Basics of Computers

BASIC OPERATIONAL CONCEPTS

• To perform a given task,an appropriate program consisting of a list of instructions is stored in memory. Individual instructions are brought from the memory to the processor,which executes the specified operations. Data to be used as operands are also stored in the memory.

Slide 2

• For example :
• Add LOCA,R0
• This instuction adds the operand at memory location LOCA to the operand in the register(present inside processor),R0 and Places the sum in R0. The original contents of LOCA are preserved whereas the R0 is overwritten.
• The above happens in the following steps :
• 1. Instruction fetched from memory to the processor.
• 2. The operand at LOCA is fetched and added to the contents of R0.
• 3.Resulting sum is stored in R0.
• The preceding 'ADD' instruction combines a memory access operation with an ALU operation.
• But in many modern computers these are performed by separate operations for better performance. <In next slide>

Slide 3

• Load LOCA,R1
• Add R1,R0
• 1. Transfers contents of memory location LOCA to processor register R1
• 2. Adds registers R1,R0 and places the resultant in R0
• In this way the original contents of memory LOCA is preserved.Transfer between memory and processor starts by sending address of memory location to memory unit and issuing control signals.
• This process also involves a number of registers. Namely :
• i. Instruction registers (IR) - holds instruction to be currently executed
• ii. Program Counter (PC) - keeps track of the execution of the program. Holds the address of next instruction to be fetched and executed.
• (Next two registers facilitate communication with the memory)
• iii. Memory Address register (MAR) - holds address of location to be accessed
• iv. Memory data register (MDR) - contains the data to be written into or read out of the addressed location

OPERATING STEPS

• STEP 1 : Programs reside in the memory which is usually got there through an input unit
• Execution starts when PC points to the first instruction of the program.
• STEP 2 : The contents of PC are transferred to the MAR and a Read control signal is sent to the memory. After the time required to access the memory lapses, the addressed word is read out of memory and loaded into MDR. Contents of MDR are transferred into IR. Then the instruction register is ready to be decoded and executed.
• Hint = PC -> MAR-> Memory -> MDR -> IR
• STEP 3 : If the instruction involves an operation to be performed by ALU >>
• 1. Obtain operands, if it resides in memory fetch it by sending its address to the MAR and initiating a read cycle.
• 2. When operands are read from memory to MDR, it is transferred into ALU.
• 3. After one or more operands are fetched (by 1&2) the ALU performs the desired operation.
• The address of the location where the result is to be stored is sent to MAR and the write cycle is intiated. (If the result of the operation is to be stored in memory then the result is sent to MDR)
• 4. At some point during execution of the current instruction, the contents of PC are incremented and PC points to next instruction to be executed.
• 5. Thus new instruction is fetched and may start execution in the order given above.
• NOTE : Sometimes an interrupt signal is raised during execution due to a request from I/O devices for service. This alters the internal state of the processor and an interrupt -service routine is implemented. Once done the program execution continues.

BYTE ADDRESSABILITY

• Three basic information quantities : the bit, byte and word.
• Byte = 8 bits
• Word = ranges from 16 to 64 bits
• The most practical assignment is to have successive addresses refer to successive byte memory locations in the memory. The term byte-addressable memory is used for this assignment. Byte locations have addresses 0,1,2.....thus if word length of machine is 32 bits, successive words are located at addresses 0,4,8.......with each word consisting of four bytes.
• Two ways that byte addresses can be assigned across words :
• 1. BIG ENDIAN : is used when lower byte addresses are used for the more significant bytes (leftmost bytes) of the word.
• Byte addresses - 0 1 2 3
• 4 5 6 7
• 2. LITTLE ENDIAN : is used for opposite ordering where the lower byte addresses are used for the less significant bytes (rightmost bytes) of the word.
• Byte addresses - 3 2 1 0
• 7 6 5 4