Microprocessors and Microcontrollers - short answer type questions from AMIE exams (Winter 2020)

Answer all questions (2 x 10)

What are the uses of PUSH and POP operations in 8085 microprocessors?

  • In a stack, all operations take place at the "top" of the stack.
  • The "push" operation adds an item to the top of the stack.
  • The "pop" operation removes the item on the top of the stack and returns it.

How many I/O ports are there in 8051? Which port is bit addressable?

  • 8051 microcontrollers have 4 I/O ports each of 8-bit, which can be configured as input or output. Hence, total 32 input/output pins allow the microcontroller to be connected with the peripheral devices.
  • All ports of 8051 are bit addressable

List the interrupts in the order of priority for 8051 MCU.

  • Interrupts are the events that temporarily suspend the main program, pass the control to the external sources and execute their task. It then passes the control to the main program where it had left off.
  • 8051 has 5 interrupt signals, i.e. INT0, TFO, INT1, TF1, RI/TI. Each interrupt can be enabled or disabled by setting bits of the IE register and the whole interrupt system can be disabled by clearing the EA bit of the same register.

List the interrupts used in the 8085 microprocessor.

Hardware and Software Interrupts
When microprocessors receive interrupt signals through pins (hardware) of microprocessors, they are known as Hardware Interrupts. There are 5 Hardware Interrupts in the 8085 microprocessor. They are – INTR, RST 7.5, RST 6.5, RST 5.5, TRAP

Software Interrupts
These are inserted in between the program which means these are mnemonics of microprocessor. There are 8 software interrupts in the 8085 microprocessor. They are – RST 0, RST 1, RST 2, RST 3, RST 4, RST 5, RST 6, RST 7.

What are the flags associated with ALU operations in 8051?

  • Flags are a modified kind of register that records the condition of a microprocessor's calculation. For instance, a "zero status" flag is activated only when the microprocessor's calculation concludes with a "zero" status. The status of each flag determines the microprocessor's next action, thus enabling it to make decisions.
  • One part of architecture PSW( program status word ) is also known as the Flag register of 8051 microcontrollers. 
  • Flag register in 8051 microcontrollers is an 8-bit register and it is bit addressable.

What are the flags associated with the 8085 microprocessor?

The Flag register is a Special Purpose Register. Depending upon the value of the result after any arithmetic and logical operation, the flag bits become set (1) or reset (0). In the 8085 microprocessor, the flag register consists of 8 bits and only 5 of them are useful.

The 5 flags are:

  • Carry flag (Cy),
  • Auxiliary carry flag (AC),
  • Sign flag (S),
  • Parity flag (P), and
  • Zero flag (Z).

Name the different buses used in the 8085 microprocessor. What are their roles in the 8085 microprocessor?

A bus is a group of conducting wires which carries information, all the peripherals are connected to a microprocessor through Bus.


Address Bus
An address bus is a group of conducting wires which carries address only. The address bus is unidirectional because data flow in one direction, from the microprocessor to memory or from the microprocessor to Input/output devices. The length of the Address Bus of the 8085 microprocessor is 16 Bit.

Data bus
It is a group of conducting wires which carries Data only. The data bus is bidirectional because data flow in both directions, from the microprocessor to memory or Input/Output devices and from memory or Input/Output devices to microprocessor. The length of the Data Bus of the 8085 microprocessor is 8 Bit.

Control bus
It is a group of conducting wires, which is used to generate timing and control signals to control all the associated peripherals, microprocessor uses a control bus to process data, which is what to do with the selected memory location.

What is the role of accumulator resister in 8051 MCU? What is its bit length?

  • The accumulator, register A, is used for all arithmetic and logic operations. If the accumulator is not present, then every result of each calculation (addition, multiplication, shift, etc.) is to be stored in the main memory. Access to main memory is slower than access to a register like an accumulator because the technology used for the large main memory is slower (but cheaper) than that used for a register.
  • The accumulator on an 8051 is 8 bits (1 byte).

What is the role of the program counter in the 8085 microprocessor?

  • The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point to the next memory location.
  • The program counter acts as a pointer to the next instruction to be executed and always contains the 16-bit address of the memory location of the next instruction
  • It is a 16-bit register as 8085 has 16 address lines
  • The program counter is updated by the processor and points to the next instruction after the processor has fetched the complete instruction

Among 8085, 8086. 8051 and 8031, which processor structure is pipelined?

  • 8085 doesn’t support a pipelined architecture while 8086 supports a pipelined architecture.
  • Pipeline in 8086 is a technique that is used in advanced microprocessors, where the microprocessor executes a second instruction before the completion of the first. That is many instructions are simultaneously pipelined at the different processing stages.
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