25. Directives BYTE PTR, WORD PTR, DWORD PTR

• There are times when we need to assist assembler in translating references to data in memory.

• For example, instruction

          mov     [ESI], al  ; Store a byte-size value in memory location pointed by ESI
  

  suggests that an 8-bit quantity should be moved because AL is an 8-bit register.

• When instruction has no reference to operand size,

          mov     [ESI], 5   ; Error: operand must have the size specified
  

• To get around this instance, we must use a pointer directive, such as

          mov     BYTE PTR [ESI], 5  ; Store 8-bit value
          mov     WORD PTR [ESI], 5  ; Store 16-bit value
          mov     DWORD PTR [ESI], 5 ; Store 32-bit value
  

• These instructions require operands to be the same size.

• In general, PTR operator forces expression to be treated as a pointer of specified type:

          .DATA
          num  DWORD   0
  
          .CODE
          mov     ax, WORD PTR [num] ; Load a word-size value from a DWORD