, variables and intermediate results temporarily.


ii. Special purpose registers: They are used as segment registers, pointers, index register or as offset storage registers for particular addressing modes.


10.            What are general data registers?

The registers AX,BX,CX and DX are the general data registers.


L and H represents the lower and higher bytes of particular register. AX register is used as 16-bit accumulator.


BX register is used as offset storage for forming physical addresses in case of certain addressing modes.


CX register is used as a default counter in case of string and loop instructions.


DX register is used as an implicit operand or destination in case of a few instructions.



11.            Give the different segment registers. The four segment registers are:


i. Code segment register: It is used for addressing a memory location in the code segment of the memory, where the executable program is stored.


ii. Data segment register: It points to the data segment of the memory, where data is resided. iii. Extra segment register: It also contains data.


iv. Stack segment register: It is used for addressing stock segment of memory. It is used to store stack data.


12.            What are pointers and index registers?


IP, BP and SP are the pointers and contain offsets within the code, data and stack segments respectively. SI and DI are the index registers, which are used as general purpose registers

and also for offset storage in case of indexed, based indexed and relative based indexed addressing modes.



13.How is the physical address calculated? Give an example.


The physical address, which is 20-bits long is calculated using the segment and offset registers, each 16-bits long. The segment address is shifted left bit-wise four times and offset address is added to this to produce a 20 bit physical address.


14. What is meant by memory segmentation?


Memory segmentation is the process of completely dividing the physically available memory into a number of logical segments. Each segment is 64K byte in size and is addressed by one of the segment register.


15. What are the advantages of segmented memory? The advantages of segmented memory are:


i.  Allows the memory capacity to be 1Mbyte, although the actual addresses to be handled are of 16-bit size.


ii. Allows the placing of code, data and stack portions of the same program in different parts of memory for data and code protection.


iii. Permits a program and/or its data to be put into different areas of memory, each times program is executed i.e., provision for relocation may be done.


16. What is pipelining?

Fetching the next instruction while the current instruction executes is called pipelining


17. What are the two parts of a flag register? The two parts of the 16 bit flag register are: i. Condition code or status flag register:


It consists of six flags to indicate some condition produced by an instruction. ii. Machine control flag register:


It consists of three flags and are used to control certain operations of the processor


18.Draw the format of 8086 flag register. 8086 flag register:

19. Explain the three machine control flags. i. Trap flag:


If this flag is set, the processor enters the single step execution.

ii. Interrupt flag: If this flag is set, the markable interrupts are recognized by the CPU,

otherwise they are ignored.


iii. Direction flag: This is used by string manipulation instructions. If this flag bit is „0‟,the string is processed from the lowest to the highest address i.e., auto incrementing mode. Otherwise, the string is processed from highest address to lowest address, i.e., auto decrementing mode.



20. What are the three groups of signals in 8086? The 8086 signals are categorized in three groups. They are:


i. The signals having common functions in minimum and maximum mode. ii. The signals having special functions for minimum mode.


iii. The signals having special functions for maximum mode.



21. What are the uses of AD15 – AD0 lines?

AD15 – AD0 are time multiplexed memory I/O address and data lines. Address remains on the lines during T1 state, while data is available on data bus during T2, T3, Tw and T4 states. These lines are active high and float to a tristate during interrupt acknowledge and local bus hold acknowledge cycles.


22. What is the operation of RD signal?


RD is an active low signal. When it is low, it indicates the peripherals that the processor is performing a memory or I/O read operation.


23.Give the function of i. Ready and ii. INTR signal.


i.  Ready signal: It is an acknowledgement from slow devices of memory that they have completed data transfer. The signal is synchronized by 8284 A clock generator to give ready input to 8086. The signal is active high.


INTR signal: It is a level triggered input. This is sampled during the last cycle of each instruction to determine the availability of the request. If any interrupt request is pending, the processor enters the interrupt acknowledge cycle. This can be internally masked by resetting the interrupt enable flag. The signal is active high and internally synchronized.


24. What is the operation performed when TEST input is low?


When the TEST input is low, execution will continue, else, the processor remains in an idle state.


25. What is NMI (Non-Maskable Interrupt)?


NMI is an edge-triggered input, which causes a type 2 interrupt. It is not maskable internally by software and transition from low to high initiate the interrupt response at the end of the current instruction. This input is internally synchronized.


22. What is the purpose of clock input?


The clock input provides the basic timing for processor operation and bus control activity. It is an asymmetric square wave with 33% duty cycle. The range of frequency varies from 5MHz to 10MHz.


23. What is the function of MN/MX pin?


The logic level at MN/MX pin decides whether processor operates in minimum or maximum mode.


24. What happens when a high is applied to RESET pin?


When a high is given to RESET pin, the processor terminates the current activity and starts executing from FFFF0H. It must be active for at least four clock cycles. It is internally synchronized.


25.            What will happen when a DMA request is made, while the CPU is performing a memory or I/O cycles?


When a DMA request is made, while the CPU is performing a memory or I/O cycles, it will request the local bus during T4 provided:


i. The request occurs on or before T2 state of the current cycle.


ii. The current cycle is not operating over the lower byte of a word.


iii. The current cycle is not the first acknowledge of an interrupt acknowledge sequence. iv. A lock instruction is not being executed.


26.            What is multiprogramming?


If more than one process is carried out at the same time, then it is know as multiprogramming. Another definition is the interleaving of CPU and I/O operations among several programs is called multiprogramming. To improve the utilization of CPU and I/O devices, we are designing to process a set of independent programs concurrently by a single CPU. This technique is known as multiprogramming


27.            Write the advantages of loosely coupled system over tightly coupled systems?


1. More number of CPUs can be added in a loosely coupled system to improve the system performance

2. The system structure is modular and hence easy to maintain and troubleshoot.

3. A fault in a single module does not lead to a complete system breakdown.


28.            What is the different clock frequencies used in 80286?


Various versions of 80286 are available that run on 12.5MHz, 10MHz and 8MHz clock frequencies.


29. Define swapping in?


The portion of a program is required for execution by the CPU, it is fetched from the secondary memory and placed in the physical memory. This is called ‘swapping in’ of the program.


30 What are the different operating modes used in 80286? The 80286 works in two operating modes

1. Real addressing mode

2. Protected virtual address mode.


31. What are the CPU contents used in 80286?

The 80286 CPU contains almost the same set of registers, as in 8086

• Eight 16-bit general purpose register

• Four 16-bit segment registers

• Status and control register

•  Instruction pointer.


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