Protected Mode

1
Protected Mode
2
Protected Mode (1 of 2)

• 4 GB addressable RAM
• (00000000 to FFFFFFFFh)
• Each program assigned a memory partition which is
  protected from other programs
• Designed for multitasking
• Supported by Linux MS-Windows

3
Protected Mode

• memory segmentation is defined by a set of tables
  (descriptor tables) and the segment registers
  contain pointers into these tables. Each table
  entry is 8-bytes wide.
• Global Descriptor Table (GDT), all applications
  can access
• Local Descriptor Table (LDT), specific to a task
  or program.
• Segment registers don't form part of the physical
  address in protected mode, but instead are used
  as pointers to table entries in the GDT or LDT

4
Protected mode (2 of 2)

• Segment descriptor tables
• Program structure
• code, data, and stack areas
• CS, DS, SS segment descriptors
• global descriptor table (GDT)
• MASM Programs use the Microsoft flat memory model

5
Memory Management

• Protected Mode (flat model)

• Allows the microprocessor to address up to 4G
  byte of memory space
• http//www.intel.com/design/pentium4/manuals/index
  _new.htm1

• Selector, located in segment register, selects
  one of 8192 descriptors from one of two tables of
  descriptors

6
Memory Management

• Protected Mode (flat model)

• Descriptor, describes the location, length, and
  access rights of the segment of memory

• Global descriptor table (GDT)
• Local descriptor table (LDT)

7
Memory Management

• Protected Mode (flat model)

• Base address indicates the starting location of
  the memory segment

• 24-bit for the 80286 segments begin at any
  location in its 16M bytes of memory
• 32-bit for the 80386 and above - segments begin
  at any location in its 4G bytes of memory

• Segment limit contains the last offset address
  found in segment
• Granularity bit (for 80386 and above)---- G0,
  the limit specifies a segment limit of 00000H to
  FFFFFH G1, the value of limit is multiplied by
  4K bytes (00000XXXH to FFFFFXXXH)

8
Memory Management

• Protected Mode (flat model)

• AV bit used by operating systems

• AV1, segment is available
• AV0, segment is not available

• D bit indicate how the instructions access
  register and memory data in protected mode

• D0, instructions are 16-bit instructions, with
  16-bit offsets and 16-bit registers. Compatible
  with 8086 80286
• D1, 32-bit is assumed

9
Memory Management

• Protected Mode (flat model)

• The Access Right

10
Which memory locations will be used as the data
segment?
11
Flat Segment Model

• Single global descriptor table (GDT).
• All segments mapped to entire 32-bit address space

12
Multi-Segment Model

• Each program has a local descriptor table (LDT)
• holds descriptor for each segment used by the
  program

13
Paging

• Supported directly by the CPU
• Divides each segment into 4096-byte blocks called
  pages
• Sum of all programs can be larger than physical
  memory
• Part of running program is in memory, part is on
  disk
• Virtual memory manager (VMM) OS utility that
  manages the loading and unloading of pages
• Page fault issued by CPU when a page must be
  loaded from disk

14
Assembling, Linking and Running
Source file (.asm)
Object file (.obj)
Listing file (.lst)
ML
Link library (.lib)
Executable program(.exe)
Map file (.map)
Dos Loader
Output
15
Assembling, Linking and Running

• Assembling

• Two staged process
• Assemble Converts assembly language into
  machine language. Uses source file, outputs
  object file and optionally the list file
• Link Connects (links) the different .obj and
  library files together to create the executable
  file. Uses the object file(s) and optionally a
  library. Outputs executable file and optionally
  map file
• Both done with ML command

16
MASM Installation

• Insert CD in CD-Rom
• Go to Desktop, select My Computer
• Select CD-ROM directory
• Double click index.html
• On the new screen, follow the links, install
  MASM615
• Install TextPad Editor and follow the link to
  install appropriate Tool Menu Command for 32-bit
  MASM Program (See the following link for details
• http//www.nuvisionmiami.com/books/asm/ide/textpa
  d4/index.htmlAddTools)

17
MASM Installation

• Corrections for the installation of Tool Commands
  in TextPad

Should be BaseName.exe

• Run MASM Prog Command