Modes of Addressing

• Modes of Addressing:

  • Immediate
  • Direct
  • Indirect
  • Indexed
• • Immediate Addressing 
  • Operand is part of the instruction itself 

  MOV AX, 1234h // Moves the immediate hex value 1234h to the AX register
• • Direct Addressing 
  • The memory address of the operand is directly specified 

  MOV AX, [1234h] // Take the value stored in memory location 1234h and move to the AX register 
• Indirect Addressing #1

  • A register contains the memory address of the operand 
  • If BX contains the value 2000h: 

  MOV AX, [BX] // Moves the value from memory location 2000h to the AX register 

  • This does not mean "Move the value 2000h into AX" 
  • Instead, it means, "Look in the memory address 2000h (the value currently stored in the BX register) and move whatever value you find into the AX register." 
• Indirect Addressing

  • When brackets [ ] are around a register in assembly language (like [BX]), it's an instruction to treat the value inside that register as a memory address and to use the data at that memory address for the operation 
• • Indexed Addressing 
  • Combines a base address with an index to compute the effective address 
  • If BX contains 0050h and SI has a base address 1000h: 

  MOV AX, [BX + SI] // Move the value at memory location 1050h to AX 

  • Fetches data from the effective address (because 1000h + 0050h is 1050h) and moves it into the AX register