1. The Stack

2. The Stack ARMSim memory space: 0x11400 + Unused 0x09400-0x11400 Stack
Heap
0x?????-0x01400
Data
0x x?????
Text
0x x01000
Reserved

3. Stack possibilites How to use the space in stack? Grow up
Keep track of top item in stack (Full Ascending)
Keep track of first empty slot (Empty Ascending)

4. Stack possibilites How to use the space in stack? Grow down
Keep track of top item in stack (Full Descending)
Keep track of first empty slot (Empty Descending)

5. Stack possibilites ARM Convention : Full Descending
R13 Holds "top" of stack

6. Push Recipe Subtract 4 from sp … 81818181 0x113F4 0x113F8 0x113FC
????????? sp
0x11400

7. Push Recipe Subtract 4 from sp … 81818181 0x113F4 0x113F8 0x113FC
????????? sp
0x113FC

8. Push Recipe Subtract 4 from sp STR data … 81818181 0x113F4 0x113F8
0x113FC AA
0x11400
????????? sp
0x113FC

9. Push Recipe Subtract 4 from sp … 81818181 0x113F4 0x113F8 0x113FC AA
????????? sp
0x113F8

10. Push Recipe Subtract 4 from sp STR data … 81818181 0x113F4 0x113F8 BB
0x113FC AA
0x11400
????????? sp
0x113F8

11. Pop Recipe LDR data … 81818181 0x113F4 0x113F8 BB 0x113FC AA 0x11400
????????? sp
0x113F8

12. Pop Recipe LDR data Add 4 to sp … 81818181 0x113F4 0x113F8 BB 0x113FC
????????? sp
0x113FC

13. Pop Recipe LDR data … 81818181 0x113F4 0x113F8 BB 0x113FC AA 0x11400
????????? sp
0x113FC

14. Pop Recipe LDR data Add 4 to sp … 81818181 0x113F4 0x113F8 BB 0x113FC
????????? sp
0x11400

15. Push / Pop Always tear down stack in reverse order Push A Push B Pop B
Time

16. Machine Push/Pop Common need : machine instructions
Store Multiple to Full Descending use sp and update it (!) store r1 and r2

17. Machine Push/Pop Common need : machine instructions
Load Multiple from Full Descending use sp and update it (!) load r2 then r1 (reverse order)

18. Pseudo Instructions
Easier pseudinstructions: PUSH {register list} POP {register list}

19. Function Calls

20. Assembly Function
Assembly Functions are branches

21. Assembly Function Assembly Functions are branches
Need to know where to return to when done!

22. Assembly Function BL label Branch and Link Branch (change PC)
But store Store next instruction address in R14 (lr = link register)

23. Assembly Function BL label Branch and Link Branch (change PC)
But store Store next instruction address in R14 (lr = link register)

24. Assembly Function
Return
Copy lr back into pc

25. Assembly Function
Return
Copy lr back into pc

26. UDiv
Unsigned division function
Parameters in r0, r1
Results in r0, r1

27. UDiv
Calling from main
Parameters in r0, r1
Results in r0, r1

28. Using Functions GetTime program Get system time
Do lots of divisions to get Hour, Min, Sec

29. Calling Conventions

30. Calling Convetions Calling Convention
Agreement for how subroutines work
How we pass info
What registers we can use …
Differs between platforms, compilers, etc…

31. ARM Conventions ARM Register Conventions r0-r3 r4-r9 r10-r15
Register Range
Use
r0-r3
-Passing parameters & results
-Temporary values
r4-r9
-Storing information
r10-r15
Special purpose

32. ARM Conventions ARM Register Conventions r0-r3 r4-r9 r10-r15
Register Range
Use
Caller
Function
r0-r3
-Passing parameters & results
-Temporary values
These may get destroyed. Store/restore values if you care about them!!!
Change these all you want…
r4-r9
-Storing information
No worries…
If you want to use these you MUST store/restore data that was there!!!
r10-r15
Special purpose
Only use for intended purpose

33. ABS Absolute Value Does what it wants with r0-r3
Makes use of r4 and r5 – must save

34. ABS Main that calls abs must store any r0-r3 used across function call
r4+ are safe

35. Smarter ABS
If function ONLY uses r0-r3 to do work, nothing to save:

36. Smarter ABS
If main only uses r0-r3 between function calls, nothing to save

37. Nested Calls

38. Nested Calls
Real functions often call other functions:

39. Nested Calls Multiple "return addresses" needed Proc_A needs address D
Proc_B needs address B

40. Nested Calls Multiple "return addresses" needed
Proc_A needs address D
Proc_B needs address B
Store return addresses on stack

41. Calling Convention Update
If function call other functions:
Save/restore your return address (lr) to stack with registers r4+

42. Process Review Subroutine Call (done by the caller):
Push any of  r0-r3 that are in use
Put argument values into r0-r3
Call the subroutine using bl

43. Process Review Subroutine Prolog (done to start subroutine):
If we want to use r4+ push them
If we will call other functions, push lr also
Get parameters from r0-r3

44. Process Review
Subroutine Body 
Use r0-r3 and any from r4+ you saved

45. Process Review Subroutine Epilog (done at end of sub):
Put returned values in r0-r3
Pop from the stack anything you saved (r4+, lr)
Return by moving lr to pc

46. Process Review Regaining Control (done by the caller):
Get results from r0-r3
Pop any old r0-r3 values you stored before call