Module 8: Atmega32 Stack & Subroutine. Stack Pointer Subroutine Call function

Transcription

1 Module 8: Atmega32 Stack & Subroutine Stack Pointer Subroutine Call function

2 Stack

3 Stack o Stack is a section of RAM used by the CPU to store information temporarily (i.e. data or address). o The CPU needs this storage because there are only limited number of registers. o Usually, a stack is a Last In First Out (LIFO) buffer containing a number of data items usually implemented as a block of n consecutive bytes, words or long words in memory. o The address of the last data item placed into the stack is pointed to by the Stack Pointer (SP). o Application of stacks: o Temporary storage of variables o Temporary storage of program addresses o Communication with subroutines

4 How Stack Are Accessed In The AVR o Since the stack is a section of RAM, so it needs a register inside the CPU to point to it (the stack) called Stack Pointer (SP) register. o The SP is 16-bit wide and is implemented as two registers that are SPL (the low byte of SP) and SPH (the high byte of SP). o The SP must be wide enough to address the RAM, thus: o AVRs > 256 bytes of memory, the SP is made of two 8-bit registers (SPL and SPH). o AVRs < 256 bytes, the SP is only made of SPL.

5 How Stack Are Accessed In The AVR o The are two types of operations involving SP: o PUSH the storing of CPU information such (i.e. the program counter on the stack) o POP the loading of stack contents back into a CPU register.

6 Pushing On The Stack o The SP always points to the top of the stack. o As we push data to the stack, the data are saved where SP points to, and the SP is decremented by one. o Other words, SP is decremented when data is pushed onto the stack. o To push a register onto stack we use PUSH instruction. o Example: to store value of R10 onto SP

7 Popping From The Stack o Popping the contents of the stack back into a given register is the opposite process of pushing. o When POP instruction is executed, the SP is incremented and the top location of the stack is copied back to register. o This is due the stack is LIFO memory. o To retrieve a byte of data from stack we use the POP instruction. o Example: to copies value of R16 from SP

8 Initializing The SP o When AVR is powered up, the SP register contains the value 0, which is the address of R0. o Thus, need to initialize SP so that it points to somewhere in the internal SRAM. o Since in AVR, the stack grows from higher memory location to lower memory location (i.e. push onto stack, the SP decreases), so it is common to initialize SP to the uppermost memory location. o Different AVRs have different amounts of RAM, thus in the AVR assembler, RAMEND represents the address of the last RAM location. o Thus to initialize SP to the last memory location, simply load RAMEND into SP.

9 Initializing The SP o Since, SP is made of two registers, thus we load: o SPH load high byte of RAMEND into SPH o SPL load low byte of RAMEND into SPL

10 Push & Pop Example o Example: the example shows the stack and SP and the registers used after the execution of each instruction.

11 Push & Pop Example

12 Push & Pop Example

13 Push & Pop Example Address Code ORG LDI R16,HIGH(RAMEND) R20: R21: $10 $00 $00 $20 R22: R0: $30 $00 $ OUT SPH,R16 LDI R16,LOW(RAMEND) OUT SPL,R LDI R20,0x LDI R21, 0x20 SP LDI R22,0x30 PUSH R20 $ PUSH R21 $ PUSH R22 $30 000A POP R21 000B POP R0 000C POP R20 000D L1: RJMP L1 Memory

14 SP Upper Limit o As mentioned earlier, we can define the stack anywhere in the general purpose memory. o In AVR, the stack can as big as its RAM. o Note that we must not define the stack in the register memory, nor in the I/O memory. o So, the SP must be set to point above 0x60. o Stack content is important as it used to store information when we calling a subroutine. o Stack overflow will occur when the content of the stack is exceed the upper limit.

15 Subroutine

16 Subroutine o Subroutine is a sequence of, usually, consecutive instructions that carries out a single specific function or a number of related functions needed by calling programs. o A subroutine can be called from one or more locations in a program. o Why Subroutines? To use the same set of instructions sequence.

17 Subroutine: CALL Instruction o When a subroutine is called, the processor first saves the address of the instruction just below the CALL instruction on the stack and then transfer control to that subroutine.

18 Subroutine: CALL Instruction o CALL is long call to subroutine. o It calls to a subroutine within the entire Program memory. o The return address (to the instruction after the CALL) will be stored onto the Stack. Flag affected : None CPU cycle: 4, 5

19 o Example: Subroutine: CALL Instruction MOV R16,R0 ;Copy r0 to r16 CALL check ;Call subroutine NOP check: CPI R16, $42 ;Check if r16 has a special value BREQ error ;Branch if equal RET ;Return from subroutine error: RJMP error ;Infinite loop

20 Subroutine: RET Instruction o When the execution of the function is finishes, the RET instruction at the end of the subroutine is executed, the address of the instruction below the CALL is loaded into the PC, and the instruction below the CALL instruction is executed.

21 Subroutine: RET Instruction o RET is return from subroutine. o The return address is loaded from the Stack. Flag affected : None CPU cycle: 4, 5

22 o Example: Subroutine: RET Instruction MOV R16,R0 ;Copy r0 to r16 CALL check ;Call subroutine NOP check: CPI R16, $42 ;Check if r16 has a special value BREQ error ;Branch if equal RET ;Return from subroutine error: RJMP error ;Infinite loop

23 Subroutine: Role of Stack o Stack is used to temporarily store address when CPU execute the CALL instruction. o This is how the CPU know where to resume when it return from the called subroutine. o Hence, we must very careful when manipulating the stack contents. o For AVRs whose program counter is not longer than 16 bits (e.g. ATmega32), the value of the program counter is broken into 2 bytes. o The higher byte is pushed onto the stack first, and then the lower byte is pushed. o For AVRs whose program counter is longer than 16 bit but shorter than 24 bit, the value of the program counter is broken up into 3 bytes. o The highest byte is pushed first, then the middle byte is pushed, and finally the lowest byte is push.

24 Why use subroutines? Subroutine o Code re-use o Easier to understand code (readability) o Divide and conquer Complex tasks are easier when broken down into smaller tasks o Simplify the code debugging process. How do we call a subroutine in assembly? o Place the parameters somewhere known o JSR or BSR to jump to the subroutine o RTS to return Examples of subroutines: o Convert binary to ASCII o Convert Fahrenheit to Celsius o Perform output to 7-segment display

25 Subroutine: RCALL Instruction o RCALL is Relative call to subroutine. o In which, it relatively call to an address within PC-2K+1 and PC+2K (words). o The return address (the instruction after RCALL) is stored on Stack. Flag affected : None CPU cycle: 3, 4

26 Subroutine: RCALL Instruction o Not much different between CALL and RCALL. It just address CALL can be anywhere within 4M address ($0 - $3FFFFF), while RCALL must be within 4K range. o Example: RCALL routine ;Call subroutine routine: PUSH R14 ;save r14 on the stack POP R14 ;Restore r14 RET ;Return from subroutine

27 Subroutine: ICALL Instruction o ICALL is Indirect call to subroutine. o In which, it call to a subroutine within entire 4M (words) Program memory that specifies by the Z register. o The return address (to the instruction after the ICALL) is stored on Stack. Flag affected : None CPU cycle: 3, 4

28 Subroutine: ICALL Instruction o Example: MOV R30, R0 ICALL ;Set offset to call table ;Call routine pointed to by r31:r30

29 Subroutine Example o Example: write a program to count up from $00 to $FF and send the count to Port B. Use one CALL subroutine for sending the data to Port B and another one for time delay. Put time delay between each issuing of data to Port B.

30 Subroutine Example

31 o What happen to the SP: Subroutine Example