x86_1.ppt

Introduction to
The x86 Microprocessor
Prof. V. Kamakoti
Digital Circuits And VLSI Laboratory
Indian Institute of Technology, Madras
Chennai - 600 036.
http://vlsi.cs.iitm.ernet.in

Protected Mode
 Memory Segmentation and Privilege Levels
• Definition of a segment
• Segment selectors
• Local Descriptor Tables
• Segment Aliasing, Overlapping
• Privilege protection
• Defining Privilege Levels
• Changing Privilege levels

Organization
 Basic Introduction
 Structured Computer Organization
 Memory Management
 Architectural Support to Operating Systems and users
 Process Management
 Architectural Support to Operating Systems
• Task Switching and Interrupt/Exception Handling
 Legacy Management
 Instruction set compatibility across evolving processor
Architectures
 Evolution of Instruction Sets – MMX Instructions

Intel Processor operation modes
 Intel processor runs in five modes of operations
 Real Mode
 Protected Mode
 Virtual 8086 mode
 IA 32e – Extended Memory model for 64-bits
• Compatibility mode – execute 32 bit code in 64-bit mode
without recompilation. No access to 64-bit address space
• 64-bit mode – 64-bit OS accessing 64-bit address space and
64-bit registers
 System Management mode
 Real mode is equivalent to 8086 mode of operation with some
extensions
 Protected Mode is what is commonly used
 Virtual 8086 mode is used to run 8086 compatible programs
concurrently with other protected mode programs

Structured Computer Organization
Programming Language level
Assembly Language level
Operating Systems level
Microprogramming level
Digital Logic level
Computer
Architecture
Compilers ask
for features from the
Architecture to
induce more
sophistication
in the Programming
Languages
Compiled code/
Assembly code
Advanced Addressing
modes
Sophisticated
Instruction set
Support for Memory
Management
and Task Management
Multiuser OS - Protection,
Virtual
Memory, Context Switching
Intel
Understanding
How I manage
these demands makes
my
biography
interesting

Memory Management
 Multi User Operating Systems
 Ease of Programming
 Process Mobility in the Address Space
 Multiprocess Context switching
 Protection across Processes
• Intra process protection: Separation of Code,
Data and Stack
• Inter process protection
 Virtual Memory
 4GB address space for every process
Ensured
by
Segmenta
tion
Ensured
by
Paging

if (j>k)
max = j
else
max = k
Code_Segment:
mov EAX, [0]
mov EBX, [4]
cmp EAX,EBX
jle 0x7 //Label_1
mov [8], EAX
jmp 0x5 //Label_2
Label_1: mov [8], EBX
Label_2: ….
Ease Of Programming
Data Segment:
0: // Allocated for j
4: // Allocated for k
8: // Allocated for max
Code and Data
segments are
separate
and both
assumed
to start from 0
Operating System
(Kernel)
Main Memory
Other User
Process
Our Code
Segment
Vacant
Space
Our Data
Segment
Vacant
Space
0000
0700
0900
1900
2100
2300
2500
Address of j: 2100
Address of k: 2104
Address of max: 2108
2100
Segment Register (Data)
Every Memory Data
Access should add
the value stored in
Data Segment
Register
By default.

if (j>k)
max = j
else
max = k
Code_Segment:
mov EAX, [0]
mov EBX, [4]
cmp EAX,EBX
jle 0x7 //Label_1
mov [8], EAX
jmp 0x5 //Label_2
Label_1: mov [8], EBX
Label_2: ….
Process Mobility
Data Segment:
0: // Allocated for j
4: // Allocated for k
8: // Allocated for max
Operating System
(Kernel)
Main Memory
Other User
Process
Our Code
Segment
Vacant
Space
Our Data
Segment
Vacant
Space
0000
0700
0900
1900
2100
2300
2500
Address of j: 2100
Address of k: 2104
2100
Segment Register (Data)
A new process needs
a
segment of size 260
The space is
available
but not contiguous
Our Data
Segment
Vacant
Space
2300
New User
Process
2160
Vacant Space
Address of j: 2300
Address of k: 2304

