23. Direct Memory Operands

• A direct memory operand specifies the data at a given address.

• The instruction acts on the contents of the address, not the address itself.

• Except when size is implied by another operand, you must specify the size of a direct memory operand so the instruction accesses the correct amount of memory.

• The following example shows how to explicitly specify data size with the BYTE directive:

              .DATA?                  ; Segment for uninitialized data
      var     BYTE   ?                ; Reserve one byte, labeled "var"
              .CODE
              .
              .
              .
              mov    var, al          ; Copy AL to byte at var
  

• Any location in memory can be a direct memory operand as long as a size is specified (or implied) and the location is fixed.

• The data at the address can change, but the address cannot.

• By default, instructions that use direct memory addressing use the DS register.