8086 Software Aspects - Question and Answer

1. What do you mean by non maskable interrupt?

Non-maskable interrupt (NMI) is the highest priority hardware interrupt that triggers on the positive edge. This interrupt cannot be disabled or masked. It is used to save program data or processor status in case of system power failure.

2. List out the segment registers of 8086?

Code segment register
Data segment register
Stack segment register
Extra segment register

3. What is the function of bus interface unit?

The bus interface unit (BIU) interfaces the 8086 with external devices including memory via the bus. BIU fetches instructions, reads data from memory and ports, and writes data to memory and I/O ports.

4. What are maximum address space and data bus width of 8086 processor?

Maximum address apace = 2²⁰ = 1 MB
Data bus width = 16 bit

5. What is the storage space required to store the interrupt vectors of 8086?

In 8086,256 interrupt types required (256 x 4) 1024 bytes storage space.

6. What do these 8086 instructions do? – STD, IRET.

STD – Set the direction flag in flag register (D=1)
IRET – Interrupt on Return. It is used to exist any interrupt procedure, whether activated by software or hardware.

7. What is Macro?

Macro is a short sequence of instructions. It is a small subroutine. A macro can be defined anywhere in a program using the directives MACRO and ENDM.

8. What is the use of HOLD and HLDA signals?

HOLD – This signal indicates that another master is requesting the host 8086 to handover the system bus.
HLDA – (Hold acknowledge) – On receiving HOLD signal, 8086 outputs HLDA signal high as an acknowledgement.

9. What is an assembler directive?

An assembler directive is a statement to give direction to the assembler to perform the task of assembly process.

10. What is meant by software interrupt in 8086 ?

The software interrupts are program instructions. These instructions are inserted at desired locations in a program. While running a program, if a software interrupt is encountered then the processor executes an interrupt service routine (ISR).

11. What is the purpose of CLK signal in an 8086 system?

The clock signal provides the basic timing for processor operation and bus control activity. The clock frequency may be 5MHz or 8MHz of 10MHz for different 8086 versions.

12. What are the different between macros and procedures?

Procedures	Macros
To use a procedure use CALL and RET instructions are needed	To use a macro, just type its name.
It occupies less memory.	It occupies more memory.
Stack is used.	Stack is not used.
To mark the end of the procedure, type the name of the procedure before the ENDP directive.	To mark the end of the macro ENDM directive is enough.
Overhead time is required to call the procedure and return to the calling program.	No overhead time during the execution

13. What is the function of T and D flags in 8086?

D Flag-String Direction Flag : It is used to set direction in string operation.
T Flag – Single Step Trap Flag : It is used for single stepping through a program.

14. What is the operation carried out when 8086 executes the instruction MOVSW?

MOVSW – Move word string.
This instruction transfers a word from the source string (addressed by SI) to the destination string (addressed by DI) and updates SI and DI to point to the next string element.

15. What is the role of TF and IF flags in the flag register of 8086?

TF (Trap Flag) – Setting TF puts the 8086 in the single step mode. In this mode, the 8086 generates an internal interrupt after execution of each instruction.
IF (Interrupt Flag) – Setting IF causes the 8086 to receive external maskable interrupts through INTR pin. Clearing IF disables these interrupts.

16. What is pipelining?

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

17. What are the difference between 8085 and 8086?

8085	8086
8 Bit microprocessor	16 bit processor
2¹⁶ memory locations	2²⁰ memory locations
Sequential facility	Pipelined architecture available
Low speed	High speed