64-bit and above Registers
 RAX, RBX, RCX, RDX, RSI, RSP, RDI, RBP – 64 bit
General purpose registers sharing space with its
corresponding 32-bit registers
 R8-R15, additional general purpose registers
 R8D – R15D (32 bit counter part)
 R8W – R15W (16 bit counter part)
 ST0-ST7, 80 bit floating point
 MMX0-MMX7, 64-bit multi media
 XMM0-XMM7, 128-bit registers – used for floating point
and packed integer arithmetic

Multiple Segments
 The segment register can change its values to point to different
segments at different times.
 X86 architecture provides additional segment registers to access multi
data segments at the same time.
 DS, ES, FS and GS
 X86 supports a separate Stack Segment Register (SS) and a Code
segment Register (CS) in addition.
 By default a segment register is fixed for every instruction, for all the
memory access performed by it. For eg. all data accessed by MOV
instruction take DS as the default segment register.
 An segment override prefix is attached to an instruction to change the
segment register it uses for memory data access.

Multiple Segments
DS
SS
E
S
C
S
0000
0500
1500
2500
3500
mov [10], eax
- this will move the
contents of eax register to
memory location 0510
Opcode: 0x89 0x05 0x10
mov [ES:10], eax
-this will move the contents
of eax register to memory
location 3510
Opcode
0x26 0x89 0x05 0x10
“0x26” is the segment
override prefix.

Multiprocess Context switching
Process 1
CS
Process 1
DS
Process 2
CS
Process 2
SS
Process 2
DS
Process 1
SS
C
S
D
S
S
S
Process 1
in
Execution
Process 2
in
Execution

Other Registers
 EFLAGS – 32 Bit Register
CF
PF
AF
ZF
SF
TF
IF
DF
OF
IO
PL
IO
PL
NT
RF
VM
Bits 1,3,5,15,22-31 are RESERVED.
18: AC, 19:VIF, 20: VIP, 21:ID

Details of the flags
 CF – Carry Flag
 Set by arithmetic instructions that generate a
carry or borrow. Also can be set, inverted and
cleared with the STC, CLC or CMC
instructions respectively.
 PF – Parity Flag
 Set by most instructions if the least significant
eight bits of the destination operand contain an
even number of 1 bits.

 AF – Auxiliary Flag
 If a carry or borrow from the most significant
nibble of the least significant byte – Aids BCD
arithmetic
 ZF – Zero Flag
 Set by most instructions if the result of the
arithmetic operation is zero

 SF – Sign Flag
 On signed operands, this tells whether the result
is positive or negative
 TF – Trace Flag
 On being set it allows single-step through
programs. Executes exactly one instruction and
generates an internal exception 1 (debug fault)

 IF – Interrupt Flag
 When set, the processor recognizes the external
hardware interrupts on INTR pin. On clearing, anyway
has not effect on NMI (external non maskable interrupt)
pin or internally generated faults, exceptions, traps etc.
This flag can be set and cleared using the STI and CLI
instructions respectively
 DF – Direction Flag
 Specifically for string instructions. DF = 1 increments
ESI and EDI, while DF = 0 decrements the same. Set
and cleared by STD and CLD instructions

 OF – Overflow Flag
 Most arithmetic instructions set this flag to
indicate that the result was at least 1 bit too
large to fit in the destination
 IOPL – Input Output Privilege Level flags
 For protected mode operations – indicates the
privilege level, 0 to 3, at which your code must
be running in order to execute any I/O-related
instructions

 NT – Nested Task Flag
 When set, it indicates that one system task has
invoked another through a CALL instruction as
opposed to a JMP. For multitasking this can be
manipulated to our advantage
 RF – Resume Flag
 It is related to Debug registers DR6 and DR7.
By setting this, you can selectively mask some
exceptions while you are debugging code

 VM – Virtual 8086 mode flag
 When it is set, the x86 processor is basically converted into a high-
speed 8086 processor.
 AC (bit 18) Alignment check flag — Set this flag and the
AM bit in the CR0 register to
 enable alignment checking of memory references; clear the AC
flag and/or the
 AM bit to disable alignment checking.
 VIF (bit 19) Virtual interrupt flag — Virtual image of
the IF flag. Used in conjunction
 with the VIP flag. (To use this flag and the VIP flag the virtual
mode extensions
 are enabled by setting the VME flag in control register CR4.)

