what does PC have to do with load or link address?

Question

Link address is the address where execution of a program takes place, while load address is the address in memory where the program is actually placed.

Now i'm confused what is the value in program counter? is it the load address or is it the link address?

Answer 1

Link address is the address where execution of a program takes place

No, it's not.

while load address is the address in memory where the program is actually placed.

Kind of. The program usually consists of more than one instruction, so it can't be placed at a single "load address".

When people talk about load address, they usually talk about relocatable code that can be relocated (at runtime) to an arbitrary load address.

For example, let's take a program that is linked at address 0x20020, and consists of 100 4-byte instructions, which all execute sequentially (e.g. it's a sequence of ADDs followed by a single SYSCALL to exit the pogram).

If such a program is loaded at address 0x20020, then at runtime the program counter will have value 0x20020, then it will advance to the next instruction at 0x20024, then to 0x20028, etc. until it reaches the last instruction of the program at 0x201ac.

But if that program is loaded at address 0x80020020 (i.e. if the program is relocated by 0x80000000 from its linked-at address), then the program counter will start at 0x80020020, and the last instruction will be at 0x800201ac.

Note that on many OSes executables are not relocatable and thus have to always be loaded at the same address they were linked at (i.e. with relocation 0; in this case "link address" really is the address where execution starts), while shared libraries are almost always relocatable and are often linked at address 0 and have non-zero relocation.

Answer 2

Both are different concepts, used in different context. The Linker/Loader is mainly responsible for code relocation and modification; the PC is a digital counter which indicates the positioning of program sequence(not a type's address/location like linker/loader).

Linking & Loading :-

The heart of a linker or loader's actions is relocation and code modification. When a compiler or assembler generates an object file, it generates the code using the unrelocated addresses of code and data defined within the file, and usually zeros for code and data defined elsewhere. As part of the linking process, the linker modifies the object code to reflect the actual addresses assigned. For example, consider this snippet of x86 code that moves the contents of variable a to variable b using the eax register.

mov a,%eax 
mov %eax,b

If a is defined in the same file at location 1234 hex and b is imported from somewhere else, the generated object code will be:

A1 34 12 00 00 mov a,%eax 
A3 00 00 00 00 mov %eax,b

Each instruction contains a one-byte operation code followed by a four-byte address. The first instruction has a reference to 1234 (byte reversed, since the x86 uses a right to left byte order) and the second a reference to zero since the location of b is unknown.

Now assume that the linker links this code so that the section in which a is located is relocated by hex 10000 bytes, and b turns out to be at hex 9A12. The linker modifies the code to be:

 A1 34 12 01 00 mov a,%eax  
 A3 12 9A 00 00 mov %eax,b

That is, it adds 10000 to the address in the first instruction so now it refers to a's relocated address which is 11234, and it patches in the address for b. These adjustments affect instructions, but any pointers in the data part of an object file have to be adjusted as well.

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Program Counter :-

The program counter (PC) is a processor register that indicates where a computer is in its program sequence.

In a typical central processing unit (CPU), the PC is a digital counter (which is the origin of the term "program counter") that may be one of many registers in the CPU hardware. The instruction cycle begins with a fetch, in which the CPU places the value of the PC on the address bus to send it to the memory.

The memory responds by sending the contents of that memory location on the data bus. (This is the stored-program computer model, in which executable instructions are stored alongside ordinary data in memory, and handled identically by it).

Following the fetch, the CPU proceeds to execution, taking some action based on the memory contents that it obtained. At some point in this cycle, the PC will be modified so that the next instruction executed is a different one (typically, incremented so that the next instruction is the one starting at the memory address immediately following the last memory location of the current instruction).