To be specified explicitly in these cases the 16 bit

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Unformatted text preview: mponents are restricted in the following manner: The ESP register cannot be used as an index register. When the ESP or EBP register is used as the base, the SS segment is the default segment. In all other cases, the DS segment is the default segment. The base, index, and displacement components can be used in any combination, and any of these components can be NULL. A scale factor may be used only when an index also is used. Each possible combination is useful for data structures commonly used by programmers in high-level languages and assembly language. The following addressing modes suggest uses for common combinations of address components. Displacement A displacement alone represents a direct (uncomputed) offset to the operand. Because the displacement is encoded in the instruction, this form of an address is sometimes called an absolute or static address. It is commonly used to access a statically allocated scalar operand. Base A base alone represents an indirect offset to the operand. Since the value in the base register can change, it can be used for dynamic storage of variables and data structures. Base + Displacement A base register and a displacement can be used together for two distinct purposes: -- As an index into an array when the element size is not 2, 4, or 8 bytes--The displacement component encodes the static offset to the beginning of the array. The base register holds the results of a calculation to determine the offset to a specific element within the array. -- To access a field of a record: the base register holds the address of the beginning of the record, while the displacement is a static offset to the field. Vol. 1 3-31 BASIC EXECUTION ENVIRONMENT An important special case of this combination is access to parameters in a procedure activation record. A procedure activation record is the stack frame created when a procedure is entered. Here, the EBP register is the best choice for the base register, because it automatically selects the stack segment. This is a compact encoding for this common function. (Index Scale) + Displacement This address mode offers an efficient way to index into a static array when the element size is 2, 4, or 8 bytes. The displacement locates the beginning of the array, the index register holds the subscript of the desired array element, and the processor automatically converts the subscript into an index by applying the scaling factor. Base + Index + Displacement Using two registers together supports either a two-dimensional array (the displacement holds the address of the beginning of the array) or one of several instances of an array of records (the displacement is an offset to a field within the record). Base + (Index Scale) + Displacement Using all the addressing components together allows efficient indexing of a two-dimensional array when the elements of the array are 2, 4, or 8 bytes in size. Specifying an Offset in 64-Bit Mode The offset part of a memory address in 64-bit mode can be spe...
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