Lecture-9.1: Addressing Modes related Mathematics in 8086 Microprocessor - Part-1(MPU)

Addressing Modes related Mathematics in 8086 Microprocessor

Logical Address

A logical address is generated by the CPU while a program runs. It represents the address from the process’s perspective and does not exist physically; hence it is also called a virtual address.

The logical address space is the set of all logical addresses a process can generate. Programs use logical addresses to reference memory, and the MMU translates them into physical addresses when accessing actual memory.

 

Physical Address

A physical address is the real location in main memory (RAM) where data or instructions are stored. The physical address space consists of all physical addresses corresponding to logical addresses.

The MMU performs address translation using a page table, mapping each logical page to a physical frame. This allows processes to access memory transparently, without knowing actual memory locations.

 

Memory Management Unit

The Memory Management Unit (MMU) is a hardware component in the computer system that handles all memory and caching operations associated with the CPU. Its main function is to translate logical (virtual) addresses generated by the CPU into physical addresses in main memory.

Difference Between Logical address and Physical Address:

Parameter	LOGICAL ADDRESS	PHYSICAL ADDRESS
Basic	generated by CPU	location in a memory unit
Address Space	Logical Address Space is set of all logical addresses generated by CPU in reference to a program.	Physical Address is set of all physical addresses mapped to the corresponding logical addresses.
Visibility	User can view the logical address of a program.	User can never view physical address of program.
Generation	generated by the CPU	Computed by MMU
Access	The user can use the logical address to access the physical address.	The user can indirectly access physical address but not directly.
Editable	Logical address can be change.	Physical address will not change.
Also called	virtual address.	real address.

In the 8086 microprocessors, the effective address (EA) is a 16-bit offset value that, when combined with a 16-bit segment register, produces the 20-bit physical memory address. The EA is also known as the offset address because it indicates the distance from the beginning of a memory segment to the actual memory location of the operand.

The EA is not a single value but is calculated differently depending on the instruction's addressing mode. The calculation can involve up to three components: 

• Displacement: An immediate 8-bit or 16-bit value included in the instruction.
• Base: The value of a base register, either BX (Base Register) or BP (Base Pointer).
• Index: The value of an index register, either SI (Source Index) or DI (Destination Index). 

 

Effective address calculation by addressing mode 

Here is how the EA is calculated in different 8086 memory addressing modes:

Direct addressing

The 16-bit effective address is directly specified as part of the instruction.

Ø Example: MOV AX, [5000H]

Ø     Effective Address (EA): 5000H

Ø Physical Address:  DS × 10H + 5000H

Register indirect addressing 

The EA is held in one of the bases (BX, BP) or index (SI, DI) registers. 

• Example: MOV AX, [BX]
• Effective Address (EA): The value of BX.
• Physical Address: (DS X 10H) + [BX] ; (if BX, SI, or DI is used). If BP is used, the default segment is SS. 

Based addressing 

The EA is the sum of a base register (BX or BP) and an 8-bit or 16-bit displacement. 

• Example: MOV DX, [BX+04H]
• Effective Address (EA):  [BX] + 04H
• Physical Address:  (DS X 10H) + [BX] + 04H

Indexed addressing 

The EA is the sum of an index register (SI or DI) and an 8-bit or 16-bit displacement. 

• Example: MOV BX, [SI+16H]
• Effective Address (EA):  [SI] + 16H
• Physical Address:  (DS X 10H) + [SI] + 16H

Based-indexed addressing 

The EA is the sum of a base register (BX or BP) and an index register (SI or DI). 

• Example: MOV AX, [BX][SI] or MOV AX, [BX+SI]
• Effective Address (EA):  [BX] + [SI]
• Physical Address:   (DS X 10H) + [BX] + [SI] + 16H

Relative based-indexed addressing 

The EA is the sum of a base register (BX or BP), an index register (SI or DI), and an 8-bit or 16-bit displacement. 

• Example: MOV DL, [BX+SI+50H]
• Effective Address (EA):  [BX] + [SI] + 50H
• Physical Address:  (DS X 10H) + [BX] + [SI] + 50H

 

Physical address calculation 

Once the 16-bit effective address is determined, the 8086 calculates the 20-bit physical address using one of the four segment registers (CS, DS, SS, ES): 

Physical Address = (Segment Register X 10H) + Effective Address (EA) / Offset Address

The default segment register depends on the memory access: 

• For data accesses, the default is the DS (Data Segment) register.
• For stack operations (PUSH, POP), the default is the SS (Stack Segment) register.
• For instruction fetches, the CS (Code Segment) register is used.
• For string operations, the DI register uses the ES (Extra Segment) register.