 VIP (bit 20) Virtual interrupt pending flag —
Set to indicate that an interrupt is pending;
 clear when no interrupt is pending. (Software sets and
clears this flag; the
 processor only reads it.) Used in conjunction with the
VIF flag.
 ID (bit 21) Identification flag — The ability of a
program to set or clear this flag indicates
 support for the CPUID instruction.

Protected Mode Registers
 LDTR – Local Descriptor Table Register –
16 bits
 GDTR – Global Descriptor Table Register –
48 bits
 IDTR – Interrupt Descriptor Table Register
– 48 bits
 TR – Task register – 16 bits

Other System Registers
 Control – CR0, CR2, CR3 (each 32-bits)
 CR0 is very important
• Bit 0 – PE bit – when set processor in protected
mode else real mode
• Bit 3 – TS bit – The processor sets this bit
automatically every time it performs a task switch.
This can be cleared using a CLTS instruction
• Bit 31 – PG bit – when set paging MMU is enabled
else it is disabled

 Control – CR0, CR2, CR3 (each 32-bits)
 CR2 – Read only register – deposits the last 32-
bit linear address that caused a page-fault
 CR3 – Stores the physical address of the PDB –
Page Directory Base register. The paging tables
are to be 4KB aligned and hence the 12 least
significant bits are not stored and ignored

 Debug Registers
 DR0, DR1, DR2, DR3, DR6, DR7
 DR0-DR3 can hold four linear address breakpoints so
that of the processor generates these addresses a debug
exception (Interrupt 1) is caused
 DR6 – Debug status register indicating the
circumstances that may have caused the last debug fault
 DR7 – Debug control register. By filling in the various
fields of this register, you can control the operation of
the four linear address breakpoints

 Test Registers – TR6 and TR7
 Used to perform confidence checking on the
paging MMU’s Translation Lookaside Buffer
(TLB).

Test Your Understanding
 There are ------- GPRs in the x86
 The x86 system in protected mode has ------
-- enabled by default
 Three salient features of using
Segmentation

Answers
 1. Eight GPRs
 2. Segmentation
 3. Three Features
 Code Mobility
 Logically every segment can start with zero
 Inter and Intra process protection ensuring data
integrity.

 Learnt so far
 Intel Memory Management fundamentals
• Motivation from a Computer Organization standpoint
• Intel Register set – General Purpose Registers, Segment
registers and system registers
• x86 modes of operations

x86 Memory Management
 To Learn
 Real and protected mode addressing in x86
 Virtual Memory and paging
 Addressing
 Task switching and Interrupt handling
 Legacy issues

Real Mode - Memory Addressing
•Segment << 4 + offset = 20 bit EA
DS = 0x1004 mov [0x1000], EAX
The mov will store the content of EAX in
0x10040 + 0x1000 = 0x11040
Why this stuff? - To get 1 MB addressing using 16-
bit Segment Registers
•Segment size is a fixed 64K

Protected Mode Addressing
 mov [DS:1000], EAX
 Let value of DS be 0x10. This is used to select a
segment descriptor in a descriptor table.
 The segment descriptor contains information
about the base address of the segment, to which
1000 is added to get the effective address.
 The value stored in DS is called a selector.
 Henceforth we discuss protected mode.

Protected Mode Addressing
SELECTOR OFFSET
Descriptor Table
Base Address
Linear
Address
Logical
Address
Segment Descriptor

Intra and Inter process Protection
•A process always executes from Code segment. It should
not execute by accessing from adjoining Data or stack area
or any other code area too.
•A stack should not overgrow into adjoining segments
Every segment is specified a
start address and limit.
Architecture checks if limit is
not exceeded.
C
S
ES
SS
500
1000
1500
2000
jmp CS:250 //This is fine
jmp CS:501 //This is a violation as limit is 500
mov [ES:498], AX //This is fine
mov [ES:498], EAX //This is a violation!!!
PUSH AX //Let SP be 498, it is fine
PUSH EAX //Let SP be 498, violation
POPAX //Let SP be 2, it is fine
POP EAX //Let SP be 2, Violation!!!

Interprocess Protection
Process 1
CS
Process 1
DS
Process 2
CS
Process 2
SS
Process 2
DS
Process 1
SS
C
S
D
S
S
S
Process 1 should be
prevented from loading
CS, such that it can
access the code of
Process 2
Similarly for the DS,SS,
ES, FS and GS
Privilege levels: [0-3]
assigned to each
segment.
0: Highest privilege
3: Lowest privilege

Privilege levels and Protection
 Every segment has an associated privilege level
and hence any code segment will have an
associated privilege level.
 The CPL (Current Privilege Level) of a process is
the privilege level of the code segment, the code
stored in which, it is executing.
 A process can access segments that have privilege
levels numerically greater than or equal to (less
privileged than) its CPL.

Protection Implementation
 Every segment is associated with a descriptor
stored in a descriptor table.
 The privilege level of any segment is stored in its
descriptor.
 The descriptor table is maintained in memory and
the starting location of the table is pointed to by a
Descriptor Table Register (DTR).
 The segment register stores an offset into this
table.

Updating Segment registers
 Segment registers (DS, ES, SS, GS and FS) are
updated by normal MOV instructions.
 MOV AX, 0x10 ; MOV DS, AX
 The above command is successful if and only if
the descriptor stored at the offset 0x10 in the
descriptor table has a privilege level numerically
greater than or equal to the CPL.
 A process with CPL = 3 cannot load the segment
descriptor of CPL <= 2, and hence cannot access
the segments.

Updating segment registers
 The code segment register is updated by normal
jump/call operations.
 jmp 0x20:0x1000
 This updates the CS by 0x20, provided the descriptor
stored at offset 0x20 has a privilege level numerically
greater than or equal to CPL
 Other modes of updating CS register
 Numerically higher to lower Privilege Levels using
CALL gates – useful for system calls.
 Any privilege level to any other privilege level using
task switch.

Descriptor Tables
 There are two descriptor tables
 Global Descriptor Tables
 Local Descriptor Tables
 The global descriptor table’s base address is stored
in GDTR
 The local descriptor table’s base address is stored
in LDTR
 The two privileged instructions LGDT and LLDT
loads the GDTR and LDTR.

Structure of a Selector
Index T1
0
2
15
T1 = 0 GDT
= 1 LDT
Since segment descriptors are each 8 bytes, the last
three bits of the selector is zero, in which one of
them is used for LDT/GDT access.

Two process each of PL = 3 should be allotted segments such
that one should not access the segments of other.
GDT
R GDT
All descriptors in GDT
have
PL = 0,1,2
LDTR
LDTR
Per process
Per process
If at all each process should access memory, it has to use the descriptors in its LDTR
only and it cannot change the LDTR/LDT/GDTR/GDT contents as they would be
maintained in a higher privileged memory area.

Did You Note!!
 There is an 100 % degradation in Memory
access time – because every memory access
is two accesses now, one for getting the
base address and another for actually
accessing the data.
 A solution indeed: Along with the segment
registers, keep a shadow registers which
stores additional necessary information.

Base Address,
Limit, DPL.
Segment selector
Visible part Hidden part
CS
SS
DS
ES
FS
GS

Be Careful
0x10 20
120
Descriptor Table
Base Address
Linear
Address
Logical
Address
Base = 100
Changing Base
Linear address
will still be 120
Have to execute
mov DS,0x10
again to get the
answer as 220,
as this would
update the
hidden part
Base = 200
add [DS:20],eax

Virtual Memory and Paging
 It is always enough if the next instruction to
be executed and the data needed to execute
the same are available in the memory.
 The complete code and data segment need
not be available.
 Use of paging to realize the stuff!
 By using segmentation the processor
calculates an 32-bit effective address.

Paging fundamentals
 Each page is 4096 bytes
 Physical RAM has page frames like photo frames,
which is also 4096 bytes.
 A page is copied into the page frame, if needed
and removed to accommodate some other page.
 By this, a 4 GB code can run on a 128MB physical
memory
 This is also called demand paging.

Protected Mode Addressing with paging
DIR TABLE OFFSET
CR3 REG
DIR ENTRY
PG TBL
ENTRY
PHYS ADDRS
PAGE DIRECTORY PAGE TABLE
PAGE FRAME
12
10
10
4KB entries
with 4 bytes
per entry
4KB entries
with 4 bytes
per entry
If 20 bytes are used as a single level
paging then page table alone is 4 MB
which is inefficient. So two level paging.
Develop the page table on demand
TLB’s used to improve performance
Dirty bit accommodated in each page
entry

Protected Mode Addressing - Paging entries

Task Switching
 There are different types of descriptors in a
Descriptor table.
 One of them is a task state segment descriptor.
 jmp 0x10:<don’t’care> and that 0x10 points to a
TGD, then the current process context is saved and
the new process pointed out by the task state
segment descriptor is loaded.
 A perfect context switch.
 TSS descriptor only in a GDT.

