Introduction to Programming - (C Language) - Unit : 1 - Program counter & Data measurement

 

PROGRAM COUNTER

 

The program counter (PC), also known as the instruction pointer (IP) in some architectures, is a vital component of a computer's central processing unit (CPU) or microprocessor. It is a special-purpose register that keeps track of the memory address of the next instruction to be fetched and executed by the CPU. In other words, it points to the location in memory where the CPU should read the next machine instruction.

Here's how the program counter works within the context of executing a program:

Initialization: When a program is loaded into memory and execution begins, the program counter is initialized to point to the memory address of the first instruction in the program.

Instruction Fetch: The CPU reads the instruction at the memory address pointed to by the program counter and increments the program counter to point to the next instruction. This is often a simple increment operation, although in some cases, it may be more complex, depending on the instruction set architecture (ISA) of the CPU.

Execution: The CPU decodes and executes the instruction it just fetched. This instruction might include arithmetic operations, data transfers, control flow instructions (like jumps and branches), or other operations.

Repeat: Steps 2 and 3 are repeated in a loop until the program completes or encounters a control flow instruction that changes the program counter's value (e.g., a jump or branch instruction). Such control flow instructions can alter the flow of execution by causing the program counter to point to a different memory address.

The program counter is crucial for maintaining the sequential execution of instructions in a program. It ensures that the CPU knows where to fetch the next instruction and keeps the program's execution on track.

 

Program counter’s role in computer architecture:

 

Size: The size of the program counter register depends on the computer architecture. In 32-bit systems, it's typically 32 bits long, while in 64-bit systems, it's 64 bits long. This determines the range of memory addresses it can point to.

Incrementing: After fetching an instruction, the program counter is typically incremented by the size of the instruction or the size of the memory address (e.g., 4 bytes in a 32-bit system, 8 bytes in a 64-bit system) to point to the next instruction's address. This is known as the "program counter increment."

Exception Handling: During program execution, exceptions or interrupts (e.g., hardware interrupts or software exceptions like divide-by-zero errors) can occur. In such cases, the program counter's value can be temporarily saved to handle the exception and then restored to resume normal program execution once the exception is handled.

Branching and Control Flow:

In addition to simple incrementing, the program counter can be modified explicitly by branching and control flow instructions.

Conditional Branches: Conditional branch instructions allow the program counter to be set to different addresses based on certain conditions. For example, in an if-else statement, the program counter might be set to the address of the "if" block or the "else" block depending on the condition's outcome.

Unconditional Jumps: Unconditional jump instructions can directly set the program counter to a specific memory address, enabling non-sequential execution. This is commonly used for loops and function calls.

Pipelining: In modern CPUs, instruction fetching and execution often occur in pipelines to maximize performance. The program counter plays a crucial role in ensuring that the correct instructions are fetched in the correct order and at the right time.

Privilege Levels: Some CPUs, especially in multi-core and multi-threaded systems, have different privilege levels (e.g., user mode and kernel mode). The program counter may have separate versions or restrictions depending on the privilege level to control access to sensitive parts of the system.

Context Switching: In multi-tasking operating systems, the program counter is saved during a context switch when the CPU switches from executing one process to another. This allows the CPU to resume execution of the previously running process from where it left off.

 

Security: Security features like address space layout randomization (ASLR) can randomize the base address of a program, making it harder for attackers to predict the location of specific functions. The program counter is involved in managing this address translation.

 

Overall, the program counter is a vital component in the CPU's execution of instructions, managing control flow, and enabling the sequential execution of programs. Its operation and features may vary depending on the specific architecture of the CPU.

 

DATA MEASUREMENT

Computer doesn’t know our language. It can understand only Machine Language (0,1). Computer reads the data in binary form. 0,1 are Called bits or binary digits. 0 means False, Off, No and 1 means True, On, Yes. Data converts into ASCII code and then converts into Binary code. ASCII means (American Standard code for Information Interchange). ASCII codes are 256 (0 – 255)

8 Bits (0,1) are equal to 1 Byte (1 Character)

 

8 bits	=	1 Byte
1024 Bytes	=	1 K.B ( Kilo Byte )
1024 K.B	=	1 M.B ( Mega Byte )
1024 M.B	=	1 G.B ( Giga Byte )
1024 G.B	=	1 T.B ( Terra Byte )
1024 T.B	=	1 P.B ( Peta Byte )
1024 P.B	=	1 E.T ( Exa Byte )
1024 E.T	=	1 Z.T ( Zeta Byte )
1024 Z.T	=	1 Y.T ( Yotta Byte )