Stack

1
Stack

• Special data structure
• Last-In, First-Out (LIFO) list
• Pointer to next (empty) place in list top of
  stack
• Base of stack
• Push operation add item to list at top and
  update top
• Pop/Pull operation remove item from top of list
  and update top return value of top item

2
Stack

• Special block of memory
• Base - Usually initialized to a large address
• Stack usually grows toward lower addresses
• Push operation decrement top of stack
• Pop operation increment top of stack
• Top stored in a CPU register called the Stack
  Pointer (SP)
• GPR R1 in CodeWarrior
• SP is like a pointer variable in C it is the
  address of the top of the stack

3
Stack

• Use of a stack
• Temporary storage during program runtime
• Not fixed in size grows and shrinks using push
  and pop operations
• Not typed elements of list not defined as one
  type
• Subroutines (functions, procedures)
• Linkage saving a link back to the caller
  (address of calling routine)
• Data return value, parameters, local variables
• Other saved information
• Managed at the assembly language level
• Rules for compiler, AL programmer

4
Stack

• Example

SP 0x7FFF   SP points to top - next empty
location to push data to
5
Stack

• Example

Push 15   Takes up 1 byte on the stack SP is now
pointing to 0x7FFE (next location to push data to)
 
6
Stack

• Example

Push -3   Takes up 1 byte on the stack SP is now
pointing to 0x7FFD (next location to push data to)
 
7
Stack

• Example

Push 0x1808   0x1808 is a 16-bit value Takes up
two bytes on stack Pushed LSB, then
MSB Why? Little Endian LSB is at higher
address Stack grows toward lower addresses   SP
is now 0x7FFB
 
 
8
Stack

• Example

Pop a byte value   Update SP first by
incrementing, then read value Value read
0x18 SP is now 0x7FFB
 
 
 
9
Stack

• Example

Pop a half-word value   Update SP, read MSB, read
LSB Value MSB, LSB 0x08, 3 0x08,
0xFD 0x08FD SP is now 0x7FFE
 
 
 
 
10
Stack

• Pushing and Popping
• Should be symmetric what goes on, must come off
• When you pop an item from the stack, the value is
  still there in memory (i.e., it is not cleaned
  out)
• Declare local variables in C - initial values are
  whatever was left on the stack
• Stack does not know that a byte is part of a
  32-bit, 16-bit, or 8-bit value
• For PPC, stack grows minimum /- 4 bytes at a
  time
• Word alignment is enforced, where word boundaries
  have addresses with least-significant nibbles of
  0, 4, 8, C
• SP assigned word-aligned addresses only,
  incremented and decremented by a minimum of 4
  bytes

 
 
 
 
11
Stack

• Notes - section 4.6
• Assembly language view of the stack
• Revisit again later
• Selected topics
• Subroutine linkage using Link Register
• Nested subroutines
• Local variables

12
Stack

• Subroutine call and return
• LR Link Register
• Call Do_Tasks
• Return address in main
• RA saved in LR
• Inside Do_Tasks
• Call Do_Calc
• Return address in Do_Tasks
• Saved?
• Need to use stack

main () int j Do_Tasks () Do_Tasks2
() void Do_Tasks () ... fResult
Do_Calc(15) ...   float Do_Calc (float
fNum)
13
Stack
14
Stack

• Local variables
• long lVal
• char byVal
• char szName10
• Total Space 4 1 10 15 bytes
• Decrease SP by 16 to make space
• Why 16 bytes?
• PowerPC is word-aligned
• Need to keep stack pointer at an even multiple of
  4 bytes

15
Stack

• Local variables

   
16
Rules

• EABI Embedded Application Binary Interface
• Specifies the interface between compiled
  application programs and the processor
• API how software is used in applications
• ABI how hardware (or low-level software) is used
  in applications
• Reduce memory usage and optimize execution speed
• Describes conventions for register usage,
  parameter passing, stack organization, small data
  areas, object file, and executable file formats

17
Rules

• Data types and alignment
• All data types are aligned in memory on addresses
  that are a multiple of their size
• Word-aligned Since size is 4 bytes, a word is
  aligned on an address evenly divisible by 4

18
Rules

• Register usage conventions
• Will learn more later about processor registers
  in assembly language programming

Recall GPR Register View in CodeWarrior R
General PurposeF FloatsCR Conditions
19
Rules

• Stack frame
• Organizes or delineates a functions stack space
• Supports parameter passing, register
  preservation, local variables, and code debugging
• Place various data into a stack frame in a
  consistent manner
• Important for functions that call other functions
  (calling chain)
• SP always points to the lowest addressed word of
  the currently executing functions stack frame.

20
Rules

• Stack frame
• New frame is created adjacent to the most
  recently allocated frame lower in memory
  (pushed).
• Creation decrement the SP just once, in the
  function prologue, by the total amount of space
  required by that function
• store-with-update (stwu) instruction insure the
  SP update is an atomic operation that cannot be
  interrupted
• Removed in the function's epilogue by adding the
  current stack frame size to the SP before the
  function returns to the calling function

21
Rules

• Stack frame example

22
Rules

• Stack frame example

LR Link Register, stores return address of caller
23
Rules

• Parameter passing
• For PowerPC processors, it is more efficient to
  pass arguments (parameters) in registers than
  using memory.
• Up to eight scalar values are passed by using R3
  through R10
• Return values are passed back in R3 and R4