Q2Microprocessor and Interfaces

Addressing modes define how the operand's effective address (offset) is determined:

• Immediate: Operand is given in the instruction (e.g., MOV AX, 1234H).
• Register: Operand is in a register (e.g., MOV AX, BX).
• Direct: 16-bit offset address is given in the instruction (e.g., MOV AX, [5000H]).
• Register Indirect: Offset is held in a base/index register like BX, SI, DI (e.g., MOV AX, [BX]).
• Based: Offset is sum of base register and displacement (e.g., MOV AX, [BX+10H]).
• Indexed: Offset is sum of index register and displacement (e.g., MOV AX, [SI+20H]).
• Based Indexed: Offset is sum of base, index, and optional displacement (e.g., MOV AX, [BX+SI+5H]).

Segmentation is the mechanism of dividing the 1MB physical address space of the 8086 into 64KB logical blocks called segments. The start address of a segment is stored in a Segment Register (CS, DS, SS, ES).

The exact location of data within a segment is given by a 16-bit Offset Address (or Effective Address). The Bus Interface Unit (BIU) computes the actual 20-bit physical address by shifting the segment register value 4 bits to the left (multiplying by 16) and adding the offset. Physical Address = (Segment Address * 10H) + Offset Address. Advantages include independent code and data areas, easy relocation of programs, and protection.