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During this same period of time, Parnas had been advocating the concept of 
"information hiding" to improve the quality and maintainability of software 
and to improve programmer productivity (e.g., Clements, et al., 1983). The 
principle involved is to isolate each type of software module from all other 
types. In particular, the application programmer often does not have to know 
anything about the user interface, nor the operating system or specific 
hardware on which his application program will be run. This principle is 
inherent in the design of EASI/IMP. 
Figure 2 illustrates this third major stage in the evolution of software 
architecture. The concept of a user interface system is that it is not a 
layer on the operating system, but rather a separate, complementary and 
conceptually independent entity between the user and the application 
programs. 
  
STAGE III 
  
  
USER INTERFACE HARDWARE 
(user INTERFACE SYSTEM ^ 
  
  
  
  
    
  
APPLICATION 
SOFTWARE 
  
  
( UTILITIES (IMP) 
md wu 
C OPERATING SYSTEM 
  
  
COMPUTER HARDWARE 
and RESOURCE 
  
Figure 2: The Emergence of User Interface Systems 
(Source: Morland, 1985) 
  
EASI/IMP 
Long before Morland and Parnas advocated this splitting up of user interface 
from machine from application programs, PCI had recognized the need to do so. 
The company set itself the goal of establishing a software system that was 
useable, useful, and transportable; a base on which application packages 
could be built to ensure maximum portability and minimum difficulty for both 
users and programmers. The result was EASI/IMP, a user interface and 
application manager. 
IMP -- The Interface Management Package 
IMP is a software engineering tool. It consists of two parts: a library of 
FORTRAN subroutines used to interface application programs to the outside 
world, and a collection of system utilities and accounts designed to automate 
the compiling, linking and management of tasks. To the programmer, it offers 
the advantages of reduced coding effort, reduced program maintenance costs 
and reduced design requirements. These reductions in effort result from the 
elimination of the need to be concerned about the user interface which 
results in reduced applications code required. 
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