为什么我们在汇编中添加一个值来初始化堆栈指针（R7）？

Question

I'm currently new into assembly. I know that we first need to allocate the dimension of the stack (in the example below is 1000). However I struggle to understand why we should add a value (in this case #999) to initialize the stack pointer. Here's the pseudo-assembly:

STACK: .RES 1000

MOV #STACK, SP
ADD #999, SP 

Answer 1

To allocate space on the stack, the stack pointer is decremented (ie the stack grows down). Thus, in order to use the buffer STACK for the stack, the stack pointer has to initially point to the end of STACK so decrementing the stack pointer makes it point to new parts of STACK .