1. Indirect Memory Operands

• Like direct memory operands, indirect memory operands specify the contents of a given address.

• However, the processor calculates the address at run time by referring to the contents of registers.

• Since values in the registers can change at run time, indirect memory operands provide dynamic access to memory.

• Indirect memory operands make possible run-time operations such as pointer indirection and dynamic indexing of array elements, including indexing of multidimensional arrays.

• For example, the following instruction moves into AX the word value found at the address in DS:BX.

          mov     ax, WORD PTR [bx]
  

  where

  • WORD specifies the data size

  • PTR re-casts memory location pointed by [BX] into the WORD-sized value.

• When you specify more than one register, the processor adds the contents of the two addresses together to determine the effective address (the address of the data to operate on):

          mov     ax, [bx+si]