How to access Memo field in MSSQL7?

I know if I want to access a Memo(Text) field in MSSQL7, I should use the
SQL statement of ReadText/WriteText/UpdateText.  And I got example in SQL
online book.

However, what I want to do is access them in Delphi.  In the source code, I
found inprise read the Tblobfield by using FMemo.Text:=TField(xxx).AsString,
I don't understand.  Why is it ok?  A memo field may growth up to 4k, even
more, why does it access normally?

In my mind, using AsString to access TBlobField is not correct.  Am I right?

If so, what is the correct procedure?  Any simple example?

Thanks very much.

Ajax.