@Override
    void setStartAddress(Address addr) {
		message = "";
		startAddr = addr;
		
		if (startAddr == null) {
			message = "Invalid Address";
		}

		else if (startAddr.compareTo(block.getStart()) >= 0) {
			message = "Start must be less than " +  
						  block.getStart();
		}
		else {
			length = block.getEnd().subtract(startAddr) +1;
		}
		listener.stateChanged(null);		
	}
@see ghidra.app.plugin.core.memory.ExpandBlockModel#setStartAddress(ghidra.program.model.address.Address)
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/memory/ExpandBlockUpModel.java