The behavior of reserved bits should be regarded as

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Unformatted text preview: cant byte. Figure 1-1 illustrates these conventions. 1.5.2. Reserved Bits and Software Compatibility In many register and memory layout descriptions, certain bits are marked as reserved. When bits are marked as reserved, it is essential for compatibility with future processors that software treat these bits as having a future, though unknown, effect. The behavior of reserved bits should be regarded as not only undefined, but unpredictable. Software should follow these guidelines in dealing with reserved bits: • • • • Do not depend on the states of any reserved bits when testing the values of registers which contain such bits. Mask out the reserved bits before testing. Do not depend on the states of any reserved bits when storing to memory or to a register. Do not depend on the ability to retain information written into any reserved bits. When loading a register, always load the reserved bits with the values indicated in the documentation, if any, or reload them with values previously read from the same register. NOTE Avoid any software dependence upon the state of reserved bits in Intel Architecture registers. Depending upon the values of reserved register bits will make software dependent upon the unspecified manner in which the processor handles these bits. Programs that depend upon reserved values risk incompatibility with future processors. Highest 31 Address Data Structure 87 24 23 16 15 0 28 24 20 16 12 8 4 0 Bit offset Byte 3 Byte 2 Byte 1 Byte 0 Lowest Address Byte Offset Figure 1-1. Bit and Byte Order 1-6 ABOUT THIS MANUAL 1.5.3. Instruction Operands When instructions are represented symbolically, a subset of the Intel Architecture assembly language is used. In this subset, an instruction has the following format: label: mnemonic argument1, argument2, argument3 where: • • • A label is an identifier which is followed by a colon. A mnemonic is a reserved name for a class of instruction opcodes which 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 arithmetic or logical instruction, 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. Some assembly languages put the source and destination in reverse order. 1.5.4. Hexadecimal and Binary Numbers Base 16 (hexadecimal) numbers are represented by a string of hexadecimal digits followed by the character H (for example, F82EH). A hexadecimal digit is a character from the following set: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F. Base 2 (binary) numbers are represented by a string of 1s and 0s, sometimes followed by the character B (for example, 1010B). The “B” designation is only used in situations where confusion as to the type of number might arise. 1.5.5. Segmented Addressing The processor uses byte addressing. This means memory is organized and accessed as a sequence of bytes. Whether one or more bytes are being accessed, a byte address is used to locate the byte or bytes of memory. The range of memory that can be addressed is called an address space. The processor also supports segmented addressing. This is a form of addressing where a program may have many independent address spaces, called segments. For example, a program can keep its code (instructions) and stack in separate segments. Code addresses would always 1-7 ABOUT THIS MANUAL refer to the code space, and stack addresses would always refer to the stack space. The following notation is used to specify a byte address within a segment: Segment-register:Byte-address For example, the following segment address identifies the byte at address FF79H in the segment pointed by the DS register: DS:FF79H The following segment address identifies an instruction address in the code segment. The CS register points to the code segment and the EIP register contains the address of the instruction. CS:EIP 1.5.6. Exceptions An exception is an event that typically occurs when an instruction causes an error. For example, an attempt to divide by zero generates an exception. However, some exceptions, such as breakpoints, occur under other conditions. Some types of exceptions may provide error codes. An error code reports additional information about the error. An example of the notation used to show an exception and error code is shown below. #PF(fault code) This example refers to a page-fault exception under conditions where an error code naming a type of fault is reported. Under some conditions, exceptions which produce error codes may not be able to report an accurate code. In this case, the error code is zero, as shown below for a general-protection exception. #G...
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