eB. 
disregarding the underlying mathematical model. 
These improvements are mostly the consequence of several 
properties of mini-computers that are of particular interest when these 
machines are considered for the role of dedicated computers in analyti- 
cal instruments. The memory access time in the range of 300 to 900 
nsec is quite adequate for the overwhelming majority of predictable 
uses in photogrammetry. So is the transfer rate to peripheral devices 
that is of the order of one 16 bit word per 400 nsec (about 2.5 million 
words per second). In most of these computers the registers of peri- 
pheral devices can be handled by the central processor in the same way 
as the core memory. The same set of instructions that is used for 
manipulation of data in core memory may be used equally well for data 
in peripheral devices. For instance, data in an external device 
register can be tested or modified directly by the central processing 
unit without transferring these data into memory and without disturbing 
the general registers. A multi-level automatic priority interrupt 
system that permits the processor to respond automatically to outside 
conditions is also quite helpful. It, for example, enables the operator 
to branch to the beginning of any application program at any time and | 
during any operation of the instrument. This feature is equally useful 
in cases of errors made by the operator or by the computer in the course 
of an operation, as well as in cases when different sequences of appli- 
cation programs are needed. It also simplifies the programming by 
reducing the necessity for constant status checking of the operator's 
control switches. The 16 bit word length that is most commonly found 
in mini-computers is one of the characteristics that is not suited for 
the analytical instruments. Due to the range of magnitude of the 
numbers encountered in photogrammetric data processing use of the 
double precision mode is required, causing a significant loss of speed. 
Under these circumstances it is inevitable to resort to the use of 
floating point processors. Such processors (that fit integrally into 
the central processor) with single and double precision (32 or 64 bit) 
floating modes are available on some mini-computers. As for the memory 
capacity, 16 - 28 K core memory is sufficient for most applications of 
analytical instruments. The availability of general-purpose interfaces 
that allow for software-controlled parallel transfer of data between 
the computer and external devices simplifies considerably the inter- 
facing of optical-mechanical and other components of an analytical 
instrument. Finally the upward compatibility of the so-called computer 
families that are manufactured by larger computer companies, allow for 
relatively easy upgrading of performance of analytical instruments 
originally built with smaller or slower computers from the same family. 
The large variety of input-output devices, displays and exter- 
nal storage devices that are designed for use with mini-computers offer 
an almost unlimited choice of peripheral configurations, from a minimal 
one with a teletype and a slow tape read-punch up to those more complex 
ones using different types of magnetic tape units, fixed head discs 
(~ 256 K words), moving head discs (~ 1.2M to 10M words) and CRT inter- 
active graphic displays. Thus, the configuration of peripheral devices 
can be chosen to fit the requirements of a particular task and can be 
added to the basic configuration of an analytical instrument whenever 
the need arises. 
The software operating systems of mini-computers provide the