CBW (convert byte to word) has no operands. It copies the top bit of AL, the sign bit, into all eight bits of AH. AX then holds the same signed value as AL did: 05h becomes 0005h, and FBh (−5) becomes FFFBh, which is still −5 as a word. No flags change.

AL beforeAX afterValue
05h0005h5
7Fh007Fh127
80hFF80h−128
FBhFFFBh−5

When to use it

Use it before a byte IDIV, which divides all of AX: put the dividend in AL, CBW, then divide. Use it too whenever a signed byte has to take part in word arithmetic, such as adding a signed byte offset to a word.

With MOV AH, 0 instead of CBW, AX would be 00FBh = 251 and the sum 260.
MOV     AL, -5   ; a signed byte: FBh
CBW              ; AX = FFFBh, still -5
ADD     AX, 9    ; AX = 4
ADD     AL, '0'  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print DL
INT     21h

Unsigned values

For an unsigned byte, extend with zeros instead: MOV AH, 0 (or XOR AH, AH). CBW would turn 200 (C8h) into FFC8h, which is not 200 any more.

Pitfalls

CBW works only on AL and AH; there is no form for other registers. To sign-extend BL into BX on the 8086, go through AL (MOV AL, BL / CBW / MOV BX, AX), or test the sign and set the high byte yourself.

Later CPUs

The 386 adds CWDE (AX into EAX) and MOVSX, which sign-extends any byte or word register or memory operand into any wider register.