user with tools for and e TE ent development and execution of 
application e : 0 ting systems have, besides assemblers 
and compilers (e.g. Forse ege capabilities for editing, linking, 
overlay segment creation, and on line debugging. The librarian programs 
associated with these systems allow for placing of debugged object 
modules in a library for later inking of different object modules prior 
to loading nd execution. Executive program subroutines are usually 
modular in structure so that the user may select the modules he needs 
and eliminate others. Users application programs are also treated by 
the executive as device independent. Devices can therefore be assigned 
just before the program is executed and the same program may use diffe- 
rent devices at different times. In other words the tendency in the 
design of these software operating systems is to provide maximal effi- 
ciency in the use of hardware resources. Most of these systems also 
have a number of utility programs such as those for manipulation, main- 
tenance and verification of files and for copying the entire contents 
of discs or magnetic tapes on other discs or magnetic tapes. 
Some of these systems, as for example the single user, low 
overhead, real-time operating system, do have properties that make their 
use in analytical instruments quite attractive. These systems normally 
provide two monitors i.e. a single job monitor and a foreground/back- 
ground monitor. The latter allows for simultaneous operation of two 
programs. Consequently the real-time operations may be controlled by a 
high priority foreground program while the functions which do not have 
critical response time requirements may be accomplished with the back- 
ground program which operates whenever the foreground program is not 
busy. Although these two programs operate independently, they can 
communicate with each other through job communication areas in the 
memory or through disc files. The advantages of using a foreground/ 
background monitor on analytical instruments is quite evident since the 
real-time programs are active only part of the time, many off-line parts 
of application programs may be run intermittently in the background 
while maintaining the internal exchange of information with real-time 
programs. 
As already mentioned, the higher languages of these software 
operating systems combined with the use of double precision floating 
point processors will allow for writing of programs of sufficiently 
short operating times for most of the real-time requirements. 
Naturally, in exceptional cases, one can resort to writing the real- 
time programs in assemblers that would, by a conservative estimate, 
increase their speed of operation at least three times. 
2.2. Optical-mechanical component and interface of positioning devices 
For reasons explained earlier, the optical-mechanical component 
of an analytical instrument is essentially a stereocomparator equipped 
with sampled data positioning devices. These are the analog components 
of an analytical instrument and as such the limiting factors of accu- 
racy, speed of operation and universality. Although, thanks to the 
relatively simple construction of these components and the digitizing 
of the photo-coordinates, it is possible to match, with some effort, 
the accuracy attainable by high precision photography. The separation 
of measuring components from those of locomotion in positioning devices 
are for example a good way to increase the speed without negative