[ixp1200] [ixp2400] Trasmit control word in TX?

[paragon]

Hello~
I don't understand to following.
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Writing the Transmit control word.

MSF_TBUF_CONTROL_BASE is associated with TBUF element.

I thought MSF_TBUF_CONTROL_BASE + tbuf_element,
But MSF_TBUF_CONTROL_BASE + (tbuf_element <<3) in IXP2400/2800 Programming chapter 5 

Um... What is reason about tbuf_elemnt <<3? 

Please, advice me~

ref. MSF_TBUF_CONTROL_BASE = 0x1800

Thanks.