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Unformatted text preview: bered starting from the least significant byte. See Figure 1-1. 1-4 Vol. 1 ABOUT THIS MANUAL Highest Address Data Structure 32 24 23 16 15 8 7 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 1.3.2 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 that 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 64 and IA-32 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. Vol. 1 1-5 ABOUT THIS MANUAL 126.96.36.199 Instruction Operands When instructions are represented symbolically, a subset of the IA-32 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. 1.3.3 Hexadecimal and Binary Numbers Base 16 (hexadecimal) numbers are represented by a string of hexadecimal digits followed by the character H (for example, 0F82EH). A hexadecimal digit is a character from the f...
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This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.
- Winter '11