In this part we will look into a special memory region of the process called the Stack. This chapter covers Stack’s purpose and operations related to it. Additionally, we will go through the implementation, types and differences of functions in ARM.

堆棧是進程的一個特殊內存區域。堆棧的使用對於不同處理器的實現是不一樣的。介紹堆棧的實現，類型以及。。。

Generally speaking, the Stack is a memory region within the program/process. This part of the memory gets allocated when a process is created. We use Stack for storing temporary data such as local variables of some function, environment variables which helps us to transition between the functions, etc. We interact with the stack using PUSH and POP instructions. As explained in Part 4: Memory Instructions: Load And Store PUSH and POP are aliases to some other memory related instructions rather than real instructions, but we use PUSH and POP for simplicity reasons.

堆棧是屬於某個程序或進程的。當進程創建時，這部分堆棧內存也被分配。用堆棧存儲局部變量，用於幫助我們在函數之間轉移的環境變量，等。為簡便起見，用PUSH和POP來訪問堆棧，類似Sparc的助記符。

Before we look into a practical example it is import for us to know that the Stack can be implemented in various ways. First, when we say that Stack grows, we mean that an item (32 bits of data) is put on to the Stack. The stack can grow UP (when the stack is implemented in a Descending fashion) or DOWN (when the stack is implemented in a Ascending fashion). The actual location where the next (32 bit) piece of information will be put is defined by the Stack Pointer, or to be precise, the memory address stored in the SP register. Here again, the address could be pointing to the current (last) item in the stack or the next available memory slot for the item. If the SP is currently pointing to the last item in the stack (Full stack implementation) the SP will be decreased (in case of Descending Stack) or increased (in case of Ascending Stack) and only then the item will placed in the Stack. If the SP is currently pointing to the next empty slot in the Stack, the data will be first placed and only then the SP will be decreased (Descending Stack) or increased (Ascending Stack).

In our examples we will use the Full descending Stack. Let’s take a quick look into a simple exercise which deals with such a Stack and it’s Stack Pointer.

  按照堆棧的生長方向和堆棧指針SP指向的位置，堆棧可以分為4種。例子中使用Full descending Stack，即上圖第二種，堆棧向低地址生長，SP指向最後一個數據。

To understand functions in ARM we first need to get familiar with the structural parts of a function, which are:

1. Prologue，起始，序曲
2. Body
3. Epilogue，結束，尾聲

The purpose of the prologue is to save the previous state of the program (by storing values of LR and R11 onto the Stack) and set up the Stack for the local variables of the function. While the implementation of the prologue may differ depending on a compiler that was used, generally this is done by using PUSH/ADD/SUB instructions. An example of a prologue would look like this:

函數起始：

（1）保存之前的狀態（將LR和R11保存到堆棧，下面第1句）

（2）設置堆棧的fp，一般是將fp=sp+4（因為之前push已經移動了2個單位）

（3）設置堆棧的sp，sp現在已經移動了2個單位，再移動剩餘所需的空間即可。

1 push   {r11, lr}    /* Start of the prologue. Saving Frame Pointer and LR onto the stack */  2 add    r11, sp, #0  /* Setting up the bottom of the stack frame */  3 sub    sp, sp, #16  /* End of the prologue. Allocating some buffer on the stack. This also allocates space for the Stack Frame */

The body part of the function is usually responsible for some kind of unique and specific task. This part of the function may contain various instructions, branches (jumps) to other functions, etc. An example of a body section of a function can be as simple as the following few instructions:

1 mov    r0, #1       /* setting up local variables (a=1). This also serves as setting up the first parameter for the function max */  2 mov    r1, #2       /* setting up local variables (b=2). This also serves as setting up the second parameter for the function max */  3 bl     max          /* Calling/branching to function max */

The sample code above shows a snippet of a function which sets up local variables and then branches to another function. This piece of code also shows us that the parameters of a function (in this case function max) are passed via registers. In some cases, when there are more than 4 parameters to be passed, we would additionally use the Stack to store the remaining parameters. It is also worth mentioning, that a result of a function is returned via the register R0. So what ever the result of a function (max) turns out to be, we should be able to pick it up from the register R0 right after the return from the function. One more thing to point out is that in certain situations the result might be 64 bits in length (exceeds the size of a 32bit register). In that case we can use R0 combined with R1 to return a 64 bit result.

