The type of 3270 emulation that most CERN users have been used to is that
done by the IBM 7171 systems: any CERN user having used Index class 103/125
or DECserver service name CERNVM will fall into this category.
Unfortunately, neither real IBM 3270 displays nor most other emulators
(e.g. IBM PCs, Macs or other workstations) behave like this 7171 emulation.

The main difference seen when using a real 3270, or emulation, is that
blank characters and null (non-existent) characters are NOT the same (even
though both look like blanks on the screen). Thus, if you are using a real
3270 then some programs, notably XEDIT, require to be told to treat nulls
properly. This is done with the command SET NULLS ON. This command may be
executed whilst in XEDIT or (preferably) included in the PROFILE XEDIT file.

Another difference is that spacing with cursor movement produces null
characters, not blanks. If these null characters are WITHIN a line then
the IBM removes them when it receives them.

TAGIBM now gives the user the option to choose between these two modes of
emulation, as well as other useful options : see the options item of the
help screen for more details.