mac hdmi声音输出设置:请帮俺解释一下do循环结构代码段即可，谢谢

Private Sub command1_Click()
Dim no As Integer, flag As Integer
Dim m As Integer, top As Integer, bott As Integer
flag = -1 '置未找到标志
top = 1: bott = 10 '设定范围
no = Val(Text1.Text) '取学号
If no < h(top) Or no > h(bott) Then
flag = -2 '若超出学号范围, 置特殊标志-2
End If

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Do While flag = -1 And top <= bott
m = (top + bott) / 2 '取中点
Select Case True
Case no = h(m) '找到
flag = m '置找到标志
Text2.Text = h(m)
Text3.Text = d(m, 1)
Text4.Text = d(m, 2)
Text5.Text = (d(m, 1) + d(m, 2)) / 2
Case no < h(m) '小于中间数据
bott = m - 1 '上半部
Case no > h(m) '大于中间数据
top = m + 1 '下半部
End Select