Slide 6.3: Zero/sign extension of integers

Zero/Sign Extension of Integers

Zero/Sign extension of integers mainly deals with the problems caused by the restriction of MOV instruction: Both operands must be the same size.

Zero Extension of an Integer
Source (8 bits)	Destination (16 bits)
`1 0 0 1 1 0 0 1`	`0 0 0 0 0 0 0 0 1 0 0 1 1 0 0 1`

Sign Extension of an Integer
Source (8 bits)	Destination (16 bits)
`1 0 0 1 1 0 0 1`	`1 1 1 1 1 1 1 1 1 0 0 1 1 0 0 1`

Copying Smaller Values to Larger Ones
The following three examples show the problems of copying smaller values to larger ones.

The example on the left: Both operands must be the same size.
The example in the middle: It is able to solve the problem if the variable var is an unsigned integer.
The example on the right: However, this shows if the variable var is negative, the method is not working.

`EAX = WORD`	`AX = WORD`	`AX = SWORD`
.data var WORD 100h .code mov eax, var call WriteHex	.data var WORD 100h .code mov eax, 0 mov ax, var call WriteHex	.data var SWORD 0F1F0h ; F1F0h = -3600 .code mov eax, 0 mov ax, var call WriteHex
Output	Output	Output