ra寄存器定位core

$ra寄存器中存入的是pc的值（程序运行处的地址），调用函数时，在跳转前，必须保存当前地址（pc的值），以便后来返回。jal $ra 保存后跳转，jr $ra,返回到跳转前，通过$ra保存进入上层栈地址。

jal
    直接跳转指令，并带有链接功能，指令的跳转地址在指令中，跳转发生时要把返回地址存放到R31寄存器（ra）中。
jr
    使用寄存器的跳转指令，跳转地址在寄存器中。

所以根据ra定位core位置时，可以直接在ra位置前找jal便可以找到当前正在执行的函数。

转自http://user.it.uu.se/~justin/Teaching/NewDarkSlides/lecture5.pdf

•  As in high level languages , when programming in assembly language you should split up your program into smaller functions, that you can reuse.

•  One of the key ideas with functions is that you can call them
from any where and return back to where you called the function from.

•  The MIPS processor has two instructions that enable you to call functions, jr and jal.
•  Jump and link.
                    jal  label
   Copies the address of the next instruction into the register $ra
(register 31) and then jumps to the address label.

•  jr  $register jumps to the address in $register most common use
                    jr  $ra

            .data
str:        .asciiz  "Hello  mum!.\n"
            .text
            .globl  main  #necessary  for  the  assembler
main:        jal  message
            jal  message
            li  $v0,
            syscall    #exit  the  program  gracefully
message:    la  $a0,str
            li  $v0,
            syscall    #Magic  to  printhings  on  the  screen.
            jr  $ra

•  There are many way of passing values to functions, but there is a convention that most programs on the MIPS follow.
•  $a0-$a3 (registers 4 to 7) arguments 1-4 of a function.

•  $v0-$v1 (registers 2 and 3) results of a function.

        li  $a0,
        li  $a1,
        li  $a3,
        jal  silly  #Now  the  result  of  the  function  is  is  $v0.
        li  $v0,
        syscall
silly:  add  $t0,$a0,$a1
        sub  $v0,$a3,$t0
        jr  $ra