不超過4個的輸入參數可以通過寄存器傳遞，若超過4個參數，則超過的需要通過堆棧傳遞。函數返回值也是通過R0傳遞。

The last part of the function, the epilogue, is used to restore the program’s state to it’s initial one (before the function call) so that it can continue from where it left of. For that we need to readjust the Stack Pointer. This is done by using the Frame Pointer register (R11) as a reference and performing add or sub operation. Once we readjust the Stack Pointer, we restore the previously (in prologue) saved register values by poping them from the Stack into respective registers. Depending on the function type, the POP instruction might be the final instruction of the epilogue. However, it might be that after restoring the register values we use BX instruction for leaving the function. An example of an epilogue looks like this:

函數結束，恢復初始狀態：

（1）設置堆棧的sp，一般通過r11=fp來設置，通常應該是sp=r11+4。

（2）恢復之前保存的r11=fp和lr到r11和PC。

1 sub    sp, r11, #0  /* Start of the epilogue. Readjusting the Stack Pointer */  2 pop    {r11, pc}    /* End of the epilogue. Restoring Frame Pointer from the Stack, jumping to previously saved LR via direct load into PC. The Stack Frame of a function is finally destroyed at this step. */

So now we know, that:

1. Prologue sets up the environment for the function;
2. Body implements the function’s logic and stores result to R0;
3. Epilogue restores the state so that the program can resume from where it left of before calling the function.

Another key point to know about the functions is their types: leaf and non-leaf. The leaf function is a kind of a function which does not call/branch to another function from itself. A non-leaf function is a kind of a function which in addition to it’s own logic’s does call/branch to another function. The implementation of these two kind of functions are similar. However, they have some differences. To analyze the differences of these functions we will use the following piece of code:

另一個關於函數的要點是，函數分葉子函數和非葉子函數。葉子函數里不再繼續調用其它函數，非葉子函數里會繼續調用其它函數

 1 /* azeria@labs:~$ as func.s -o func.o && gcc func.o -o func && gdb func */   2 .global main   3   4 main:   5     push   {r11, lr}    /* Start of the prologue. Saving Frame Pointer and LR onto the stack */   6     add    r11, sp, #0  /* Setting up the bottom of the stack frame */   7     sub    sp, sp, #16  /* End of the prologue. Allocating some buffer on the stack */   8     mov    r0, #1       /* setting up local variables (a=1). This also serves as setting up the first parameter for the max function */   9     mov    r1, #2       /* setting up local variables (b=2). This also serves as setting up the second parameter for the max function */  10     bl     max          /* Calling/branching to function max */  11     sub    sp, r11, #0  /* Start of the epilogue. Readjusting the Stack Pointer */  12     pop    {r11, pc}    /* End of the epilogue. Restoring Frame pointer from the stack, jumping to previously saved LR via direct load into PC */  13  14 max:  15     push   {r11}        /* Start of the prologue. Saving Frame Pointer onto the stack */  16     add    r11, sp, #0  /* Setting up the bottom of the stack frame */  17     sub    sp, sp, #12  /* End of the prologue. Allocating some buffer on the stack */  18     cmp    r0, r1       /* Implementation of if(a<b) */  19     movlt  r0, r1       /* if r0 was lower than r1, store r1 into r0 */  20     add    sp, r11, #0  /* Start of the epilogue. Readjusting the Stack Pointer */  21     pop    {r11}        /* restoring frame pointer */  22     bx     lr           /* End of the epilogue. Jumping back to main via LR register */

The example above contains two functions: main, which is a non-leaf function, and max – a leaf function. As mentioned before, the non-leaf function calls/branches to another function, which is true in our case, because we branch to a function max from the function main. The function max in this case does not branch to another function within it’s body part, which makes it a leaf function.

