Chapter 1   Introduction to the 80386

Chapter 1 Introduction to the 80386


Chapter 1  Introduction to the 80386

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The 80386 is an advanced 32-bit microprocessor optimized for multitasking
operating systems and designed for applications needing very high
performance. The 32-bit registers and data paths support 32-bit addresses
and data types. The processor can address up to four gigabytes of physical
memory and 64 terabytes (2^(46) bytes) of virtual memory. The on-chip
memory-management facilities include address translation registers,
advanced multitasking hardware, a protection mechanism, and paged virtual
memory. Special debugging registers provide data and code breakpoints even
in ROM-based software.



1.1 Organization of This Manual

1.1 Organization of This Manual This book presents the architecture of the 80386 in five parts: Part I -- Applications Programming Part II -- Systems Programming Part III -- Compatibility Part IV -- Instruction Set Appendices These divisions are determined in part by the architecture itself and in part by the different ways the book will be used. As the following table indicates, the latter two parts are intended as reference material for programmers actually engaged in the process of developing software for the 80386. The first three parts are explanatory, showing the purpose of architectural features, developing terminology and concepts, and describing instructions as they relate to specific purposes or to specific architectural features. Explanation Part I -- Applications Programming Part II -- Systems Programming Part III -- Compatibility Reference Part IV -- Instruction Set Appendices The first three parts follow the execution modes and protection features of the 80386 CPU. The distinction between applications features and systems features is determined by the protection mechanism of the 80386. One purpose of protection is to prevent applications from interfering with the operating system; therefore, the processor makes certain registers and instructions inaccessible to applications programs. The features discussed in Part I are those that are accessible to applications; the features in Part II are available only to systems software that has been given special privileges or in unprotected systems. The processing mode of the 80386 also determines the features that are accessible. The 80386 has three processing modes: 1. Protected Mode. 2. Real-Address Mode. 3. Virtual 8086 Mode. Protected mode is the natural 32-bit environment of the 80386 processor. In this mode all instructions and features are available. Real-address mode (often called just "real mode") is the mode of the processor immediately after RESET. In real mode the 80386 appears to programmers as a fast 8086 with some new instructions. Most applications of the 80386 will use real mode for initialization only. Virtual 8086 mode (also called V86 mode) is a dynamic mode in the sense that the processor can switch repeatedly and rapidly between V86 mode and protected mode. The CPU enters V86 mode from protected mode to execute an 8086 program, then leaves V86 mode and enters protected mode to continue executing a native 80386 program. The features that are available to applications programs in protected mode and to all programs in V86 mode are the same. These features form the content of Part I. The additional features that are available to systems software in protected mode form Part II. Part III explains real-address mode and V86 mode, as well as how to execute a mix of 32-bit and 16-bit programs. Available in All Modes Part I -- Applications Programming Available in Protected Part II -- Systems Programming Mode Only Compatibility Modes Part III -- Compatibility

1.1.1 Part I -- Applications Programming

1.1.1 Part I -- Applications Programming This part presents those aspects of the architecture that are customarily used by applications programmers. Chapter 2 -- Basic Programming Model: Introduces the models of memory organization. Defines the data types. Presents the register set used by applications. Introduces the stack. Explains string operations. Defines the parts of an instruction. Explains addressing calculations. Introduces interrupts and exceptions as they may apply to applications programming. Chapter 3 -- Application Instruction Set: Surveys the instructions commonly used for applications programming. Considers instructions in functionally related groups; for example, string instructions are considered in one section, while control-transfer instructions are considered in another. Explains the concepts behind the instructions. Details of individual instructions are deferred until Part IV, the instruction-set reference.

