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$17[Example I: A leaf cell editor for VLSI design]
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VLSI design systems are geared towards the management of complex,
integrated circuit designs. By the end of this decade, using available
technology, Mead [Mead 80] compares the design of integrated circuits 
with the complex task of designing an urban density road network the
size of an entire continent. The scope for error in such a project, if no 
proper tools are available, is
immense. Current fabrication technology also imposes a constraint on the
number of iterations designers can make to their circuit layouts as financial 
expenditure increases on each iteration.
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Management is currently handled by a process of abstraction, i.e. 
of hierarchical structuring of the chip design into manageable "cells". Each
cell can in turn be individually created with geometric design rule checking
applied at each separate stage. A composition tool, either graphic or
textual, is then applied to
put together individually designed cells and form the overall design.
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[Gray 79], [Buchanan 80], [Whitney 81], [Rees 83] have
proposed a methodology of circuit design based on such a hierarchical approach.
Recognising that there is a striking similarity between structured programming
techniques and VLSI design, we can apply the ideas of MODULARITY, (the
partition 
of the design into manageable "cells" with well defined functions and 
interfaces), HIERARCHY, (where modularity extends over partitioned levels of
the design), and ITERATION (the repetition of basic modules in both one and
two dimensions).
Using this design methodology, the complete design is created from a "top"
down" partition and a "bottom up" integration, with verification and 
simulation applied at various levels of the layout.
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CAD may play an important role at all levels of the hierarchy, from composition tools
at root nodes of the design system down to "leaf cell" graphics editors.
In this section we provide a short presentation of
an editor for structured design of VLSI leaf cells which has been created
around the experimental graphics interface being presented in this paper. 
The editor is a component of a major design system [Rees 83] which
comprises a high level composition language, on-line geometric design rule
checking and automatic stretching and compaction of composition cells.
We outline some of the capabilities of the editor, with particular 
emphasis on the ease with
which we can produce a leaf cell cicuit design when tools such as the
appropriate representing data structure and its efficient intepreter are 
available.
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$H[Editor design environment]
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There are certain features of a circuit design environment which are fundamental
to the user. It must exhibit: fast interactive response, the ability to 
view the design at arbitrary geometric scales ( window management and 
multiple viewports ) together with a clean and comprehensive user interface.
The designer should feel intimately coupled to his design environment, with
both control and the ability to perform structured operations such as
viewing, panning and zooming at the touch of a button.
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The editor is menu driven, and uses a mouse as the manipulating device. 
The mouse is used for pointing and editing individual 
objects on the screen, or as an area selector where whole groups of
objects are operated upon. The menu consists of representations of
components the designer can paint with (wires, transistors, contacts)
together with a set of high level functions (view, move, clone, input, 
output etc.)
which he can use to operate upon and compose design layouts. 
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While operating within its own allocated window, the editor is a multiwindow
system in itself, with one menu and potentially more than one workspace
subwindow.