Apr 1999 CS202: Computer Architecture and Data Communication (Question 1)

(a)	Define the following terms:
	(i) MBR	[1]
	(ii) MAR	[1]
	(iii) SP	[1]
	(iv) PC	[1]
	(v) DMA	[1]
	(vi) CPU	[1]
	(i) MBR stands for Memory Buffer Register It is used to hold temporary data or instructions from and to the memory. (ii) MAR stands for Memory Address Register It is used to hold an address of data or instruction of the memory. (iii) SP stands for Stack Pointer It is used as a pointer to the last element that is pushed onto the stack. (iv) PC stands for Program Counter (v) DMA stands for Direct Memory Access It is one of kind of I/O module. Here using DMA, I/O devices can directly make a communication with the memory without the help of the CPU. CPU stands for Central Processing Unit It is used to coordinate and control the whole operation of the computer, it is a kernel of the computer.
(b)	What is the importance of a Program Status Word (PSW) or Status Register (SR)?	[3]
	Program Status Word (PSW) or Status Register (SR) is the computer program that is used to check the status of the programs or external devices. It is used to check whether the devices are ready to do the job or not, and if it is not then the PSW or SR will return an error message. CPU also use PSW or SR to read something from the I/O devices. CPU sends the command to PSW or SR to check the status of the I/O devices. If they are ready, then I/O module will read the data and then the CPU will write that data.
(c)	List four devices commonly found in a network, and explain the purpose of each.	[4]
	Host computer Host computer is used to calculate, store and retrieve information, and coordinate the network activities. FEP (Front End Processor) It is used to unload some control and processing functions from host computer to the programmed or specialized network processor to start the application of the network activity. Modem It stands for Modulator Demodulator Modem here is used to convert the electrical pulses or digital signal that is used in computer to the analog signal for transformation, and at the end it will then convert back the analog signal to the digital signal so that the other computer can understand and process. Multiplexer It is used to combine together the devices that have a slow transmission rate into one and common line of the high speed so that it can help the devices transfer the information faster than before.
(d)	For the following instruction mnemonics, identify the appropriate addressing modes:	[3]
	(i) ADD AX, 10
	(ii) ADD AX [BX + 3]
	(iii) MOV AX [VAR]
	(i) It is immediate addressing. (ii) It is a displacement addressing. (iii) It is a direct addressing.
(e)	To operation ADD ACC, MEM uses the ALU to add the value stored in the accumulator to the value that is stored in the memory location specified in the instruction, storing the result in the accumulator. Show the steps of the execution cycle on the operation.	[4]
	Execution cycle IR ® decoding circuitry MEM ® MAR Remarks: MAR = MEM MAR ® address bus read control line is asserted ¬ [MEM] MEM ® data bus ® MBR MBR ® AX AX = Acc + MEM