LCD problem

Sounds like you might have a "backwards" MSB How many lines on your screen? Any chance 2/3 of that number works out to a power of two?

Think about things that could cause the high bit to be backwards:

Wiring error Mixing up signed and unsigned numbers starting from an initial value other than zero (you did initialze the value, right?) Wrong electrical sense

The last one is fun - you would expect that all bits would have the same electrical sense, but that isn't necessarily so. Some LCDs are implemented with multiple controllers sharing the actual display. The high bit is then not really an address line, but a select between the two regions. Nobody said the assigment of high/low there had to be consistent with the line addresses within each region...