Microcontroller Systems: Stack

last updated: 06/06/19

Song of this chapter: Yosemite Sam > The Looney Tunes Show Blow The Stack Song Boomerang > Blow My Stack!

Stack (wiki)

The stack is a part of the memory always needed if we use subroutines (functions) or interrupts (interrupt service routines (ISR)) in our program.

To explain the working of the stack we will first look at the mechanism how the hardware executes a program.

Program Counter

The Program Counter PC in our ATmega328 microcontroller is a a 14 bit processor address register that is apart of all central processing unit (CPU) and indicates where a computer is in its program sequence. The PC holds (stores) the address of the next instruction to be executed (points to the next instruction). The PC is not accessible directly.

Normally the PC is incremented after fetching an instruction. But if we have jump (jmp), relative jump (rjmp) or conditionally jump instructions (called branch instructions, e.g. brne (branch if not equal)), the PC is loaded with the address of the label we want to jump. Let's look at an example:

program counter

In our previous Assembly language example, we used a rcall instruction to jump to our DELAY subroutine (function). Why didn't we use a jmp instruction instead?

Let's look at a modified example:

jump vs call

Call and return

This can not work! The first jump works because the subroutine brings us back to the main program, one instruction behind the initial jump (red arrows). The second jump (green) will not work, because the subroutine doesn't bring us back to the instruction behind the jump instruction (dashed green arrow) but to the label BACK and so causing faulty endless loop.

So why does it work with a call instruction?

The rcall (relative call) instruction (or call instruction) and the ret (return) instruction are super instructions, doing more things at the same time. The rcall works only in combination with the ret instruction.

The rcall instruction first saves the return address (address of the instruction following the rcall) in SRAM memory. Only after doing this the jump is executed. The ret function fetches automatically the return address from the SRAM memory and jumps the to this address.

The part in SRAM where the return addresses are stored is called the stack. The stack is in great parts managed by the hardware. There exists a 2 byte SPR register called the stack pointer. This register holds the address where return addresses and variables are stored in SRAM. The stack pointer is decremented and incremented automatically by the hardware, but the initialisation of the stack pointer must be done by the programmer! This is obligatory if we use subroutines (rcall, ret), interrupts (reti, return from interrupt) or push and pop instructions (see below).

So it is possible to locate the stack at will in SRAM. But we have to consider an important fact. The stack pointer is decremented when storing data on the stack (red arrow). For the SRAM data it is the contrary. Addresses are normally incremented when storing normal data to SRAM (green-blue arrow). So the only logically start address for the stack is the highest SRAM address (RAMEND).

memory atmega328 SRAM data

An initialisation of the stack pointer in assembler would look like this:

    ; initialise the stack pointer
    ldi     r16,HIGH(RAMEND)       ;RAMEND = 0x08FF for ATmega328
    out     SPH,r16
    ldi     r16,LOW(RAMEND)
    out     SPL,r16

The rcall instruction saves the return address (2 byte) on the stack. Automatically the the stack pointer is decremented the byte is saved. So an rcall decrements the stack pointer by two. The ret instruction retrieves the return address and increments the stack pointer by two.

After the call and execution of a subroutine (function, method), the stack is again empty, even if the last address is still saved on the stack. The stack pointer is again on it's initial position.

stack lifo

Push and pop

Beside rcall and ret we have two other instructions to work with the stack. With the push and pop instructions we can save working registers to the stack. The push instruction saves the content of a working register to the stack and decrements the stack pointer. The pop instruction retrieves the content from the stack and saves it to a working register and increments the stack pointer.

The name "stack" comes from the analogy to a set of physical items stacked on top of each other. We can easily take an item off the top of the stack. To get an item deeper in the stack we have to take off multiple other items first.

So the stack works with the [LIFO method] (https://en.wikipedia.org/wiki/Stack(abstractdata_type)), (Last In, First Out)p

stack lifo

This can be easily seen in our subroutine. We need 4 working register r26-r29 (XL, XH, YL, YH) inside our DELAY subroutine to kill time. But the same register could be used by our main program. So we save the content of these register to the stack when entering the subroutine. Inside the subroutine we can now use the register as local variables at will. Before getting back to the main loop, we restore the 4 register and the main loop does not notice the register were used inside the subroutine.

The order of pushing and poping must be reversed!

    DELAY:  push    XL              ;save the 4 used (global) register to stack, so
            push    XH              ;that they are not changed by the subroutine
            push    YL              ;the 2 double reg. X and Y (16 bit) are now free
            push    YH              ;for local use
            pop     YH              ;recover 4 used (global) register from stack
            pop     YL
            pop     XH
            pop     XL
            ret                     ;return to main loop (return address on stack)

Here a little example how the stack behaves with one subroutine (SR1) and 3 local variables (ZH, ZL abd r16):

stack example

Global and local variables

A local variable is only valid inside a piece of code (.e.g. subroutine) but can't be accessed by the main program or other subroutines or interrupt service routines. A local variable can also only local to the main program.

A global variable can be accessed from everywhere.

Avoid collision of stack and heap

This is a real danger when programming in Assembly language. If we forget one push or pop, the stack pointer will possibly address the complete SRAM and so overwrite the data memory, the SPRs and the GPR causing the program to crash. This is called a stack buffer overflow.

Also if we don't take care of our data memory, the data could overwrite the stack, and so falsify e.g. return addresses, causing the program to crash.

The stack may also be a security risk because it stores return addresses. If a virus program gets the return address from stack it can inject own code and run after this the originally program, so the execution of the virus code is not noticed.

"Just do it" S1:

Here is the list file of a program with a nested loop. Fill the blank sheet (copy and print it) with the contents of the register and return addresses step by step. Mark the position of the stack pointer and fill in the stack pointer addresses as shown above.

         ;       main loop
         002E   E111  ldi      r17,0x11        ; initialise r17
         002F   E222  ldi      r18,0x22        ; initialise r18
         0030   D002  rcall    SR1             ; call subroutine SR1
         0031   0000  nop                      ; no operation
         0032   CFFB  rjmp     MAIN            ; endless loop
         ;       subroutine SR1
    SR1: 0033   931F  push     r17             ; save main variables
         0034   932F  push     r18
         0035   E313  ldi      r17,0x33        ; use r17 locally in SR1
         0036   D003  rcall    SR2             ; call subroutine SR2
         0037   912F  pop      r18             ; restore main register
         0038   911F  pop      r17
         0039   9508  ret                      ; return to main
    SR2: 003A   931F  push     r17             ; save SR1 variables
         003B   E414  ldi      r17,0x44        ; use r17 locally in SR2
         003C   911F  pop      r17             ; restore SR1 register
         003D   9508  ret                      ; return to SR1

stack blank sheet

"Just do it" S2:

Interesting links: