Segment Descriptor Describes a Memory Area in Detail
by Bob Colwell, Tom Shanley
The Unabridged Pentium 4 IA32 Processor Genealogy
- Copyright
- At-a-Glance Table of Contents
- Figures
- Tables
- Acknowledgments
- About This Book
- Introduction
- Single-/MultiTask OS Background
- The 386
- 386 Real Mode Operation
- Protected Mode Introduction
- Intro to Segmentation in Protected Mode
- Code Segments
- Data and Stack Segments
- Creating a Task
- Mechanics of a Task Switch
- 386 Demand Mode Paging
- Problem—Loading Entire Task into Memory is Wasteful
- Solution—Load Part and Keep Remainder on Disk
- Problem—Running Two (or more) DOS Programs
- Solution—Redirect Memory Accesses to Separate Memory Areas
- Global Solution—Map Linear Address to Disk Address or to a Different Physical Memory Address
- The Paging Unit Is the Translator
- Three Possible Page Lookup Methods
- IA32 Page Lookup Method
- Enabling Paging
- Page Directory and Page Tables
- Finding the Location of a Physical Page
- Eliminating the Directory Lookup
- Checking Page Access Permission
- Page Faults
- Usage of the Dirty and Accessed Bits
- Demand Mode Paging Evolution
- The Flat Model
- Interrupts and Exceptions
- Special Note
- General
- Hardware Interrupts
- Software-Generated Exceptions
- Interrupt/Exception Priority
- Real Mode Interrupt/Exception Handling
- Protected Mode Interrupt/Exception Handling
- Interrupt/Exception Handling in VM86 Mode
- Exception Error Codes
- The Resume Flag Prevents Multiple Debug Exceptions
- Special Case—Interrupts Disabled While Updating SS:ESP
- Detailed Description of the Software Exceptions
- Virtual 8086 Mode
- A Special Note
- DOS Application—Portrait of an Anarchist
- Solution—Set a Watchdog on the DOS Application
- The Virtual Machine Monitor (VMM)
- Entering or Reentering VM86 Mode
- An Interrupt or Exception Causes an Exit From VM86 Mode
- A Task Switch Causes an EFlags Update
- DOS Task's Memory Usage
- The Privilege Level of a VM86 Task
- Restricting IO Accesses
- IOPL-Sensitive Instructions
- Interrupt/Exception Generation and Handling
- Registers Accessible in Real/VM86 Mode
- Instructions Usable in Real/VM86 Mode
- VM86 Mode Evolution
- The Debug Registers
- 486
- Pentium®
- Intro to the P6 Core and FSB
- Pentium® Pro Software Enhancements
- Pentium® Pro Software Enhancements
- MicroCode Update Feature
- The Problem
- The Solution
- The Microcode Update Image
- Matching the Image to a Processor
- The Microcode Update Loader
- Updates in a Multiprocessor System
- The Image Management BIOS
- When Must the Image Upload Take Place?
- Determining if a New Update Supersedes a Previously-Loaded Update
- Effect of RESET# Or INIT# on a Previously-Loaded Update
- Pentium® II
- Pentium® II Hardware Overview
- The Pentium® Pro and Pentium® II: Same CPU, Different Package
- Dual-Independent Bus Architecture (DIBA)
- IOQ Depth
- Pentium® Pro/Pentium® II Differences
- One Product Yields Three Product Lines
- The Pentium® II/Xeon/Celeron Roadmap
- The Cartridge
- The Core
- The FSB and BSB
- The Introduction of the Celeron
- Miscellaneous Hardware Stuff
- Pentium® II Power Management Features
- Pentium® II Software Enhancements
- Pentium® II Xeon Features
- Pentium® II Hardware Overview
- Pentium® III
- Pentium® 4
- Pentium® 4 Road Map
- Pentium® 4 System Overview
- Pentium® 4 Processor Overview
- The Pentium® 4 Processor Family
- Pentium® III/Pentium® 4 Differences
- Pentium® 4/Pentium® 4 Prescott Differences
- Pentium® 4 Processor Basic Organization
- The FSB is Tuned for Multiprocessing
- Intro to the FSB Enhancements
- IA Instructions Vary in Length and Are Complex
- The Trace Cache
- There Are Two Pipeline Sections
- The μop Pipeline
- The IA32 Data Register Set Was Small
- Speculative Execution
- Pentium® 4 PowerOn Configuration
- Configuration on Trailing-Edge of Reset
- Setup and Hold Time Requirements
- Built-In Self-Test (BIST) Trigger
- Assignment of IDs to the Processor
- Error Observation Options
- In-Order Queue Depth Selection
- Power-On Restart Address
- Tri-State Mode
- Processor Core Speed Selection
- Bus Parking Option
- Hyper-Threading Option
- Program-Accessible Startup Features
- Pentium® 4 Processor Startup
- Pentium® 4 Core Description
- Hyper-Threading
- General
- Background
- The HT Approach
- Overview of HT Resource Usage
- HT and the Data TLB
- HT and the FSB
- The IOQ Depth Was Increased
- HT Performance Issues
- HT and Serializing Instructions
- HT and the Microcode Update Feature
- HT Cache-Related Issues
- HT and the TLBs
- HT and the Thermal Monitor Feature
- HT and External Pin Usage
- The Pentium® 4 Caches
- Pentium® 4 Handling of Loads and Stores
- The Pentium® 4 Prescott
- Introduction
- Increased Pipeline Depth
- Trace Cache Improvements
- Increased Number of WCBs
- L1 Data Cache Changes
- Increased L2 Cache Size
- Enhanced Branch Prediction
- Store Forwarding Improved
- SSE3 Instruction Set
- Increased Elimination of Dependencies
- Enhanced Shifter/Rotator
- Integer Multiply Enhanced
- Scheduler Enhancements
- Fixed the MXCSR Serialization Problem
- Data Prefetch Instruction Execution Enhanced
- Improved the Hardware Data Prefetcher
- Hyper-Threading Improved
- Pentium® 4 FSB Electrical Characteristics
- Intro to the Pentium® 4 FSB
- Pentium® 4 CPU Arbitration
- Pentium® 4 Priority Agent Arbitration
- Pentium® 4 Locked Transaction Series
- Pentium® 4 FSB Blocking
- Pentium® 4 FSB Request Phase
- Pentium® 4 FSB Snoop Phase
- Pentium® 4 FSB Response and Data Phases
- A Note on Deferred Transactions
- The Purpose of the Response Phase
- The Response Phase Signal Group
- The Response Phase Start Point
- The Response Phase End Point
- The Response Types
- The Response Phase May Complete a Transaction
- The Data Phase Signal Group
- Five Example Scenarios
- Data Phase Wait States
- The Response Phase Parity
- Data Bus Parity
- Pentium® 4 FSB Transaction Deferral
- Pentium® 4 FSB IO Transactions
- Pentium® 4 FSB Central Agent Transactions
- Pentium® 4 FSB Miscellaneous Signals
- Pentium® 4 Software Enhancements
- The Foundation
- Miscellaneous New Instructions
- Enhanced CPUID Instruction
- The SSE2 Instruction Set
- The SSE3 Instruction Set
- Local APIC Enhancements
- The Thermal Monitoring Facilities
- FPU Enhancement
- The MSRs
- The Machine Check Architecture
- Last Branch, Interrupt, and Exception Recording
- The Debug Store (DS) Mechanism
- New Exceptions
- The Performance Monitoring Facility
- Pentium® 4 Xeon Features
- Pentium® M
- Pentium® M Processor
- Background
- The Pentium® M and Centrino
- Characteristics Overview
- The FSB Characteristics
- Enhanced Power Management Characteristics
- Three Different Packaging Models
- Improved Thermal Monitor Mode
- Enhanced Branch Prediction
- μop Fusion
- Advanced Stack Management
- Miscellaneous
- The Data Cache and Hyper-Threading
- The Next Pentium® M
- Pentium® M Processor
- Additional Topics
- CPU Identification
- System Management Mode (SMM)
- What Falls Under the Heading of System Management?
- The Genesis of SMM
- SMM Has Its Own Private Memory Space
- The Basic Elements of SMM
- A Very Simple Example Scenario
- How the Processor Knows the SM Memory Start Address
- Protected Mode, Paging and PAE-36 Mode Are Disabled
- The Organization of SM RAM
- Entering SMM
- Exiting SMM
- Caching from SM Memory
- Setting Up the SMI Handler in SM Memory
- Relocating the SM RAM Base Address
- SMM in an MP System
- The Local and IO APICs
- Before the Advent of the APIC
- MP Systems Need a Better Interrupt Distribution Mechanism
- A Short History of the APIC
- Detecting the Presence and Version of the Local APIC
- Enabling/Disabling the Local APIC
- Local Cluster and APIC ID Assignment
- An Introduction to the Interrupt Sources
- Introduction to Interrupt Priority
- An Intro to Edge-Triggered Interrupts
- An Intro to Level-Sensitive Interrupts
- The Local APIC Register Set
- Locally Generated Interrupts
- Task and Processor Priority
- Interrupt Messages
- The IO APIC
- Message Signaled Interrupts (MSI)
- Message Format
- The Spurious Interrupt Vector
- The Agents in an Interrupt Message Transaction
- BSP Selection Process
- The APIC, the MPS and ACPI
- Acronyms
- CD-ROM Warranty
- Index
Segment Descriptor Describes a Memory Area in Detail
In a multitasking environment, the OS programmer must be able to specify the following characteristics of each segment:
The task that it belongs to.
Its start address anywhere in the 4GB memory address range.
Its length (anywhere from one byte to 4GB in length).
How it may be accessed: read-only, execute-only, read/writable.
The minimum privilege level a program must have to access the segment.
Whether it's a code or data segment, or a special segment that is only used by the OS.
Whether the segment of information is currently present in memory or resides on a mass storage device.
Figure 7-1 on page 111 illustrates the manner in which the processor interprets the contents of a segment register while operating in Real Mode. The only thing it contains is the upper 16 bits of the 20-bit start address of the segment within the first megabyte of memory space. The processor automatically appends the lower four bits of the start address and always sets them to zero. As an example, if the programmer moved the value 1010h into the DS register
mov ax, 1010 mov ds, ax
Figure 7-1. Segment Register Contents in Real Mode
this would set the start address of the data segment to 10100h.
As stated earlier in this chapter, when in Protected Mode the OS programmer must be able to define many more properties of a segment in addition to its start memory address. It should be obvious that it would not be possible to define all of these characteristics in the 16-bit segment register.
In Protected Mode, it requires eight bytes of information to describe all of the characteristics associated with a particular segment of memory space. The protected mode OS must provide an eight byte descriptor for each memory segment to be used by each program (including those used by the OS itself). It would consume a great deal of processor real estate to keep descriptors for all segments used by all programs in registers on the processor chip itself. For this reason, the descriptors are stored in special tables in memory. The next section provides a description of the descriptor tables.
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