汇编代码中在调用c函数前的准备工作

调用c语言函数前，需要把sp指针设置到栈底，这样c函数里面压栈出栈才能正常使用
还有的会初始化BSS段

 ldr x0, =Image$$EL3_STACKS$$ZI$$Limit   这里是取的链接器导出的符号，即栈地址

//
    // That's the last of the control settings for now
    //
    // Note: no ISB after all these changes, as registers won't be
    // accessed until after an exception return, which is itself a
    // context synchronisation event
    //

    //
    // Setup some EL3 stack space, ready for calling some subroutines, below.
    //
    // Stack space allocation is CPU-specific, so use CPU
    // number already held in x19
    //
    // 2^12 bytes per CPU for the EL3 stacks
    //
    ldr x0, =Image$$EL3_STACKS$$ZI$$Limit
    sub x0, x0, x19, lsl #12
    mov sp, x0

    //
    // we need to configure the GIC while still in secure mode, specifically
    // all PPIs and SPIs have to be programmed as Group1 interrupts
    //

    //
    // Before the GIC can be reliably programmed, we need to
    // enable Affinity Routing, as this affects where the configuration
    // registers are (with Affinity Routing enabled, some registers are
    // in the Redistributor, whereas those same registers are in the
    // Distributor with Affinity Routing disabled (i.e. when in GICv2
    // compatibility mode).
    //
    mov x0, #(1 << 4) | (1 << 5) // gicdctlr_ARE_S | gicdctlr_ARE_NS
    mov x1, x19
    bl  SyncAREinGICD