1.1.2 Part II -- Systems Programming

1.1.2 Part II -- Systems Programming This part presents those aspects of the architecture that are customarily used by programmers who write operating systems, device drivers, debuggers, and other software that supports applications programs in the protected mode of the 80386. Chapter 4 -- Systems Architecture: Surveys the features of the 80386 that are used by systems programmers. Introduces the remaining registers and data structures of the 80386 that were not discussed in Part I. Introduces the systems-oriented instructions in the context of the registers and data structures they support. Points to the chapter where each register, data structure, and instruction is considered in more detail. Chapter 5 -- Memory Management: Presents details of the data structures, registers, and instructions that support virtual memory and the concepts of segmentation and paging. Explains how systems designers can choose a model of memory organization ranging from completely linear ("flat") to fully paged and segmented. Chapter 6 -- Protection: Expands on the memory management features of the 80386 to include protection as it applies to both segments and pages. Explains the implementation of privilege rules, stack switching, pointer validation, user and supervisor modes. Protection aspects of multitasking are deferred until the following chapter. Chapter 7 -- Multitasking: Explains how the hardware of the 80386 supports multitasking with context-switching operations and intertask protection. Chapter 8 -- Input/Output: Reveals the I/O features of the 80386, including I/O instructions, protection as it relates to I/O, and the I/O permission map. Chapter 9 -- Exceptions and Interrupts: Explains the basic interrupt mechanisms of the 80386. Shows how interrupts and exceptions relate to protection. Discusses all possible exceptions, listing causes and including information needed to handle and recover from the exception. Chapter 10 -- Initialization: Defines the condition of the processor after RESET or power-up. Explains how to set up registers, flags, and data structures for either real-address mode or protected mode. Contains an example of an initialization program. Chapter 11 -- Coprocessing and Multiprocessing: Explains the instructions and flags that support a numerics coprocessor and multiple CPUs with shared memory. Chapter 12 -- Debugging: Tells how to use the debugging registers of the 80386.

1.1.3 Part III -- Compatibility

1.1.3 Part III -- Compatibility Other parts of the book treat the processor primarily as a 32-bit machine, omitting for simplicity its facilities for 16-bit operations. Indeed, the 80386 is a 32-bit machine, but its design fully supports 16-bit operands and addressing, too. This part completes the picture of the 80386 by explaining the features of the architecture that support 16-bit programs and 16-bit operations in 32-bit programs. All three processor modes are used to execute 16-bit programs: protected mode can directly execute 16-bit 80286 protected mode programs, real mode executes 8086 programs and real-mode 80286 programs, and virtual 8086 mode executes 8086 programs in a multitasking environment with other 80386 protected-mode programs. In addition, 32-bit and 16-bit modules and individual 32-bit and 16-bit operations can be mixed in protected mode. Chapter 13 -- Executing 80286 Protected-Mode Code: In its protected mode, the 80386 can execute complete 80286 protected-mode systems, because 80286 capabilities are a subset of 80386 capabilities. Chapter 14 -- 80386 Real-Address Mode: Explains the real mode of the 80386 CPU. In this mode the 80386 appears as a fast real-mode 80286 or fast 8086 enhanced with additional instructions. Chapter 15 -- Virtual 8086 Mode: The 80386 can switch rapidly between its protected mode and V86 mode, giving it the ability to multiprogram 8086 programs along with "native mode" 32-bit programs. Chapter 16 -- Mixing 16-Bit and 32-Bit Code: Even within a program or task, the 80386 can mix 16-bit and 32-bit modules. Furthermore, any given module can utilize both 16-bit and 32-bit operands and addresses.

1.1.4 Part IV -- Instruction Set

1.1.4 Part IV -- Instruction Set Parts I, II, and III present overviews of the instructions as they relate to specific aspects of the architecture, but this part presents the instructions in alphabetical order, providing the detail needed by assembly-language programmers and programmers of debuggers, compilers, operating systems, etc. Instruction descriptions include algorithmic description of operation, effect of flag settings, effect on flag settings, effect of operand- or address-size attributes, effect of processor modes, and possible exceptions.

1.1.5 Appendices

1.1.5 Appendices The appendices present tables of encodings and other details in a format designed for quick reference by assembly-language and systems programmers.

1.2 Related Literature

1.2 Related Literature The following books contain additional material concerning the 80386 microprocessor: * Introduction to the 80386, order number 231252 * 80386 Hardware Reference Manual, order number 231732 * 80386 System Software Writer's Guide, order number 231499 * 80386 High Performance 32-bit Microprocessor with Integrated Memory Management (Data Sheet), order number 231630

1.3 Notational Conventions

1.3 Notational Conventions This manual uses special notations for data-structure formats, for symbolic representation of instructions, for hexadecimal numbers, and for super- and sub-scripts. Subscript characters are surrounded by {curly brackets}, for example 10{2} = 10 base 2. Superscript characters are preceeded by a caret and enclosed within (parentheses), for example 10^(3) = 10 to the third power. A review of these notations will make it easier to read the manual.

1.3.1 Data-Structure Formats

1.3.1 Data-Structure Formats In illustrations of data structures in memory, smaller addresses appear at the lower-right part of the figure; addresses increase toward the left and upwards. Bit positions are numbered from right to left. Figure 1-1 illustrates this convention. See Also:
Fig.1-1

1.3.2 Undefined Bits and Software Compatibility

1.3.2 Undefined Bits and Software Compatibility In many register and memory layout descriptions, certain bits are marked as undefined. When bits are marked as undefined (as illustrated in Figure 1-1), it is essential for compatibility with future processors that software treat these bits as undefined. Software should follow these guidelines in dealing with undefined bits: * Do not depend on the states of any undefined bits when testing the values of registers that contain such bits. Mask out the undefined bits before testing. * Do not depend on the states of any undefined bits when storing them in memory or in another register. * Do not depend on the ability to retain information written into any undefined bits. * When loading a register, always load the undefined bits as zeros or reload them with values previously stored from the same register. ---------------------------------------------------------------------------- NOTE Depending upon the values of undefined register bits will make software dependent upon the unspecified manner in which the 80386 handles these bits. Depending upon undefined values risks making software incompatible with future processors that define usages for these bits. AVOID ANY SOFTWARE DEPENDENCE UPON THE STATE OF UNDEFINED 80386 REGISTER BITS. ---------------------------------------------------------------------------- See Also:
Fig.1-1

1.3.3 Instruction Operands

1.3.3 Instruction Operands When instructions are represented symbolically, a subset of the 80386 Assembly Language is used. In this subset, an instruction has the following format: label: prefix mnemonic argument1, argument2, argument3 where: * A label is an identifier that is followed by a colon. * A prefix is an optional reserved name for one of the instruction prefixes. * A mnemonic is a reserved name for a class of instruction opcodes that have the same function. * The operands argument1, argument2, and argument3 are optional. There may be from zero to three operands, depending on the opcode. When present, they take the form of either literals or identifiers for data items. Operand identifiers are either reserved names of registers or are assumed to be assigned to data items declared in another part of the program (which may not be shown in the example). When two operands are present in an instruction that modifies data, the right operand is the source and the left operand is the destination. For example: LOADREG: MOV EAX, SUBTOTAL In this example LOADREG is a label, MOV is the mnemonic identifier of an opcode, EAX is the destination operand, and SUBTOTAL is the source operand.

1.3.4 Hexadecimal Numbers

1.3.4 Hexadecimal Numbers Base 16 numbers are represented by a string of hexadecimal digits followed by the character H. A hexadecimal digit is a character from the set (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F). In some cases, especially in examples of program syntax, a leading zero is added if the number would otherwise begin with one of the digits A-F. For example, 0FH is equivalent to the decimal number 15.

1.3.5 Sub- and Super-Scripts

1.3.5 Sub- and Super-Scripts This manual uses special notation to represent sub- and super-script characters. Sub-script characters are surrounded by {curly brackets}, for example 10{2} = 10 base 2. Super-script characters are preceeded by a caret and enclosed within (parentheses), for example 10^(3) = 10 to the third power.