- Problem description:
- How can you develop a
computer system
that can
load
a program
in pieces
(say chunks of
1 Kbytes)
anywhere in memory
and
execute it ???
- Furthermore, you do not need to load the entire program into the memory to execute the program !!!
This is the result if we have paging
- How can you develop a
computer system
that can
load
a program
in pieces
(say chunks of
1 Kbytes)
anywhere in memory
and
execute it ???
- When you write an assembler program,
you will no doubt has
define variables
and used them like this:
movw r0, #:lower16:x movt r0, #:upper16:x ldr rr, [r0] // r0 contains the address of x .... .... x: .4bytes 7777
- After you
assemble the program,
the assembler would have
replace
the symbolic constant with
a memory address
(= binary number)
Suppose the variable
x: .4byte 7777is stored at memory address 34708.
Then the symbol x is equal to the numeric value 34708
- The assembler program
will be translated as:
Address What is stored in memory at this address ========================================================== 0: movw r0, #:lower16:34708 (in binary) 4: movt r0, #:upper16:34708 (in binary) 8: ldr rr, [r0] // r0 contains 34708 .... .... 34708: 7777 (in binary) --- 4 byte memory for variable x(Each ARM instruction occupies 4 bytes of memory)
- You were ALSO told that
the whole program is
stored in memory,
starting at
address 0:
We keep things simple so focus teaching assembler programming....
- The reality of program execution is very different (and strange)
- Suppose we want to
execute the
same program
but putting (= loading) the
program in a
different location in
memory
Suppose we load the program at memory location 51200 and executed it:
- There will be a problem:
- The memory location
used to store
the variable X is
now:
85908 (= 51200 + 34708)However, since the program is not changed, the program will still be using the address 34708 and it will access memory cells used to store something else !!!
- The memory location
used to store
the variable X is
now: