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Replies to This Discussion

1)     Write Keyboard writing protocol?


Keyboard writing Protocol

  • • Wait till input buffer is full
  • • Write on buffer
  • • Wait till output buffer is full
  • • Check the acknowledgement byte
  • • Repeat the process if it was previously unsuccessful.

2)     Steps involve in LPT1 and LPT2 ?


Swapping LPTs

Direct Parallel Port Programming

unsigned int far * lpt = (unsigned int far *) 0x00400008 ;

unsigned int temp;


*lpt=*(lpt + 1);

*(lpt + 1)=temp;

The LPTs can be swapped i.e LPT1 can be made LPT2 and vice versa for LPT2. This can

be accomplished simply by swapping their addresses in the BIOS data area as shown in

the slide above.

Explain null modem? 5 marks

if data is to transferred from one computer to another computer through some media which can carry digital signal then modem can be eliminated and A UART can be interconnected on both the computers. Such arrangement is called a NULL modem.

3) How to send data to keyboard?

Similarly some data (as control information) can be send to the keyboard. The processor will write on the port 60H. The device driver will check the OBF( output buffer full bit of port 64H which remains set as long as the byte is not received by the keyboard. On receipt of the byte from the port 60H the keyboard device write a code 0xFA on the port 60H to indicate that the byte has been received properly.


Describe the purpose of 12h interrupt and 88h? 3Marks

Int 12H is used to determine the amount of conventional memory interfaced with the processor in kilobytes. The amount of memory above conventional memory (extended memory) can be determined using the service 15H/88H.

3: In flow control what is the working of D4?

First the low nibble of the byte is sent from the sender in bit D0 to D3 of the data port. D4 bit is cleared to indicate the low nibble is being sent. The receiver will know the arrival of the low nibble when its checks BUSY bit which should be set (by the interface) on arrival.


The lower 4 bits of this register stores a code indicating the frequency with which the RTC hardware interrupt can interrupt the processor. The next field is used to specify the time frequency i.e. the frequency with the time is sampled and hence updated. The most significant bit indicates that after time sampling if the time has been updated in to the 64 byte RAM or not.

Q4: What is the purpose of Interrupt ID register in UART? 3 marks

Once an interrupt occurs it may be required to identify the case of the interrupt. This register is used to identify the cause of the interrupt.

What are the usages of coprocessor control word in coprocessor while testing for coprocessor? 3 marks

The coprocessor control word contains some control information about the coprocessor. The bit number 7 of coprocessor control word is the Interrupt Enable Flag and bit number 8 & 9 should contain 11 on initialization.

((( inport((*lpt) +1)) & 0x80) == 0x80)

Condition will check whether the printing port is idle or not. see page #97 for more detail.


3) In LPT1 BIOS what is the meaning of " inportb*(base +1)& 0x80);?

In direct parallel port programming it indicate to print a file. While loop terminate when the file ends.

In case of printer the (base +1) is the printer status port. it will tell if its busy or not...



unsigned int far * lpt = (unsigned int far *)0x00400008 Direct parallel port programming LPTs swapping

Here we are accessing the base register address of LPT1 that is on the 40 segment 08 offset

for more info check page # 91,92 of handbook


which command before changing the typematic and LEDS? 3 marks.

0xF3 for typematic and 0xED for LED

3: In flow control what is the working of D4?

First the low nibble of the byte is sent from the sender in bit D0 to D3 of the data port. D4 bit is cleared to indicate the low nibble is being sent. The receiver will know the arrival of the low nibble when its checks BUSY bit which should be set (by the interface) on arrival.



q 1. What is the purpose of service 1 and 2 of 14h  2marks

BIOS support for COM ports


SERVICE#1 = output characters

SERVICE#2 = read in characters


RS232C is a standard for physical dimension of the connector interconnecting a DTE(Data  terminal equipment) and DCE (Data communication equipment).


Data is received through the RxD line. Data is send through the TxD line. DTR (data terminal ready) indicates that the data terminal is live and kicking. DSR(data set ready) indicates that the data set is live. Whenever the sender can send data it sends the signal RTS( Request to send) if as a result the receiver is free and can receive data it send the sender an acknowledge through CTS( clear to send) indicating that its clear to send now.


Q3: What is the RTC? 3 marks


Real time clock is a device incorporated into the PC to update time even if the computer is off. It has the characteristics shown in the slide above which enables it to update time even if the computer is off.

Step to access battery powered ram. (5 Marks)

Battery Powered RAM is accessed in two steps

• Specify the Byte no. in 70H port.

• Read/write port 71H to get/set the value of specified byte.

Following slide shown a fragment of code that can be used to read or write onto any byte within the 64 byte battery powered RAM.

outport (0x70, 0);

outport (0x70, 4);

sec = inport (0x71);

outport (0x71,hrs);


what is the purpose of self-test mode in UART? 2 Marks

What is Self-test mode of UART?. 3 Marks

Moreover if a single computer is available to a developer the UART contains a self-test mode which can be used by the programmer to self-test the software. In self-test mode the output of the UART is routed to its input. So you receive what you send.


thanks to all


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