IBM 5250, originally, was a particular moel of a terminal device sold with the IBM System/34 minicomputer system. Similar to the IBM 3270, it is a block-oriented terminal protocol, yet is incompatible with the 3270 standard. The relationship between the terminal and system was rich: The system sent a data stream containing markup bytes indicating the boundaries of data entry fields, highlighted or colored areas, and indication that the system was busy responding to the last request from the terminal. The 5250 data stream definition has been refined over time to include GUI elements like pop-up windowing, check and option boxes, mouse handling, and pull-down menus.