Understanding RISC-V Convention

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RISC-V Convention

Registers

• In RISC-V each of the 32 registers has a different name which indicates its intended usage.

1. ra holds the return address. In particulat it is useful for function calls:

understand ra

def foo():
    x = 1
    bar()
    z = 2
def bar():
    y = 7

Imagine we are at the line y = 7 in bar. When bar finishes we want to resume execution inside of foo and complete the next instruction, z = 2. We do this by storing the address of the instruction where the code should return. In this case ra would hold the address of z = 2 so execution resumes as expected after a function call.

2. sp holds the current base of the stack. When we need to add more space onto the stack, we will decrement sp which gives us more addresses at which to store data. When we exit a function we will increment sp to restore the stack back to the state when entering the function.
3. t register values are not guaranteed to exist after calling a function, while s registers should be used for values needed after a function call.

Function Calls

We make a function call using jal instruction to a label or a jalr instruction to a register rd.

In particular, jal ra label or jalr ra rd imm but sometimes we will abbreviate it with the pseudo-instruction jal label or jalr rd. What this jal does is store PC+4 in ra, which is the address of the next instruction to run after the function all and increment the PC by the offset to the label. jalr is similar except it sets the PC value to rd+imm.

Jumping to a label withou making a function call is done with the instructions jal x0 label and jalr x0 rd imm, sometimes abbreviated with the pesudo-instructions j label and jr rd.

Prologue/Epilogue

• The sp will have the same value when exiting the function that it did entering(unless we store retun values on the stack).
• All s registers will have the same value exiting the function that they did entering.
• The function will return to the value stored in ra, assuming no abnormal execution.

To achieve this we add a section before our function called the prologue and a section after called the epilogue:

def prologue():
    decrement sp by num s registers + local var space
    Store any saved registers used
    Store ra if a function call is made

def epilogue():
    Reload any saved registers used
    Reload ra (if necessary)
    Increment sp back to previous value
    Jump back to retun address

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最后更新: 2024年4月20日 22:02:37
创建日期: 2024年4月16日 22:03:28