of Microprocessors

MICROPROCESSORS 149
Text Editor
Much of the actual time a user spends with a microcomputer system is
probably spent with the text editor program. Even if he or she is a perfect
typist and never needs to actually edit, the editor is necessary to create text
files for the assembler or compiler or even music interpreter. However, since
people are seldom perfect typists and are never perfect programmers, the
editor is also used to add, delete, change, and move program text. Most
editors are limited to editing files that are small enough to fit into main
memory. Thus, large programs must be broken into acceptably small
segments and edited separately. Typically, a file would be read into a text
buffer in memory, edited as required, and a new file would be created. The
old file can then be deleted if desired. If large insertions that might cause the
file to exceed available memory are anticipated, a portion of the text buffer
contents may be written as one file, deleted from memory, and the remainder
along with the insertion would be written as another file. The assembler or
compiler can link the segments together into a single program. Having a
large amount of memory available is obviously a benefit when using this type
of editor.
Highly sophisticated editors supplied as part of some disk-operating
systems are able to handle any size file by scrolling the text through memory
forward and backward in response to user commands. Insertions and deletions
may be made in any order by scrolling the text on the screen to the
desired point and then keying in the change. The inherent editing capability
of the disk allows long files to be edited directly without creating unnecessary
copies. Less advanced editors may still allow unlimited file size and
insertions but can only scroll forward, thus requiring that editing be done in
sequence to some extent.
Assembler
The purpose ofan assembler is to convert program source text statements
into binary machine language object code and a printed listing. Assemblers
work in a variety of ways according to the size of system they were designed
for and level of sophistication.
Today, good assemblers work with a disk-operating system in a general
way that is much more convenient than the early cassette-based ones were.
Before being run, the assembler is given the name of the source file, which
already exists, and the names of two new files that it will create; one to hold
the object code and the other to hold the listing. The assembler then scans
the source file two or three times and produces the object and listing files.
Before printing the listing file, the user can quickly scan it using the editor
to see ifany errors were flagged by the assembler. Assuming there were few or
none, the editor can be commanded to print the listing if one is actually