1. Operating Modes
UQ: State and explain the operating modes of X86 family of processors. Show the mode transition diagram highlighting important features.(10 Marks)

2. Operating Modes The Intel 386DX has two modes of operation Real Mode:
Real Mode (Real Address) and
Protected Mode (Protected Virtual Address Mode)
Real Mode:
It works as a very fast 8086 with 32-bit extensions.
It is required to set up the processor for protected mode

3. Operating Modes Protected Mode:
It provides access to sophisticated memory management, paging and privilege capabilities of the processor

4. Real Mode Architecture
It has same base architecture as 8086
When a processor is reset, it is initialized in real mode.
It sets up the processor for Protected Mode.
The segment size of in real mode is 64KB
The maximum memory size is 1MB
Only address lines A2-A19 are active

5. Real Mode Architecture
Paging is not allowed in real mode and hence the physical address is same as linear.
Physical Address is formed by adding contents of segment register shifted left by 4 bits to an effective address.
This results in a physical address from to 0010FFEF (FFFF0+FFFF)
Real mode segments always start on 16-byte boundaries since they are left shifted.

6. Real Mode Addressing

7. Real Mode Architecture
All segments in Real Mode are exactly 64KB long and generate exception 13 if a data operand or instruction fetch occurs past the end of a segment.
Segments may be overlapped in Real Mode.If a particular segment does not use all 64KB another segment can be overlayed on top of the unused portion of the previous segment.

8. Protected Mode
UQs :
Draw protected mode address translation mechanism of and explain segment translation in detail.(2)

9. Protected Mode Architecture
It increases linear address space to 4GB and virtual address space to 64TB
This mode uses two components to form the logical address:
16 bit selector: to determine the linear base address of the segment
32 bit offset: Added to base address to form the linear address

10. Protected Mode: Addressing Mechanism
In this mode, the selector is used to specify an index to the operating system defined table
The table contains the 32-bit base address of a given segment.
The linear address is formed by adding base address to the offset

11. Protected Mode: Addressing Mechanism

12. Protected Mode: Addressing Mechanism
Paging provides a means of managing very large segments.
It translates linear address into physical address.

13. Protected Mode: Addressing Mechanism

14. Segmentation
Segmentation provides the basis of protection which encapsulates regions of memory which have common attributes.
All information about a segment is stored in an 8-byte data structure called a descriptor.
All descriptors are contained in tables recognized by hardware. 14

15. Selector (-Segment Register)
A selector in protected mode has 3 fields:
TI (Table Indicator): Local or Global Descriptor Table Indicator
Index(Descriptor Entry Index ): Selects one of 8K descriptors
RPL (Requestor Privilege Level): allows testing of selector’s privilege attributes
Level 0: Most Privilege Level
Level 3: Least Privilege Level 15

16. Selector 16

17. Descriptor Tables It defines all the segments used in x86 system
There are 3 types of table:
Global Descriptor Table(GDT)
Local Descriptor Table(LDT)
Interrupt Descriptor Table(IDT)
All tables are variable length memory arrays
They can range in size from 8 bytes to 64KB (213 x 23) (3 bits are implied as descriptor size is fixed)

18. Descriptor Tables
Each table can hold up to 8192(213) 8-byte descriptors.
The table has registers associated with them named GDTR, LDTR and IDTR which hold the 32-bit linear base address and 16-bit limit of each table.
These tables are manipulated by the OS using privileged instructions.

19. Global Descriptor Table
It contains descriptors which are possibly available to all of the tasks in a system.
Every Intel386 DX system contains a GDT.
It contains code and data segments used by the operating systems and task state segments and descriptors for the LDTs in a system.
The first slot of GDT corresponds to the null selector and is not used.

20. Global Descriptor Table Register