AMD Am186TMER, Am188TMER Addressing Modes, Register and Immediate Operands, Memory Operands

2.7ADDRESSING MODES

The Am186ER and Am188ER microcontrollers use eight categories of addressing modes to specify operands. Two addressing modes are provided for instructions that operate on register or immediate operands; six modes are provided to specify the location of an operand in a memory segment.

Register and Immediate Operands

νRegister Operand Mode—The operand is located in one of the 8- or 16-bit registers.

νImmediate Operand Mode—The operand is included in the instruction.

Memory Operands

A memory-operand address consists of two 16-bit components: a segment value and an offset. The segment value is supplied by a 16-bit segment register either implicitly chosen by the addressing mode or explicitly chosen by a segment override prefix. The offset, also called the effective address, is calculated by summing any combination of the following three address elements:

1.Displacement—an 8-bit or 16-bit immediate value contained in the instruction

2.Base—contents of either the BX or BP base registers

3.Index—contents of either the SI or DI index registers

Any carry from the 16-bit addition is ignored. Eight-bit displacements are sign-extended to 16-bit values.

Combinations of the above three address elements define the following six memory addressing modes (see Table 2-3):

1.Direct Mode—The operand offset is contained in the instruction as an 8- or 16-bit displacement element.

2.Register Indirect Mode—The operand offset is in one of the registers: SI, DI, BX, or BP.

3.Based Mode—The operand offset is the sum of an 8- or 16-bit displacement and the contents of a base register (BX or BP).

4.Indexed Mode—The operand offset is the sum of an 8- or 16-bit displacement and the contents of an index register (SI or DI).

5.Based Indexed Mode—The operand offset is the sum of the contents of a base register (BX or BP) and an index register (SI or DI).

6.Based Indexed Mode with Displacement—The operand offset is the sum of a base register’s contents, an index register’s contents, and an 8-bit or 16-bit displacement.

Table 2-3	Memory Addressing Mode Examples
	Addressing Mode	Example
	Direct	mov ax, ds:4
	Register Indirect	mov ax, [si]
	Based	mov ax, [bx]4
	Indexed	mov ax, [si]4
	Based Indexed	mov ax, [si][bx]
	Based Indexed with Displacement	mov ax, [si][bx]4

2-10 Programming