What is r4 used for in ppc?

Question

The title said all, if someone can explain me how it's work that would be amazing and also i'm new at PPC and i'm blocked for the R4 i'm not sure to completely understand.

Answer 1

r4 is just a fixed-point general purpose register; other than storing integer values, it has no special function on the hardware side.

However, most software ABIs use r4 for the second (integer) argument over a function call.

[Note that "integer" includes pointer types here]

Check out the "PowerISA" document for full details of the Instruction Set Architecture for POWER: https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0

Answer 2

PowerPC has 32 general purpose registers (GPR) that can generally be used as the target or source in an instruction. r4 is one of those general purpose registers. This site contains a description of common use of the registers.

Application Binary Interfaces (ABIs) provide a set of rules on calling conventions for functions. In the most common PPC ABI, r4 is used to hold the second argument to a function. For example you could have a simple set of functions:

.func1:
    # ...
    # foo = func2(8, 9);
    # ...
    # Prologue to save SP, etc. omitted
    li r3, 8
    li r4, 9
    bl .func2
    # Eplogue to restore stack, saved registers, return to caller omitted

.func2:
    # Add two numbers together
    # int32_t func2(a, b) { return a+b; }
    # r3 = a, first argument
    # r4 = b, second argument
    # return value in r3
    # Note: no need for a prologue or epilogue because we didn't modify saved registers and this is a leaf function
    add r3, r3, r4
    blr

While r3 is typically used for the return value, r3 can be combined with r4 for 64 bit return values from functions.

Check out the ST Manual for Book E processors or other such manuals. They will have much more detail about register usage, etc.