Stored Program organization

The simplest way to organize a computer is to have one processor register and an instruction code format with two parts. The first part specifies the operation to be performed and the second specifies the address. The memory address tells the control where to find the operand in the memory. This operand is read from the memory and used as data to be operated on together with the data stored in the processor register.

Figure below depicts this type of computer organization. Instructions are stored in one section of the memory and the operands or data in another. For example if we have a 4K x16 memory unit, it will require 12-bit address to access each memory word.

The instruction and the data accessed from memory is placed in special register designated for instruction code and data .

Here 4-bits in instruction format are the operation code and the rest 12-bits represent the address of memory where the operand is stored. It uses this 12-bit address to read the 16-bit operand from the data portion of the memory.

It then executes the operation specified in the operation part of the instruction. The operation is generally performed with the memory operand and the content of the processor register  which is an accumulator.

The second of the instruction not necessarily contain the address of operand, in that case it may be used to mean other operations such as clear AC, complement AC etc.

Instruction formats : Instruction format are of the following types and described in detain in Instruction Types

1. Direct register operation
2. Immediate data Operation
3. Direct memory or indirect memory operations

Block diagram of a simple computer

 

Computer Registers

Register Symbol	No. of Bits	Register Name	Function
MAR	8	Memory Address Register	Holds address of the memory
MBR	8	Memory Buffer Register	Holds contents of memory word
A	8	A register	Processor register
R	8	R register	Processor register
PC	8	Program Counter	Holds address of instruction
IR	8	Instruction register	Holds Operation Code
T	3	Timing Counter	Sequence generator

Three Instructions for basic computer

Op-Code	Mnemonics	Decription	Function
00000001	MOV R	Move R to A	A
00000010	LDI OPRD	Load Operand to A	A
00000011	LDA ADRS	Load openad specified by address into A	A

Execution of an instruction involves the following steps described in Instruction Cycle

• Instruction fetch operation
• Instruction decode
• Instruction Execution

Register Transfer statement for this simple computer

FETCH	t0:	MAR <– PC
	t1:	MBR <– M, PC <– PC+1
	t2:	IR <– MBR
MOV	q1t3:	A <–R, Tß0
LDI	q2t3:	MAR <– PC
	q2t4:	MBR <– M, PC <– PC+1
	q2t5:	A <– MBR, T <–0
LDA	q3t3:	MAR <– PC
	q3t4:	MBR <– M, PC <– PC+1
	q3t5:	MAR <– MBR
	q3t6:	MBR <– M
	q3t7:	A <– MBR, T <– 0

 

Design of a computer

Referring to the table above we find that the micro-operation MAR <– PC appears thrice i.e. in 1^st, third and fifth line, so we can combine it as:

X1= t0 + q2t3 + q3t3 : MAR <– PC

Or

X1= t0 + t3(q2 + q3) : MAR ß PC

X1= t0 + q2t3 + q3t3 :	MAR
X2 = q3t5 :	MAR
X3 = t1 + q2t4 + q3t4	PC
X4= x3 + q3t6 :	MBR
X5 = q2t5 + q3t7 :	A
X6 = q1t3 :	A
X7 = x5 + x6	T
X8 = t2:	IR

 

 

 

 

Mcq

A register in term of the register-transfer logic notation is :…

1. Storage register
2. Flip-flop
3. Counter
4. Memory units
5. All of the above

Binary information stored in a register may be :…

1. Binary or BCD numbers, alphanumeric characters
2. Control information
3. Any binary coded information such as address, instruction code etc
4. All of the above

A complex digital system such as a computer can be defined in terms of the:…..

1. Combinational and sequential logics
2. Register-transfer logic method where registers are selected to be primitice components
3. Both the above
4. None of these

We can easily specify a large digital system such as a processor or a digital computer in terms of :…

1. A state table as it is easy to work with
2. Partitioning it into modular subsystem consisting of combinational and sequential
3. Both the above
4. None of the above

Micro-operation is an elementary operation that can be performed in parallel in one clock pulse period may also be defined as ………………..

1. An operation  performed on the data stored in registers
2. An operation  performed on two variables

In register-transfer notation a control function is a binary variable that……….

1. Initiates sequence of operations consisting of timing signals that sequence the operations one at a time
2. Is a binary variable that, when in one binary state, initiate an operation and, when in other binary state, inhibits the operation
3. Specifies the control conditionsand timing sequence for the executing the listed micro-operations
4. All the above