RISC-V Conventions

Question

You are the author of the function F while your friend is the author of the function G. Both F and G are located in the same RISC-V main memory but you and your friend do not know the contents of each other's codes. However, you both mutually agree to follow the RISC-V guidelines for using the appropriate registers in function calls.

Here is what is known to both of you:

1. The function F accepts its argument in a0 and returns its result in a1

2. The function G accepts its arguments in a0 and a1 and returns its result in a0

You have written the following lines of code for function F (which itself calls G) :

Since you don’t know how your friend implements G except that they follows the RISC-V conventions, which of the following registers must you store to stack memory before calling G(push at around line 10), and restore afterwards(pop at around line 12)? 

  1)a0,t1,t0

 

  2)t1,t0

 

  3)t0, ra

 

  4)t0

 

  5)None of the options

 

  6)a1,a0,t1,t0

 

Answer 1

The image you seem to have intended to attach doesn't seem to have loaded, but I should be able to equip you to answer this question without it.

The RISC-V calling convention dictates that integer registers t0-t6 and FP registers ft0-ft11 are "scratch" registers and must be saved by the caller if the values are needed later.

Additionally, if the original value of a0 is needed after calling G, it must also be saved since it will be overwritten with the return value of G.