- Before we solve the Paging Problem:
- 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 !!!
Let's solve a simpler problem:
- Develop a computer system that can load a (whole) program anywhere in memory and execute it
- Develop a
computer system
that can
load
a program
in pieces
(say chunks of
1 Kbytes)
anywhere in memory
and
execute it ???
- Program address
- Program address =
an address used by an
instruction in
a program
Example:
Note: the address to identify an assembler/machine instruction is (also) a program address
- Program address =
an address used by an
instruction in
a program
- Memory address:
- Memory address =
the address used to
identify a
specific (byte) location
in memory
Example:
- Memory address =
the address used to
identify a
specific (byte) location
in memory
- The relationship between
memory address <----> program address
depends on the placement of a program into the computer memory
- So far in your
computer science career,
you have only seen
the following
trivial placement:
- Place the
whole program in
the computer memory
starting at the
memory location 0:
This placement will result in the following mapping between program address and memory address:
memory address = program address
- Place the
whole program in
the computer memory
starting at the
memory location 0:
- In general,
the memory address is
a complex function of
the program address:
memory address = f(program address)
- Consider the following
addressing technique:
- We add a
base register inside the
CPU
- Whenever the
CPU sends
an address to the
memory,
we add the
value of
the base register to
the address:
- We add a
base register inside the
CPU
- Consider now the
execution of
the instruction
move.l 4000, D0when we store the value 10000 in the base register:
The program will now work without having to change anything !!!
- In fact, in this example,
you see that:
memory address used = program address + 10000This is the underlying mechanism of the paging technique !!!
- We can use this
"trick" on
pieces of
a single program !!
- In paging, we cut a program up
into pages of
equal size
- Each page is
stored in
different location in
memory
- We must define a mapping for each page of the program !!!
- Example:
- Suppose we have a program that has 2000 bytes:
- We partition the
program into
2 (equal size) pieces:
- We store each fragment
separately:
- We must provide 2 offsets:
- One for addresses that are between 0 - 999
- One for addresses between 1000 - 2000
Example:
- Suppose we have a program that has 2000 bytes:
- The result is a
very flexible system where:
- You can put a page of a
program anywhere
in memory
- The program does not need to have all its pages in memory simultaneously to run !!!
- You can put a page of a
program anywhere
in memory