Short Question And Answer Microprocessor Set-17

1. What are the features used mode 2 in 8255?
Ans:The single 8-bit port in-group A is available.

  1. The 8-bit port is bi-directional and additionally a 5-bit control port is available.
  2. Three I/O lines are available at port C, viz PC2-PC0.
  3. Inputs and outputs are both latched.
  4. The 5-bit control port C (PC3=PC7) is used for generating/accepting handshake signals for the 8-bit data transfer on port A.

2. What are the modes of operations used in 8253?
Ans:Each of the three counters of 8253 can be operated in one of the following six modes of operation.

  1. Mode 0 (Interrupt on terminal count)
  2. Mode 1 (Programmable monoshot)
  3. Mode 2 (Rate generator)
  4. Mode 3 (Square wave generator)
  5. Mode 4 (Software triggered strobe)
  6. Mode 5 (Hardware triggered strobe)

3. What are the different types of write operations used in 8253?
Ans:There are two types of write operations in 8253
(1) Writing a control word register
(2) Writing a count value into a count register
The control word register accepts data from the data buffer and initializes the counters, as required. The control word register contents are used for
(a) Initializing the operating modes (mode 0-mode4)
(b) Selection of counters (counter 0- counter 2)
(c) Choosing binary /BCD counters
(d) Loading of the counter registers.
The mode control register is a write only register and the CPU cannot read its contents.

4. Give the operating modes of 8259a?
(a) Fully Nested Mode
(b) End of Interrupt (EOI)
(c) Automatic Rotation
(d) Automatic EOI Mode
(e) Specific Rotation
(f) Special Mask Mode
(g) Edge and level Triggered Mode
(h) Reading 8259 Status (i) Poll command
(j) Special Fully Nested Mode
(k) Buffered mode
(l) Cascade mode

4. Define scan counter?
Ans: The scan counter has two modes to scan the key matrix and refresh the display. In the encoded mode, the counter provides binary count that is to be externally decoded to provide the scan lines for keyboard and display. In the decoded scan mode, the counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL0-SL3.The keyboard and display both are in the same mode at a time.

5. What is the output modes used in 8279?
Ans: 8279 provides two output modes for selecting the display options.
1.Display Scan In this mode, 8279 provides 8 or 16 character-multiplexed displays those can be organized as dual 4-bit or single 8-bit display units.
2.Display Entry 8279 allows options for data entry on the displays. The display data is entered for display from the right side or from the left side.

6. What are the modes used in keyboard modes?
1. Scanned Keyboard mode with 2 Key Lockout.
2. Scanned Keyboard with N-key Rollover.
3. Scanned Keyboard special Error Mode.
4. Sensor Matrix Mode.

7. What are the modes used in display modes?
1. Left Entry mode In the left entry mode, the data is entered from the left side of the display unit.
2. Right Entry Mode.
In the right entry mode, the first entry to be displayed is entered on the rightmost display.

8. What is the use of modem control unit in 8251?
Ans: The modem control unit handles the modem handshake signals to coordinate the communication between the modem and the USART.

9. Give the register organization of 8257?
Ans: The 8257 perform the DMA operation over four independent DMA channels. Each of the four channels of 8257 has a pair of two 16-bit registers. DMA address register and terminal count register. Also, there are two common registers for all the channels; namely, mode set registers and status register. Thus there are a total of ten registers. The CPU selects one of these ten registers using address lines A0- A3.

10. What is the function of DMA address register?
Ans: Each DMA channel has one DMA address register. The function of this register is to store the address of the starting memory location, which will be accessed by the DMA channel. Thus the starting address of the memory block that will be accessed by the device is first loaded in the DMA address register of the channel. Naturally, the device that wants to transfer data over a DMA channel, will access the block of memory with the starting address stored in the DMA Address Register.

11. What is the use of terminal count register?
Ans: Each of the four DMA channels of 8257 has one terminal count register. This 16-bit register is used for ascertaining that the data transfer through a DMA channel ceases or stops after the required number of DMA cycles.

12. What is the function of mode set register in 8257?
Ans: The mode set register is used for programming the 8257 as per the requirements of the system. The function of the mode set register is to enable the DMA channels individually and also to set the various modes of operation.

13. What is interfacing?
Ans: An interface is a shared boundary between the devices which involves sharing information. Interfacing is the process of making two different systems communicate with each other.

14. List the operation modes of 8255
a) I.O Mode
i. Mode 0-Simple Input/Output.
ii. Mode 1-Strobed Input/Output (Handshake mode)
iii. Mode 2-Strobed bidirectional mode
b) Bit Set/Reset Mode.

15. What is a control word?
Ans: It is a word stored in a register (control register) used to control the operation of a program digital device.

16. What is the purpose of control word written to control register in 8255?
Ans: The control words written to control register specify an I/O function for each I.O port. The bit D7 of the control word determines either the I/O function of the BSR function.

17.What is the size of ports in 8255?
Port-A : 8-bits
Port-B : 8-bits
Port-CU : 4-bits
Port-CL : 4-bits

18. Distinguish between the memories mapped I/O peripheral I/O?
Memory Mapped I/O Peripheral Mapped I/O 16-bit device address 8-bit device address
Data transfer between any general-purpose register and I/O port. Data is transfer only between accumulator and I.O port
The memory map (64K) is shared between I/O device and system
memory. The I/O map is independent of the memory map; 256 input device and 256
output device can be connected
More hardware is required to decode 16-bit address Less hardware is required to decode 8-bit address
Arithmetic or logic operation can be directly performed with I/O
data Arithmetic or logical operation cannot be directly performed with I/O

19. Give the different types of command words used in 8259a?
Ans:The command words of 8259A are classified in two groups
1. Initialization command words (ICWs)
2. Operation command words (OCWs)

19. What is memory mapping?
Ans: The assignment of memory addresses to various registers in a memory chip is called as memory mapping.

20. What is I/O mapping?
Ans:The assignment of addresses to various I/O devices in the memory chip is called as I/O mapping.