Week 13: Stack Frame & Passing Parameters Flashcards
Computer programs and subroutines consist of
Data elements and procedures that operate on the data elements
The scope of a variable defines the
Range of its visibility or accessibility within a program
Global variables are visible…
from the moment they are loaded into memory to the moment the program stops running
Local variables and parameters are visible…
Within that procedure but invisible outside the procedure
When a language invokes a procedure , it is said to…
Activate the procedure
With each invocation of a procedure, there is an…
Activation record
Activation records contain…
All necessary information to execute the procedure
- Parameters
- Local variables
- Return address
The activation record is also known as the…
Frame
After an activation record has been used…
Executing a return from procedure deallocates or frees the storage taken up by the record
The stack provides…
A mechanism for implementing the dynamic memory allocation
The stack-frame is a region of
Temporary storage
At the beginning of a subroutine, the temporary storage will be
Pushed onto the stack
At the end of a subroutine, temporary storage will be
Popped from the stack
The two pointers associated with stack frames are
Stack Pointer, SP (r13)
Frame Pointer, FP (r11)
What register is the Stack Pointer?
r13
What register is the Frame Pointer?
r11
What is in register 11?
The Frame Pointer
What is in register 13?
The Stack Pointer
The stack pointer always points to the
Top of the stack
The frame pointer always points to the
Base of the current stack frame
The _____ may change during procedure execution, but the ______ will not
The stack pointer may change during procedure execution, but the frame pointer will not
It is strongly recommended that data in the stack frame be accessed through the
Frame Pointer
Reserving memory in the stack frame is done by
ADDing / SUBtracting x-bytes onto the Stack Pointer
Operations on the stack are ___ meaning….
Balanced, meaning if you put something onto the stack you have to remove it
Does arm have native link and unlike instructions?
NO
To create a stack frame you can…
- Push the old frame pointer onto the stack to save its value
- Make the frame pointer point to the bottom of the new stack frame
- move the stack pointer by d-bytes to create a local workplace
At the end of a subroutine call the stack frame is collapsed by
MOV sp,fp ;restore the stack pointer
LDR fp,[sp] ;restore old frame pointer from the stack
ADD sp,sp,#4 ;move stack pointer down 4 bytes to
;restore stack
To implement a program using the SP, FP, and LR…
What does a function look like that is being called by a main area that uses BL and has parameters:
If you store multiple the FP anf the LR, which will be put onto the stack first? If FD?
STMFD sp!, {fp, lr}
LR
Because it is FD, we are starting at higher memory addresses first and then descending since LDM and STM store the highest number register in the higher memory address / lowest numbered register in the lowest memory address
To call a subroutine the following steps need to be performed…
Parameters need to be passed from the caller to the subroutine (can be done via the stack)
The address of the instruction after the calling instruction needs to be saved BEFORE branching (can be done using BL or the stack or both)
Inside the subroutine we must
- Push all register values that are going to be used in the subroutine onto the stack
- Make the FP point to the bottom of the frame by copying the value of the SP to the FP
- Create a space inside the stack for local variables
- Perform subroutine instructions where local variables and parameters are relative to the FP
- At the end of the the subroutine, deallocate all created local variables
- Pop all pushed registers but use the PC instead of LR to return
Finally in the calling program, all pushed parameters need to be popped
You can pass a parameter to a subroutine by..
Value or by Reference
What does a traditional call/return mechanism with parameters look like?