Task Switching
 Every process has an associated Task State
Segment, whose starting point is stored in
the Task register.
 A task switch happens due to a jmp or call
instruction whose segment selector points to
a Task state segment descriptor, which in
turn points to the base of a new task state
segment

Interrupt Handling
 Processor generates interrupts that index into a
Interrupt Descriptor Table, whose base is stored in
IDTR and loaded using the privileged instruction
LIDT.
 The descriptors in IDT can be
 Interrupt gate: ISR handled as a normal call subroutine
– uses the interrupted processor stack to save EIP,CS,
(SS, ESP in case of stack switch – new stack got from
TSS).
 Task gate: ISR handled as a task switch
• Needed for stack fault in CPL = 0 and double faults.

Interrupt Handling
 Processor handles a total of 255 interrupts
 0-31 are used by machine or reserved
 32-255 are user definable
 0 – Divide error, goes to first descriptor in IDT
 1 – Debug
 8 – Double Fault
 12 – Stack Segment fault
 13 – General Protection Fault
 14 – Page Fault

Legacy Issues
 16-bit code in 32-bit architecture
 Address override prefix – 16-bit or 32-bit
addresses in a 32-bit or 16-bit code segment
 Operand override prefix
 Same opcode for say, add EAX,EBX and add
AX,BX
 Distinguished by the operand override prefix – 16-bit or
32-bit operands in a 32-bit ot 16-bit code segment
 D flag in the code segment descriptor tells the size
of the code segment, which is used above.

Legacy Issues
 mod r/m: says if it is a memory or register
access
 sib: says if it is memory then what
addressing is issued for effective address
calculation.

Evolving Instruction Sets
 The Multimedia Instruction set (MMX)
 First Major Extension to x86 since 1985
 57 new instructions
 Audio
 Video
 Speech Recognition and synthesis
 Data communication
 Two byte Opcode with 0F prefix
 Use of Data parallelism at the instruction opcode level to
speedup computation.

x86 Memory Management
 To learn
 Segmentation details
 Privilege levels and switching

Memory Segmentation
 Segment Descriptors
 80886 to 80386+
• In 8086, the program is not expected to generate a
non-existent memory address. If it does, then the
processor shall try to access the same and read
bogus data, or crash
• In 80386+ (and above) the segment attributes (base,
limit, privilege etc) are programmable and no matter
how privileged the code may be, it cannot access an
area of memory unless that area is described to it.

Insight into 80386+ segments
 Segments are
 Areas of memory
 Defined by the programmer
 Used for different purposes, such as code, data and
stack
 Segments are not
 All the same size
 Necessarily paragraph aligned
 Limited to 64KB

Segment Descriptors
 Describes a segment using 64-bits (0-63)
 Must be created for every segment
 Is created by the programmer
 Determines a segment’s base address (32-
bits) (Bits 16-39, 56-63)
 Determines a segment’s size (20-bits) (Bits
0-15, 48-51)

Segment Descriptors (Cont’d)
 Defines whether a segment is a system
segment (=0) or non-system (=1) (code,
data or stack) segment (System bit) (Bit 44)
 Determines a segment’s use/type (3-bits)
(Bits 41-43) after the above classification
 Determines a segment’s privilege level (2
bits) (Bits 45-46) – DPL (Descriptor
Privilege Level) Bits

Segment Descriptor (Cont’d)
 Accessed (A)-bit: Bit 40, automatically set and not
cleared by the processor when a memory reference
is made to the segment described by this
descriptor.
 Present (P)-bit: Bit 47, indicates whether the
segment described by this descriptor is currently
available in physical memory or not.
 Bits 40-47 of the descriptor is called the Access
Right Byte of the descriptor.
 User (U)-bit and X bit: Bit 52 (U-bit) not used and
Bit 53 (X-bit) reserved by Intel

Segment Descriptor (Cont’d)
 Default size (D)-bit: Bit 54, when this bit is
cleared, operands contained within this segment
are assumed to be 16 bits in size. When it is set,
operands are assumed to be 32-bits.
 Granularity (G)-bit: Bit 55, when this bit is cleared
the 20-bit limit field is assumed to be measured in
units of 1byte. If it is set, the limit field is in units
of 4096 bytes.

