hex editor formatting

Question

I'm working on a hex editor in c#, and I've got it reading in a file and showing it in hex as intended, but it is quite slow, and does not format properly (The first line always has an extra pair of hex characters). I'd like to code it to read in only the portion of the file that is shown on screen, but I haven't got the slightest idea how to code that. The code that reads files is at bitfridge.com/hex.cs Couldn't post it here because there is no way to format text here.