给一个汇编语言中的call指令嵌套的样例代码

在汇编语言中，`CALL`指令通常用于跳转到另一个子程序的开始地址。由于汇编语言的低级特性，这里是一个基本的MIPS汇编语言（RISC-V体系）中`jal` (Jump And Link) 指令嵌套的例子，它类似于`call`：

.data
start_label:
    .asciiz "Start of program.\n"

subroutine_1:
    li $v0, 4       # Load system call for print syscall number (4 = print)
    la $a0, start_label     # Load address of string into $a0 register
    syscall         # Execute the system call

subroutine_2:
    jal subroutine_1  # Jump and link to subroutine_1, passing control
                     # after execution of subroutine_1 is returned.

.text
.globl main
main:
    jal subroutine_2  # Call subroutine_2 from the main program
    li $v0, 10      # Load exit syscall number (10 = exit)
    syscall         # Terminate the program

在这个例子中，`main`程序先通过`jal`调用`subroutine_2`，`subroutine_2`再进一步调用`subroutine_1`。当`subroutine_1`结束并返回后，控制会继续执行`subroutine_2`之后的代码。