The StackKeeping Track of Function Calls

2

The Stack Pointer

• The CPU usually contains a special register, called the Stack Pointer (SP)

• It is used by the CPU whenever it needs to push something onto, or pop something off, the stack

3

Push and Pop

push

1. Decrement SP

2. Write value to new memory location that SP points to

pop

1. Read memory location pointed to by SP

2. Increment SP

4

Or in reverse…

Some CPUs reverse steps 1 and 2 so that:

• push write then decrement

• pop increment then read

On LC-3

• PUSH ADD R6, R6, #-1STR R0, R6, #0

POP LDR R0, R6, #0

ADD R6, R6, #+1

5

6

Setting up the Stack

• The CPU assumes that the SP is pointing to an area of memory that has been reserved for stack use

• It is therefore necessary for the programmer to choose where to put the stack, and to initialise the SP to the appropriate address before using it!

• For a decrement-before-write CPU, the appropriate address is one higher than the highest address in the reserved area

7

Stack Setup Example

0FF

100

1FE

1FF

200

Chosen stack area

SP

8

Stack Operation

zz

yy

xx

ww SP

zz

yy

dd

ww

zz

ee

dd

ww

zz

ee

dd

ww

SPSP

SP

Empty stack push eepush ddpop

(returns ee)

9

Stack Overflow

• If you push more data onto the stack than will fit into the area you’ve allocated to it, then you have created a stack overflow

• This is potentially disastrous, since you may overwrite (and thus destroy) important data, or instructions, outside the stack area

10

Stack Overflow

• Advanced computers have special software to detect stack overflow, but simple ones do not

• On a simple computer, you have to design the program to avoid it

• Now you know what the ‘stack overflow’ error message you may have seen means

• What might be a common cause?

11

Test Yourself!

• What is meant by Stack Overflow?

• What is the Stack Pointer (SP) used for?

• Explain push and pop with regards to the stack