Question

What is Stack Pointer (SP) register and its size in AVR? Before initializing, its default value is $0000. (i) Is it fine to keep SP register with this default value, why/why not? (ii) If we want to initialize the SP register with last RAM location, what will be the value in Hexadecimal for a RAM having total size of 16KB?

Answer 1

Stack Pointer = The Stack Pointer (SP) register is used to indicate the location of the last item put onto the stack. When you PUT something ONTO the stack (PUSH onto the stack), the SP is decremented before the item is e item is placed on the stack. When you take something OFF of the stack (PULL from the stack), the SP is incremented after the item is e item is pulled from the stack. Before you can use a stack you have to initialize th itialize the SP to point to one value higher than the highest memory location in the stack. For the HC12 use a blo e a block of memory from about $3B00 to $3BFF for the stack. For this region of memory, initialize th itialize the stack pointer to $3C00. Use LDS (Load Stack Pointer) to ter) to initialize th itialize the stack pointer. The stack pointer is initialized o itialized only one time in e time in the program.

SP register size is 8 or 16 bit

i) Default value of the stack pointer is 0x0000. Therefore programmers have to initialize a stack before use it. z The stack pointer always points to the top of the stack z Definition of the top of the stack varies: z the location of Last-In element; E.g, in 68K z the location available for the next element to be stored E.g. in AVR.