The Structure of Cedar, 1985

Machine. An original goal for Cedar was to combine a compiled, strongly-typed language with the interpretive symbolic power of lnterlisp or Smalltalk. The Abstract Machine is a step in this direction. Its facilities are all ultimately based on the symbol tables and program graphs that the compiler, binder, and program loaders produce. Its primary use at present is in support of ordinary Cedar applications that serve as interactive interpreters, debugging tools, performance monitoring, and other tools for presenting program data in a form sensible to users. The Abstract Machine (AM) implementation is based on the following concepts: • Runtime types. The unique type tags that label allocated objects are also used by all the abstract machine interfaces as runtime type values. • Program control. The AMEvents interface provides a set of low-level operations for setting breakpoints and for tracing program flow. • Type information. The AMTypes interface provides procedural access to the names and structure of data types, including a complete set of operations for analyzing the internal structure of composite types. • Value manipulation. Other AMTypes procedures permit examination and modification of runtime values. The association between the referents of REF variables and their type tags can be made safely and automatically by the system; for other values, the associations are "based on TRUSTED program assertions. These operations support interpretive programs that can operate on arbitrary data structures; they are always significantly more expensive than the corresponding compiled Cedar statements operating directly on the same objects. • Program and process structure information. The AMModel interface provides similar facilities for investigating program structure: the makeup of procedures in terms of their embedded blocks, of program modules in terms of their procedures, and of configurations in terms of their program modules and subconfigurations. A description of the loaded configurations and their associated global information within a running Cedar system is also available through AMModel. Using the AMProcess interface, one can enumerate the active processes, suspend or resume the operation of selected processes, and locate the top activation record for a given process. • Multiple virtual memory access. AM uses the WorldVM interface for all references to runtime values and to runtime program and process structures. WorldVM supports symbolic access to the local address space, to a worldswap environment (a restartable memory image saved on disk) or to a remote environment (accessed using network communications). The arms-length methods are infrequently used, but they are invaluable when the local methods fail (see §2.4). Imager. Cedar applications rely on the power and flexibility of high-resolution bitmapped display terminals. In earlier Xerox systems, system support for interactive graphics was limited to low-level bitmap operations, such as the RasterOp (BitBlt) function described by Newman and Sproull in [28]. While it is possible in Cedar to manipulate bitmaps directly, most applications instead use the Imager package, which provides support for the presentation of such graphical images as multiple-font text, lines, curves, closed outlines, and sampled images. These images can be scaled, rotated, translated, and clipped to arbitrary rectangular boundaries by providing the package with simple specifications. Programs can render images in a device-independent fashion on color or black and white display devices, or on a variety