Lecture 13:Examples of Memory

Management

MVS

• Multiple Virtual Storage Operating System (IBM)

• The byte index specifies one of 4KB within a page; the page index specifies one of 256 pages within a segment; the segment index identifies one of 2048 user-visible segments (231 bytes –2GB virtual storage)

Segment index Page index Byte index

0 11 12 19 20 31

Page table originPage table length

0 25 26 27 28 31

I C

IBM System/370

• Uses a two-level memory structure and refers o the two levels as segments and pages

• Page size: 2KB or 4KB

• Segment size (fixed): 64KB or 1MB

• 24 – bit address structure BUT

SYSTEM 370/XA• XA: extended addressing

• 31 – bit address scheme

Segment index Page index Byte index

0 11 12 19 20 31

Page table originPage table length

0 25 26 27 28 31

I C

System 370/ESA

• Latest version: Enterprise System Architecture

Page frame number

0 19 21 22 31

I P

Address space identifier

Segment number Page number Available frameUnreferenced interval count (UIC)

MVS

MVS address space structure

Task 1 Task 2 Task 3 Task n…

MVSMVS

16KB

0

MVS

MVS address space structure

Task 1 Task 2 Task 3 Task n

…

MVS

MVS/XA

16KB

Task 1 Task 2 Task 3 Task n…

0

MVS address space structure

Data

Space

M

Task n

…

MVS/ESA

16KB

Task n…

0

Data

Space

3

Data

Space

2

Data

Space

1

MVS

Task 1 Task 2 Task 3

MVS

Task 1 Task 2 Task 3

…

370/ESA address translationInstruction

Opcode R1 X2 B2 D2

Displacement

Address – Space Reference

Access RegisterGeneral – Purpose Registers

Base Address

Index

Dynamic Address

Translation

Access register

Translation

Virtual address

Real address

Segment-Table Designation+

WINDOWS NT

• Windows NT was designed to be implemented on a variety of processors. One of the most important platforms for Windows NT is the Intel 486.

• Page size: 4KB

Windows NT – Address Spaces

• Unsegmented unpaged memory: The virtual address is the same as the physical address, which is useful in low complexity, high performance controller applications

• Unsegmented paged memory: Memory is viewed as a paged linear address space. Protection and management of memory are done by paging

• Segmented unpaged memory: Memory is viewed as a collection of logical address spaces. The advantages of this view over a paged approach is that it affords protection down to the level of a single byte if necessary. It guarantees that the translation table needed is on-chip when the segment is in memory. Hence the segmented unpaged memory results in predictable access times

• Segmented paged memory: Segmentation is used to define logical memory partitions subject to access control and paging is used to manage the allocation of memory within partitions.

The format of the virtual address includes the following fields:

• Table indicator (TI): indicates whether the global segment table or a local segment table should be used for translation

• Segment number: is the number of the segment. This serves as an index into the segment table

• Offset: The offset of the address byte within the segment

• Requested privilege level (RPL): The privilege level requested for this access

Segment Offset

(a) Virtual Address

Base 31…34 G 000 P Dp1 1 Type A Base 23…16

TI Rpl

Directory Offset

(b) Linear Address

Page

47 35 31 0 31 21 11 0

(c) Segment table entry

Limit 19…16

Segment Base 15 … 0 Segment Limit 15 … 0

Page table Address 31 … 12 00 D A 00 PAvail

(d) Page table directory entry

US

RW

Page Frame Address 31…12 00 D A 00 PAvail

(e) Page table entry

US

RW

Segment Table entry

Limit Defines the size of the segment. The processor interprets the limit field in one of two ways, depending on the granularity bit: in units of one byte, up to a limit of 1MB, or in units of 4KB, up to a limit of 4GB

Base Defines the starting address of the segment within the 4GB linear address space

Accessed bit (A) Set whenever the segment is accessed. An operating system that uses segmented non-paged memory may use this bit to monitor frequency of segment usage for memory management purposes. In a paged system, this bit is ignored

Type Distinguishes between various kinds of segments and indicates the access attributes

Descriptor privilege level

Specifies the privilege level of the segment referred to by this segment table entry

Segment present bit (P)

Used for non paged systems. It indicates whether the segment is present in main memory. For paged systems, this bit is always set to 1.

Granularity bit Indicates whether the Limit field is to be interpreted in units of 1 byte or 4KB

Page table Directory Entry and Page Table Entry

Page Frame Address Provides the physical address of the page in memory if the present bit is set. Because page frames are aligned on 4K boundaries, the bottom 12 bits are zero and only the top 20 bits are included in the entry

Page table address Provides the physical address of a page table in memory if the present bit is set

Present bit (P) Indicates whether the page table or page is in main memory

Accessed bit (A) This bit is set to 1 by the processor in both levels of page tables when a read or write operation to the corresponding page occurs

Dirty bit (D) This bit is set to 1 by the processor when a write operation to the corresponding page occurs

User/Supervisor bit Indicates whether the page is available only to the operating system (supervisor level) or is available to both operating system and applications (user level)

