Ispell is fashioned after the spell program from ITS (called ispell on Twenex systems.) The most common usage is "ispell filename". In this case, ispell will display each word which does not appear in the dictionary, and allow you to change it. If there are "near misses" in the dictionary (words which differ by only a single letter, a missing or extra letter, or a pair of transposed letters), then they are also displayed. If you think the word is correct as it stands, you can type either "Space" to accept it this one time, or "I" to accept it and put it in your private dictionary. If one of the near misses is the word you want, type the corresponding number. Finally, if none of these choices is right, you can type "R" and you will be prompted for a replacement word.
When a misspelled word is found, it is printed at the top of the screen. Any near misses will be printed on the following lines, and finally, two lines containing the word are printed at the bottom of the screen. If your terminal can type in reverse video, the word itself is highlighted.
The -l or "list" option to ispell is used to produce a list of misspelled words from the standard input.
The -a is intended to be used from other programs through a pipe. In this mode, ispell expects the standard input to consist of single words. Each word is read, and a single line is written to the standard output. If the word was found in the main dictionary, or your personal dictionary, then the line contains only a '*'. If the word was found through suffix removal, then the line contains a '+', a space, and the root word. If the word is not in the dictionary, but there are near misses, then the line contains an '&', a space, and a list of the near misses separated by spaces. Also, each near miss is capitalized the same as the input words. Finally, if the word neither appears in the dictionary, and there are no near misses, then the line contains only a '#'. This mode is also suitable for interactive use when you want to figure out the spelling of a single word. (These characters are the same as the codes that the real spell program uses.)
Perhaps more than ten choices should be allowed for near misses.
The hash table is stored as a quarter-megabyte array, so a PDP-11 version does not seem likely.
Ispell should understand more troff syntax, and deal more intelligently with contractions.