imul指令of问题

The following instructions multiply -4 by 4, producing 16 in AX. AH is a sign extension
of AL so the Overflow flag is clear:
mov al,-4
mov bl,4
imul bl ; AX = FFF0h, OF = 0


The following instructions multiply 48 by 4, producing 192 in DX:AX. DX is a sign extension
of AX, so the Overflow flag is clear:
mov ax,48
mov bx,4
imul bx ; DX:AX = 000000C0h, OF = 0

符号扩展是什么？
第二个应该不是符号扩展吧，是书错了还是什么原因？

[此贴子已经被作者于2016-5-4 16:48编辑过]