Types of non-system segment
descriptors
 System bit S = 1
 000 – Data, Read only
 001 – Data, Read/Write
 010 – expand down, Read only
 011 – expand down, Read/Write
 100 – Code, Execute only
 101 – Code, Execute/Read
 110 – Conforming Code, Execute only
 111 - Conforming Code, Execute/Read

D-bit for different descriptors
 Code segment
 D = 0 then 16-bit 80286 code
 D = 1 then 32-bit 80386+ code
 Stack Segment
 D = 0 then stack operations are 16-bit wide, SP is used
as a stack pointer, maximum stack size is FFFF (64 KB)
 D = 1 then stack operations are 32-bit wide, ESP is used
as a stack pointer, maximum stack size is FFFFFFFF (4
GB)

G-bit for descriptors
 G = 0 then a limit field in descriptor of value p
indicates we can access p-1 bytes from base
 G = 1 then a limit field in descriptor of value p
indicates we can access (p * 4096) - 1 bytes
from base

Stack/expand down segments
 All offsets must be greater than limit.
 In stack descriptor, D and G bits are to be
the same, else contradiction.
Base
FFFF
Addressa
ble area
Base
FFFF
Addressa
ble area
Stack/expand-
down
Non-stack
Limit
Limit

Descriptor Tables
 Descriptors are stored in three tables:
 Global descriptor table (GDT)
• Maintains a list of most segments
• May contain special “system” descriptors
• The first descriptor is a null descriptor
 Interrupt descriptor table (IDT)
• Maintains a list of interrupt service routines
 Local descriptor table (LDT)
• Is optional
• Extends range of GDT
• Is allocated to each task when multitasking is enabled
• The first descriptor is a null descriptor

Locations of the tables
 In Memory
 Pointed out by GDTR, LDTR and IDTR for the
GDT, LDT and IDT respectively.
 The GDTR and IDTR are 48-bits in length, the
first 16-bits (least significant) storing the size
(limit) of the table and the remaining storing a 32-
bit address pointing to the base of the tables
 Limit = (no. of descriptors * 8) - 1
 LLDT stores a 16-bit selector pointing to an entry
in the GDT.

Segment Selectors
 Out of several segments described in your GDT and LDT,
which of the segment(s) that are currently being used are
pointed to by the 16-bit CS,DS,ES,FS,GS and SS registers.
 Each store a selector
 Since descriptors are at 8-byte boundaries, the 16-bit
selectors store the first most significant 13 bits to point to
the corresponding descriptor.
 The bit 2 is the T1 bit, which when 0 (1) implies the
selector is pointing to a descriptor in GDT (LDT).
 The bits (0-1) – are the Request Privilege Level (RPL) bits
used for privilege assignments.

Loading Segment Selectors into
segment registers
 Whenever segment registers are loaded, the
following rules are checked by the processor and
if violated an exception is raised thus giving high
degree of memory protection
 Rule 1: Index field of the selector within limits of
the GDT/LDT to be accessed – else raise a
General Protection Fault exception.

Loading Segment Selectors into
segment registers
 Rule 2: Loading a selector into DS,ES,FS or GS
that points to a non-readable segment results in an
exception
 Rule 3: For loading into SS, the segment pointed
to should be readable and writable
 Rule 4: For loading into CS, the segment should
be executable type
 Rule 5: Privilege level check rules to be described
later

Loading segment selectors
 All segment registers except CS may be
loaded using MOV, LDS, LES, LFS, LGS
and LSS.
 The CS is loaded using a JMP or a CALL
instruction – discussed later

Local Descriptor Table
 Is defined by a system descriptor (S=0) in
GDT which is pointed to by the LDT.
Limit
15-0
Base Address
23-0
0000010
P
Limit
19-16
0000
Base
Address
31-24
The 64-bit descriptor in GDT