Read/Write bit Fir user-level pages, indicates whether the page is read-only access or read/write access for user-level programs

Available bits Available for systems programmer use

UNIX System V

• Page table: one page table per process, with one entry for each page in virtual memory for that process

• Disk block descriptor: associated with each page of a process is an entry in this table that describes the disk copy of the virtual page

• Page frame data table: describes each frame of real memory and is indexed by frame number

• Swap-use table: there is one swap use table for each swap device, with one entry for each page on the device

Linux System

• Allocating and freeing physical memory: pages, groups of pages and small blocks of memory

• Handling virtual memory, which is memory mapped into the address space of running processes

Linux system: Management of physical memory

• Page allocator

• Buddy-heap algorithm

16KB

8KB

8KB4KB

4KB

8KB

• Maintaining the address space visible to each process

• Creates pages of virtual memory on demand• Manages the loading of those pages from

disk or their swapping back out to disk as required

• Process address space: as a set of separate regions and as a set of pages

Linux system: Management of virtual memory

• Vm_area_struct

• Policy algorithm

• Paging mechanism

• Pageout policy

Linux system: Management of virtual memory

Linux system: Management of virtual memory (memory layout)

Kernel virtual memory

Stack

Memory-mapped region

Memory- mapped region

Memory mapped region

Run-time data

Uninitialised data

Initialised data

Program text

Memory invisible to user mode code

Forbidden region

The ‘brk’ pointer

WINDOWS 2000

Page directory entry 0

Page directory entry 1023

Page directory …

Page directory entry 0

Page table entry 1023

Page table 0

…

Page directory entry 0

Page table entry 1023

Page table 1023

…

4K page

4K page

4K page

4K page

WINDOWS 2000 – Page file page-table entry

031

Page address TT P

Protection

Page file

WINDOWS 2000 – FILE I/O

File system

Disk driver

Noncached I/O

process

I/O managerI/O

Cache manager

Cached I/O

VM manager Page fault

Security Violation

• Intentional (malicious)– Unauthorised reading of data– Unauthorised modification of data– Unauthorised destruction of data

• Accidental

Security Attacks• System calls

– try illegal system calls or legal system calls with illegal parameters

• Breaking login process– start logging in and then reboot or break halfway

• Memory information– many systems do not erase the space before allocation

• Bad login program– write a login program that records other users authentication

Security Flaws

• Examples– Unix utility lpr: has an option to remove the

file after it has been printed. In early versions it was possible for anyone to use lpr to print, and then have the system remove, the password file

– link the core file to password file and force a core dump of SETUID program

– mkdir foo

Security Threats• Bacteria

– program that consumes system resources by replicating itself

• Logic Bomb– logic embedded in a program that checks for a certain set

of conditions to be present on the system. When these conditions are met, it executes some function resulting in unauthorised actions

• Trapdoor– Secret undocumented entry point into a program, used to

grant access without normal methods of authentication

Security Threats (ctd)• Trojan Horse

– secret undocumented routine embedded within a useful program. Execution of the program results in execution of the secret routine

• Virus– Code embedded within a program that causes of itself to be

inserted in one or more other programs, and performs unwanted functions

• Worm– program that can replicate itself and send a copies across

network connections

Windows NT Security

• Access Control Scheme

– governed by two entities• an access token is associated with each process

• a security descriptor is associated with each object for which inter-process access is possible

– Users log on to an NT system by name/password scheme authentication

Access Token• Security ID (SID)

– identifies the user uniquely across all the computer system

• Group SIDs– list of groups to which the

user belongs

• Default ACL– initial list of protections

applied to the object at its creation

SID

Group SIDs

Privileges

Default Owner

Default ACL

Security Descriptor• Flags

– defines the type and contents of a security descriptor

• System ACL (SACL)– specifies what kind of operations

on the object should generate audit messages

• Discretionary ACL (DACL)– Determines which users and

groups can access this object for which operations

Flags

Owner

SACL

DACL

Access Control List (ACL)

ACL Header

ACE Header

Access Mask

SID

ACE Header

Access Mask

After March break..

% File: KA.tex% Author: Eleni Mangina <elenim@roach.dai.ed.ac.uk>% Created: Thu May 28 1998\documentclass[a4paper,12pt]{article}\usepackage[dvips]{graphics}\usepackage{epsfig}% Use \includegraphics{*.eps} for pictures% Enlarge printing area a bit:\setlength{\textwidth}{16cm}\setlength{\oddsidemargin}{0cm}\setlength{\evensidemargin}{0cm}\setlength{\topmargin}{-0.94cm}\setlength{\textheight}{23cm}\begin{document}\begin{large}\bf Introduction\end{large}\paragraph{}Knowledge-based systems can be connected with the field of\newpage\begin{thebibliography}{XX}\bibitem{lamp} C.P. Yialouris, A.B. Sideridis. ``An Expert system for tomatodiseases'', ELSEVIER, 1995.\bibitem{lamp} Chris. G. Panagopoulos. ``Tomato diseases'' Course notes forPhytopathology in Agricultural University of Athens, 1992.\end{thebibliography}\end{document}