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Lecture 5: The Stack

Lecture 5: The Stack. EEN 312: Processors: Hardware, Software, and Interfacing. Department of Electrical and Computer Engineering Spring 2014, Dr. Rozier (UM). COURSE PLAN FOR TODAY Midterm I The Stack and Memory. MIDTERM I. Midterm I. Thursday, February 13 th ! Two weeks away

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Lecture 5: The Stack

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  1. Lecture 5: The Stack EEN 312: Processors: Hardware, Software, and Interfacing Department of Electrical and Computer Engineering Spring 2014, Dr. Rozier (UM)

  2. COURSE PLAN FOR TODAY Midterm I The Stack and Memory

  3. MIDTERM I

  4. Midterm I • Thursday, February 13th! • Two weeks away • Covers the following Material: • Chapters 1 – 3 • Laboratory 1

  5. THE STACK AND MEMORY

  6. A Call Chain Image by David Thomas

  7. Procedure Calling • Place parameters for procedure in registers • Transfer control to procedure • Procedure acquires storage • Procedure performs function. • Procedure places return value in appropriate register • Return control

  8. The Stack • Region of memory managed with stack discipline. • Accessed with pushand pop

  9. The Stack

  10. Stack Frames • Stack frames “belong” to a procedure. • Store local variables here (they go out of scope automatically) • Can communicate with other procedures with a stack frame.

  11. Procedures and Register Use • What if we want to use some registers in the procedure? • Caller could have data in it!

  12. Stack Discipline and ABI • Stack Discipline is important • Define an Application Binary Interface • How should procedures communicate? How should the be called? • Consistency, following standards, is the key.

  13. Conventions and Discipline Caller Callee Callee saves temporary values in its frame before using. Callee restores values in its frame after using. • Caller saves temporary values in its frame before the call. • Caller restores values in its frame after the call.

  14. Stack Frames • Contents • Local variables • Return information • Temporary space • Management • Space is allocated when entering • Set-up code • Deallocated when returning • Finish code Frame Pointer Stack Pointer

  15. Example frame ptr foo() { … bar() … } foo stack ptr foo()

  16. Example foo() { … bar() … } foo bar() { … baz … baz() … } frame ptr bar stack ptr foo() bar()

  17. Example foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar frame ptr baz stack ptr foo() bar() baz()

  18. Example foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar baz() { … baz() … } baz frame ptr baz stack ptr foo() bar() baz() baz()

  19. Example foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar baz() { … baz() … } baz baz() { … baz() … } baz frame ptr baz stack ptr foo() bar() baz() baz() baz()

  20. Example foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar baz() { … baz() … } baz frame ptr baz stack ptr foo() bar() baz() baz() baz()

  21. Example foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar frame ptr baz stack ptr foo() bar() baz() baz() baz()

  22. Example foo() { … bar() … } foo bar() { … baz … baz() … } frame ptr bar stack ptr foo() bar() baz() baz() baz()

  23. Example foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar frame ptr baz stack ptr baz() foo() bar() baz() baz() baz()

  24. Example foo() { … bar() … } foo bar() { … baz … baz() … } frame ptr bar stack ptr baz() foo() bar() baz() baz() baz()

  25. Example frame ptr foo() { … bar() … } foo stack ptr baz() foo() bar() baz() baz() baz()

  26. What is the frame pointer used for? • Given a caller and a callee, the callee will begin with: • fp = sp • A callee will use the stack to: • Save the current sp, lr, and fp before calling a function • Store the arguments for, and return value space before calling a function • Save used registers before calling a function • Store variables local to the procedure

  27. foo Frame pointer stack ptr frame ptr

  28. foo Frame pointer frame ptr a = 0x00 stack ptr PUSH

  29. foo Frame pointer frame ptr a = 0x00 b = 0xFC PUSH stack ptr

  30. foo Frame pointer frame ptr a = 0x00 b = 0xFC PUSH c = 0x87 stack ptr

  31. foo Frame pointer frame ptr a = 0x00 b = 0xFC PREPARETO CALL c = 0x87 stack ptr

  32. foo Frame pointer frame ptr a = 0x00 b = 0xFC PUSH c = 0x87 r0 lr fp a0 ret stack ptr

  33. foo Frame pointer a = 0x00 b = 0xFC RETURNFROMCALL c = 0x87 r0 lr fp a0 ret stack ptr

  34. foo Frame pointer frame ptr a = 0x00 b = 0xFC POP! c = 0x87 stack ptr

  35. 5 minute paper • Take 5 minutes to think about these tasks. Determine why a frame pointer is helpful

  36. foo Frame pointer frame ptr a = 0x00 +4 b = 0xFC +8 c = 0x87 +12 stack ptr

  37. foo Frame pointer frame ptr a = 0x00 • The frame pointer gives usa base address from which tooffset in our frame for localstorage that will automatically go out of scope. +4 b = 0xFC +8 c = 0x87 +12 stack ptr

  38. Recursion foo() { … bar() … } foo bar() { … baz … baz() … } baz() { … baz() … } bar baz() { … baz() … } baz baz() { … baz() … } baz frame ptr baz stack ptr foo() bar() baz() baz() baz()

  39. Recursion • Stack frames give each function private storage • Saved registers • Local variables • Return portions • Recursion is handled without special consideration. • Following stack discipline and call/return pattern is critical

  40. PUSH and POP • PUSH<cond> {reglist} • POP<cond> {reglist} • Really pseudo-instructions! • PUSH – STMDB • Store multiple registers, decrement address before access. • POP – LDMIA • Store multiple registers, increment address after access.

  41. foo STMDB r0 = 0x87 r0 = 0x00 r1 = 0x00 r2 = 0xFC stack ptr r2 = 0x00 sp = ??

  42. foo STMDB r0 = 0x87 r0 = 0x00 r1 = 0x00 r2 = 0xFC r2 = 0x00 stack ptr sp = ??-4

  43. foo STMIA r0 = 0x87 r0 = 0x00 r1 = 0x00 r2 = 0xFC r2 = 0x00 r3=0x87 stack ptr sp = ??-4

  44. foo STMDB r0 = 0x87 r0 = 0x00 r1 = 0x00 r2 = 0xFC r2 = 0x00 r3=0x87 stack ptr sp = ??-4

  45. foo LDMIA r0 = 0x87 r0 = 0x00 r1 = 0x00 r2 = 0xFC r2 = 0x00 r3=0x87 stack ptr sp = ??-4

  46. foo LDMIA r0 = 0x87 r0 = 0x00 r1 = 0x00 r2 = 0xFC stack ptr r2 = 0x00 r3=0x87 sp = ??

  47. Multiple Ways to Handle the Stack • Stack Pointer can • Point to the lass occupied stack (Full stack) • (needs pre-decumenting) • Point to the next address to be occupied (Empty stack) • (needs post-decrementing) • Stack type can be given as a postfix to the instruction: • STMFD/LDMFD • STMFA/LDMFA • STMED/LDMED • STMEA/LDMEA

  48. RISC Mentality • There are no special functions for stacks! • Let’s look at stack instructions: • STMDA/LDMDA

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