24. Extending Signed and Unsigned Integers

• Since moving data between registers of different sizes is illegal, you must sign-extend integers to convert signed data to a larger size.

• Sign-extending means copying the sign bit of the unextended operand to all bits of the operand's next larger size.

• This widens the operand while maintaining its sign and value.

• The four instructions presented below act only on the accumulator register (AL, AX, or EAX), as shown:

  Instruction	Sign-extend
  CBW (convert byte to word)	AL to AX
  CWD (convert word to doubleword)	AX to DX:AX
  CWDE (convert word to doubleword extended)	AX to EAX
  CDQ (convert doubleword to quadword)	EAX to EDX:EAX