An Operating Systems Vade Mecum

The command interpreter 247A new generation of terminals is now being built. In addition to characters, theycan display figures. Because they have a fairly large screen with computer control overevery dot, they are called bit-mapped displays. They typically have about 1000 dots perline and about 1000 lines.Bit-mapped displays are driven from reserved regions of physical store where a bitpattern is placed by the program. Since it can take a significant amount of computationto prepare figures (and sometimes even just letters) for such a device, such terminals arenot yet standard on most interactive-multiprogramming machines. However, they haveappeared on personal computers, where no one cares if extra computation is needed topresent a better user interface.Bit-mapped displays are often packaged with a pointing device that allows the userto specify a particular region on the display. Pointing devices have different designs.Some display screens are overlaid with a sensitive grid that can sense the touch of afinger. A separate device, usually called a ‘‘mouse,’’ can be moved over a tablet on thedesk. Its motion is conveyed to the computer, which can then display a cursor (a visualindicator of the current location) at the corresponding place on the display. The mousemay have a few keys that can be depressed to send signals to the computer.Bit-mapped displays change the character of interaction. We will discuss threeinnovations: menus, icons, and windows. A menu is a list of commands that are legal inthe current situation. Instead of typing commands to the command interpreter, the userrequests a menu to be placed somewhere on the screen. The pointing device is used topoint to the entry that is desired. Some menu entries, when selected, expand into submenus.For example, assume that the user wants to delete one.out. The current menumight have the following entries:compilefilestatuseditprintAfter the user selects file, the menu changes (or a new menu is displayed elsewhereon the screen) that looks like this:renamecopymovelinkdeletealiasviewThe user selects delete. Now a third menu appears with the names of all the files.The user selects one.out, and the file is deleted.Although this use of menus requires lots of computation (especially to provide thenames of all the files), it takes only three keystrokes for the user. Each keystroke couldinvolve one of the keys on the mouse. If the user wants to submit a command that is noton the menu, or requires flags that are not in the menu, there is usually a way to revert toline-oriented command language.Icons are graphical representations of objects, such as files or programs, andabstractions, such as today’s work or a directory. Icons can be pointed to by a mouse andmoved about on the screen. Moving an icon can have an equivalent effect on the object.For example, a file can be moved from one directory to another by dragging its icon