Another key difference is the way the prologues and epilogues are implemented. The following example shows a comparison of prologues of a non-leaf and leaf functions. The main difference here is that the entry of the prologue in the non-leaf function saves more register’s onto the stack. The reason behind this is that by the nature of the non-leaf function, the LR gets modified during the execution of such a function and therefore the value of this register needs to be preserved so that it can be restored later. Generally, the prologue could save even more registers if it’s necessary.

函數起始：對於非葉子函數，因為進一步調用其它函數會改變LR寄存器，因此，在函數起始，需要將r11和LR一起壓入堆棧存儲。而對於葉子函數，不再調用其它函數，LR不會改變，因此，不需要將LR壓入堆棧。

1 /* A prologue of a non-leaf function */  2 push   {r11, lr}    /* Start of the prologue. Saving Frame Pointer and LR onto the stack */  3 add    r11, sp, #0  /* Setting up the bottom of the stack frame */  4 sub    sp, sp, #16  /* End of the prologue. Allocating some buffer on the stack */  5  6 /* A prologue of a leaf function */  7 push   {r11}        /* Start of the prologue. Saving Frame Pointer onto the stack */  8 add    r11, sp, #0  /* Setting up the bottom of the stack frame */  9 sub    sp, sp, #12  /* End of the prologue. Allocating some buffer on the stack */

The comparison of the epilogues of the leaf and non-leaf functions, which we see below, shows us that the program’s flow is controlled in different ways: by branching to an address stored in the LR register in the leaf function’s case and by direct POP to PC register in the non-leaf function.

函數結束：對於葉子函數，可以直接bx lr，跳轉到LR處繼續執行，因為，LR未改變。BX的意思為Branch and eXchange ARM/Thumb模式。

對於非葉子函數，需要將之前保存的LR恢復給PC，來繼續執行。

1 /* An epilogue of a leaf function */  2 add    sp, r11, #0  /* Start of the epilogue. Readjusting the Stack Pointer */  3 pop    {r11}        /* restoring frame pointer */  4 bx     lr           /* End of the epilogue. Jumping back to main via LR register */  5  6 /* An epilogue of a non-leaf function */  7 sub    sp, r11, #0  /* Start of the epilogue. Readjusting the Stack Pointer */  8 pop    {r11, pc}    /* End of the epilogue. Restoring Frame pointer from the stack, jumping to previously saved LR via direct load into PC */

Finally, it is important to understand the use of BL and BX instructions here. In our example, we branched to a leaf function by using a BL instruction. We use the the label of a function as a parameter to initiate branching. During the compilation process, the label gets replaced with a memory address. Before jumping to that location, the address of the next instruction is saved (linked) to the LR register so that we can return back to where we left off when the function max is finished.

在BL的時候，將調用指令的下一條指令地址已經保存（鏈接）在了LR寄存器。

The BX instruction, which is used to leave the leaf function, takes LR register as a parameter. As mentioned earlier, before jumping to function max the BL instruction saved the address of the next instruction of the function main into the LR register. Due to the fact that the leaf function is not supposed to change the value of the LR register during it’s execution, this register can be now used to return to the parent (main) function. As explained in the previous chapter, the BX instruction  can eXchange between the ARM/Thumb modes during branching operation. In this case, it is done by inspecting the last bit of the LR register: if the bit is set to 1, the CPU will change (or keep) the mode to thumb, if it’s set to 0, the mode will be changed (or kept) to ARM. This is a nice design feature which allows to call functions from different modes.

BX LR指令中LR寄存器的最後1bit還可以用於切換ARM和Thumb模式。

最後一個關於葉子函數和非葉子函數的例子，gif動態圖很長，可以用一些gif編輯軟件，暫停看。