private void customToolStripMenuItem_Click(object sender, EventArgs e)
{
foreach (var item in speedToolStripMenuItem.DropDownItems)
(item as ToolStripMenuItem).Checked = false;
(sender as ToolStripMenuItem).Checked = true;
var csf = new ClockSpeedForm();
csf.Value = CPU.ClockSpeed;
var result = csf.ShowDialog();
if(result == DialogResult.OK)
CPU.ClockSpeed = csf.Value;
}