Privilege levels
 The need is to prevent
 Users from interfering with one another
 Users from examining secure data
 Program bugs from damaging other programs
 Program bugs from damaging data
 Malicious attempts to compromise system
integrity
 Accidental damage to data

Privilege Protection
 Continuous checking by the processor on whether
the application is privileged enough to
 Type 1: Execute certain instructions
 Type 2: Reference data other than its own
 Type 3: Transfer control to code other than its own
 To manage this every segment has a privilege level
called the DPL (Descriptor Privilege Level) Bits
45,46

Descriptor Privilege Level
 Privilege levels apply to entire segments
 The privilege level is defined in the segment
descriptor
 The privilege level of the code segment
determines the Current Privilege Level
(CPL)

Type 1: Privilege Checking
 Privileged Instructions
1. Segmentation and Protection Based (HLT, CLTS,
LGDT, LIDT, LLDT, LTR, moving data to Control,
Debug and Test registers)
2. Interrupt flag based (CLI, STI, IN, INS, OUT, OUTS)
3. Peripheral IO based
 First two types of privileged instructions can be
executed only when CPL = 0, that is, these
instructions can be in code segment with DPL =
0.

I/O instructions
 The I/O based privileged instructions are
executed only if CPL <= IOPL in EFLAGS
register.
 To add to the security the POPF/POPFD
instructions which load values into the
EFLAGS shall not touch the IOPL bit or IF
bit if CPL > 0.

 Reference data other than its own
 Load a selector into a DS, ES, FS and GS iff
max(RPL,CPL) <= DPL
 RPL may weaken your privilege level
 Decreasing RPL will not strengthen your privilege level
– Why?
 Why to decrease RPL – will discuss later
 Load a descriptor into a stack iff DPL = CPL
 All these are in addition to the rules for loading segment
selector, that were stated in Slides 87 and 88.

 Transfer control to code other than its own.
Essentially load a new selector into CS
register
 jmp across code segments with same DPL
 jmp <selector>:<offset of instruction from start
of the new segment>
 call <selector>:<offset of instruction from start
of the new segment>

 The above jmp, call and ret may be used
 To move between code segments provided
the destination segment is
 A code segment (executable permission)
 Defined with the same privilege level
 Marked present

Changing Privilege levels
 Control transfer from a code of some PL to
another code with some other different PL.
 Using conforming code segments or a special
segment descriptor called call gates.
 Conforming code segments confirms with the
privilege level of the calling code. So if a control
transfer happens from segment S to a confirming
segment T, the privilege of T would be the
privilege of S.

Conforming Code Segment
 The DPL of conforming code segment descriptor
<= CPL of invoking code.
 Therefore, CPL = 2 can invoke DPL = 1.
 CPL = 2 cannot invoke code with DPL = 3.
 Why?
 If not, you JMP back or RET to the source code
segment after executing the conforming code segment.
This should permit return from a numerically low
privilege code to a numerically high privilege code,
without check.

CALL GATE descriptor
 Is defined by a system descriptor (S=0) in
GDT which is used by the JMP or CALL.
Destination
Offset
15-0
Destination
Selector (16 bits)
WC
000
01100
P, DPL
Destination
offset
31-16
The 64-bit descriptor in GDT
•Not only the selector for the target code segment, but
also the offset in the code segment from which you
should start executing is specified. The source code
segment can only use it like a black-box

Calling Higher privileged code
SEG
CALL OFFSET
SEG
CALL OFFSET
Correct Incorrect
Gate – Sel
+ offset
Code Desc
Code Seg Code Seg
Code Desc

Call Gates
 Are defined like segment descriptors
 Occupy a slot in the descriptor tables
 Provide the only means to alter the current
privilege level
 Define entry points to other privilege levels
 Must be invoked using a CALL Instruction

Call Gate accessibility
 Target DPL <= Max (RPL, CPL) <= Gate
DPL
 For eg. CPL = 2 and the target PL = 0, you
should use a Gate with PL = 2 or 3

Privilege levels and Stacks
 The stack PL = CPL always
 When changing the CPL, the processor
automatically changes the stack!!!
 How – using the Task State Segment (TSS)
 The base of the TSS is stored in a Task register
(TR) which is updated by the privileged
instruction LTR
 The TSS associates a stack for each code for each
of the privilege levels 0, 1 and 2

x86_1.ppt

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