- The system stack of
ARM consists
the memory locations:
- Starting at the location (= address) given in the register SP
- Ending at the end of memory (in EGTAPI, this address is 503808 (= 0x7b000 (hex))
- The system stack is located
below the
memory display area:
The system stack is always displayed in EGTAPI
When the program starts running, the system stack is always empty
- When values are pushed onto the system stack, then you will see some values appear in the System Stack area
- EGTAPI can
only display
integer values stored
in the system stack
(and that's why we always
pass integer values in
the demo programs and
homework programs)
This simplification makes the implementation of EGTAPI
- In CS255,
you will (or have) learn the
use of:
- Register SP
= stack pointer
SP points to the top of the system stack
- The value at the top of the stack is the first value displayed in the system stack area
- Register FP
= frame pointer
FP points to the middle of a data structure called a procedure activation record or stack frame
- The value in the stack that is pointed to by the register FP is displayed in magenta (this value is accessed using 0 offset from FP)
We use FP to access the parameters and local variables of a function
So the location of the value in the stack that is pointed to by the register FP (in magenta) can help you find the offset of other values easily !!!
- Register SP
= stack pointer
- This example shows
how to
read the
information stored in the
system stack:
- This figure shows an
empty stack:
- This stack
contains one value (= 5):
- This stack
contains two values:
5644 and 5
The value at the top of the stack is 5644
- This stack
contains 6 values:
Notice the value 12345 is displayed in magenta:
- This value (= 12345)
is pointed to by
FP
So this value (= 12345) in the stack is accessed by using the addressing mode 0 offset from FP
Knowing this, you can now figure out how to access the other values in the stack easily:
System stack: How to access this value 5726 -12 + SP 4 -8 + SP 0 -4 + SP 12345 <--- SP 0(A6) 5644 4 + SP 5 8 + SP - This value (= 12345)
is pointed to by
FP
- This figure shows an
empty stack: