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All af this must be done using the straightforward syntax 
described earlier. The operator/analusts CANNOT be expected 
to slip in and out of the user-interface handler to the 
operating—system utilities and direct application program 
control. 
THIS MEANS THAT EVERY KEY THAT THE OPERATOR/ANALYST  TOUCHES 
WILL APPEAR TO BE INGESTED AND HANDLED BY ONE AND ONLY ONE 
PROGRAM. If you want to avoid serious maintenance problems, 
I recommend that there be, in fact. only one interface 
program. 
6. 7: HISTORY LOGGING 
HISTORY LOGGING capability should include: turning it. off 
and on. deleting it and printing it. The log should include 
time stamps for the purpose of execution timing: especially 
in AZ systems. If your system will provide the sort of 
procedural control that I have already described, it is not 
necessary that the log file actually be an executable entity 
in the manner of journal files. 
6. 8: ERROR HANDLING 
ERROR HANDLING is probably the most difficult aspect of the 
user interface. The Teason for this escapes many 
non-programmenrs, but it is due to the fact that there are 
several fundamentally distinct kinds of errors and we do not 
seem able to handle all of them in the same manner, although 
we should try to make it appear that way, if we can. 
6.8. 1 HARDWARE ERRORS. The user interface handler won't 
know about peripheral hardware other than disk and terminal. 
although the application package hosted by the interface 
might include exercise and diagnostic programs specific to 
displaus, plotters, digitizers, and film recorders, 
&.8. 2 OPERATING SYSTEM ERRORS. The user interface handler 
should be designed to make the operating sustem transparent. 
Operating system errors should not show up at this level. 
If they occur during the execution of an application 
program, they should be treated as detectable programmer 
errors. 
&. 8 3 DETECTABLE PROGRAMMER ERRORS. In the case ne 
detectable programmer errors, the application package error 
handler must produce a message (preferably hardcopy) that 
states enough, succinct information to enable the 
maintenance group (original supplier?) to solve the problem.