Answers to Quiz of 11 Sep 2011

Here are the answers to the questions of yesterday's quiz.

1.       Why should the data bus be bi-directional? 

      The data bus is used to send and receive data between various units such as the processor, memory, other peripherals etc. Thus, the bus has to be bi-directional.      

          

       What are the functions of the address bus?

       The function of an address bus is to transmit the binary address values to all the devices connected to it. The memory and I/O devices then would be able to detect if there is a request fro data from them. The specific address on the bus will tell the devices if the request is addressed to it. (if the address requested falls in its range of addresses)

      What are the control signals used in the control bus of a 8085 microprocessor?

      There are three control signals on the control bus of 8085A. These are the read (RD\), write(WR\) and the Interrupt acknowledge(INTA\) signals.

     

       Define Fetch, Decode, Execute phases of an instruction.

      Fetch is the first phase of the instruction cycle. The value available in PC is used to address memory. Content of the memory at that address is brought into the instruction register. This is the fetch phase. Depending on if the instruction is a multi-byte one, there may be more fetch cycle before execution can start.  

      How would you define a byte, nibble, and a word?

     

      A byte is 8 bits, a nibble is 4 bits and word is 16 bits( could be more in some computers).

      Describe the sequence of operations that take place when a microprocessor is powered on?

      The microprocessor is reset. A reset out signal goes out to all other blocks in the system to reset them (get them into a known starting phase). The instruction pointer/program counter is set to an initial value (most often 0000H). The microprocessor starts an op code fetch to this initial address. Content of this location is brought in, IP/PC is incremented, instruction is decoded and executed, another opcode fetch is initiated. This cycle continues unless a HLT instruction is executed or an illegal instruction is executed. In case of an illegal instruction the system will hang.

      What are the functions fulfilled by an accumulator?

      It holds variables, results of intermediate calculations.

       

      What do you understand by a flag inside a microprocessor?

      Flags are single bit hardware memory (flip-flops) that store the various status of last calculation done by the ALU. For example Z-zero, C-carry, P-parity, etc. could some flags used. Please note, different microprocessors may have a different set of flags.

      The program counter and the stack pointer inside 8085 are 16 bit in length. Why so?

      The program counter and the stack pointer are required to hold an address value which can be anywhere in the given memory space of the microprocessor. 8085 memory space is addressed by 16 bit values and hence these two registers must be 16 bit wide. 

  

      How could the various operations happening inside the 8085 MPU be identified?

      There are three status signals in 8085 that indicate the kind of machine cycle operation happening inside the microprocessors. These signals are IO/M\, S1 and S0 and the control signals. Thus they are able to indicate up to eight different types of machine cycles happening inside. Some of the possible cycles are the op code fetch, memory read, memory write, I/O read, I/O write, Interrupt Ack etc.

Special Note: Those who took this test on 10 Sep, please note that the questions 1,2,3, 4 ,5,7 and 8 are 1 mark each, questions 6 and 10 are 3 marks each and question 9 is for 2 marks for a total of 15 marks.