CSCI 264 – Assembly Language –
Memory Management – Paging (Chapter 8)
Multitasking OS vs. Batch OS
Batch OS – computer programs are executed in sequence, one after another
Multitasking OS – allows programs to take turns using processor
Task switch – task state segment – holds the frozen state of a task which is not currently
running. Task state segment includes space for all general registers, and a register CR3.
Linux is a multitasking system
Linux is protected OS
Linux uses paging to implement a memory protection.
Paging, Virtual Memory
In a computer system that does not support virtual memory, when a program generates a
memory address it is placed directly on the memory bus which causes the requested
memory location to be accessed.
On a computer that supports virtual memory, the address generated by a program goes
via a memory management unit (MMU). This unit maps virtual addresses to physical
addresses.
The virtual memory is divided into pages. The physical memory is divided into page
frames. The size of the virtual pages and the page frames are the same size – 4K.
The available physical memory is divided into pages: Pages in physical memory are
called page frames.
Each application (process) “lives” inside 4GB space. This space is divided into pages.
4GB corresponds to 1MB of pages: 4GB/4K = 4*2^30/4*2^10 = 2^20 = 1MB
The portion of hard disk used for paging is divided into pages.
To translate the virtual address to the physical address the system is using page tables.
Page tables are divided into pages as well.
Kernel addresses lie in the range from C0000000H to FFFFFFFF ( 3 to 4 GB)
User addresses in the range from 0 to BFFFFFFFH ( 0 to 3 GB)
The basic idea behind Virtual Memory is that the combined size of the program, data,
and stack may exceed the amount of physical memory available for it. The operating
system keeps those parts of the program currently in use in Main Memory, and the rest
on the disk.
Example: program uses the instruction MOV reg, [imm]
Program generates the virtual address.
Virtual address doesn’t go directly to the memory bus,
instead they go to an MMU (Memory Management Unit)
that maps the virtual address onto the physical address
Sizes Table:
1KB = 2^10
2KB = 2*2^10
3KB = 3*2^10
4KB = 4*2^10
5KB = 5*2^10
6KB = 6* 2^10
7KB = 7*2^10
8 KB = 8* 2^10
9KB
10KB
11KB
12KB
1024 bytes
2048 bytes
3072 bytes
4096 bytes
5120 bytes
6144 bytes
7168 bytes
8192 bytes
9216 bytes
10240 bytes
11264 bytes
12288 bytes
Example:
Suppose we have a computer that can generate 16 – bit addresses (from 0 to 2^16-1
=2^6*2^10 -1 = 64KB – 1). These are virtual addresses. Suppose that this computer has
only 32KB of the physical memory. The virtual address space is divided up onto units
called pages – 4KB each page. Corresponding units in the physical memory are called
page frames – the same size – 4KB each page frame.
In our example we have 16 pages and 8 page frames. (64KB / 4KB = 16, 32KB/4KB = 8)
See the picture on the next page to understand the following instructions:
MOV reg, [0]
Virtual address is 0 , this address is sent to MMU. The MMU sees that this virtual
address falls in page 0 ( 0 to 4095 = 4*2^10-1), which according to its mapping is page
frame 2 ( 8KB= 8192 to 12287 = 12KB -1).Virtual address 0 is transformed to the
physical address 8192 and this address was putted onto the bus.
MOV reg, [8192]
Virtual address 8192 belongs to virtual page 2 ( 8192 to 12287) and the corresponding
page frame is 6 ( 24K – 28K)
The physical address is 24K = 24576
MOV reg, [5], virtual address 5 was mapped to the physical address: 8197
Virtual address
space
The physical
address
60K to 64K-1
56K to 60K-1
52K to 56K -1
48K to 52K-1
44K to 48K-1
40K to 44K -1
36K to 40K -1
32K to 36K -1
28K to 32K -1
24K to 28K -1
20K to 24K -1
16K to 20K-1
12K to 16K -1
8K to 12K -1
The physical
page frame
that
x
x
x
x
7
x
5
x
x
x
3
4
0
6
4K to 8K-1
1
4K to 8K -1
0K to 4K-1
2
8K to 12K -1
No address
No address
No address
No address
28K to 32K -1
No address
20K to 24K-1
No address
No address
No address
12K to 16K-1
16K to 20K -1
0K to 4K -1
24K to 28K -1
X means that this virtual page was not mapped to the physical space.
64K (virtual space ) > 32K ( physical space)
In the actual hardware a Present/absent bit keeps track of which pages are physically
present in memory.
Instruction MOV reg, [32780] causes the CPU to trap the OS (operating system). This
trap is called page fault.
32780 belong to the virtual page 8 ( 32K to 36K -1). This page was not mapped to the
physical memory.
At this point, the OS picks a little-used page frame and writes it’s contents back to the
disk. Then, it fetches the page 8 into the page frame just freed.