Intel 80286, 80287 manual Input/Output, Memory Operand AddressingModes, BX, 51

80286 BASE ARCHITECTURE

The following four modes are used for accessing complex data structures in memory (see figure 2-13):

Based Mode. The operand is located within the selected segment at an offset computed as the sum of the displacement and the contents of a base register (BX or BP). Based mode is often used to access the same field in different copies of a structure (often called a record). The base register points to the base of the structure (hence the term "base" register), and the displacement selects a particular field. Corresponding fields within a collection of structures can be accessed simply by changing the base register. (See figure 2-13, example 1.)

•Indexed Mode. The operand is located within the selected segment at an offset computed as the sum of the displacement and the contents of an index register (SI or DI). Indexed mode is often used to access elements in a static array (e.g., an array whose starting location is fixed at transla- tion time). The displacement locates the beginning of the array, and the value of the index register selects one element. Since all array elements are the same length, simple arithmetic on the index register will select any element. (See figure 2-13, example 2.)

Based Indexed Mode. The operand is located within the selected segment at an offset computed asthe sum of the base register's contents and an index register's contents. Based Indexed mode is often used to access elements of a dynamic array (i.e., an array whose base address can change during execution). The base register points to the base of the array, and the value of the index register is used to select one element. (See figure 2-13, example 3.)

•Based Indexed Mode with Displacement. The operand is located with the selected segment at an offset computed as the sum of a base register's contents, an index register's contents, and the displacement. This mode is often used to access elements of an array within a structure. For example, the structure could be an activation record (i.e., a region of the stack containing the register contents, parameters, and variables associated with one instance of a procedure); and one variable could be an array. The base register points to the start of the activation record, the displacement expresses the distance from the start of the record to the beginning of the array variable, and the index register selects a particular element of the array. (See figure 2-13, example 4.)

Table 2-3 gives a summary of all memory operand addressing options.

2.5INPUT/OUTPUT

The 80286 allows input/output to be performed in either of two ways: by means of a separate I/0 address space (using specific I/O instructions) or by means of memory-mapped I/O (using general- purpose operand manipulation instructions).

Table 2-3. Memory Operand A'ddressingModes

Addressing Mode	Offset Calculation
Direct	16-bit Displacement in the instruction
Register Indirect	BX, 51, 01
Based	(BX or BP) + Displacement"
Indexed	(51 or 01) + Displacement"
Based Indexed	(BX or BP) + (51 or 01)
Based Indexed + Displacement	(BX or BP) + (SI or 01) + Displacement"
" The displacement can be a 0, 8 or 16-bit value.

2-21