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effort, particularly as we need to know the entry addresses. It is well worth trying to ensure that each section of the program works before creating a complex overlay structure.

The problem of locating re-entrant code is even more difficult if there is no intermediate stage in the compilation. One option would be to compile the host program several times, with different patches each time. Providing that the patch is always the last item, and that the compiler does not put anything significant immediately after the compiled code, we should then have all the patches at the same location with the necessary space allocated for them. We would obviously have to find out where this location is, which could prove somewhat tricky. I have noticed that HiSoft C has an extra (undocumented) directive #code, which I presume works like #data. Unfortunately there seems to be good reason for its being undocumented, namely that it totally confuses the compiler. Presumably it works on the non-CP/M systems for which the compiler is also available.

Operating a message overlay file is much less demanding. Once again we will want to have a reference table at the start of the file so that the program can call up the message it wants, but it can be much simpler. The only information we really need about the message is where to find it, since we can use a standard end-of-string code to stop the printing. Of the three programs where I have used a help file so far, none has needed more than 12K of help. This makes life very simple, to the extent that the first one does not even use random access. The reason is that there is less than one extent of the file, so all that is necessary is to place the sector number in the FCB current record byte and then read sequentially. Once again I arrange for each message to start at the beginning of a sector, so I only need one byte to address the start of the message. Thus by using one sector for the reference I could have up to 128 messages.

In practice I use a slightly more complex system which includes a coding seed to make the file unreadable to over-inquisitive hackers. It would be possible, of course, to use some form of data compression to shorten the file, but I did not consider this worth while. I also store the reference table backward, again for no particularly good reason, and to add extra complication each message starts with a heading which is automatically centred on screen when it is read. If a form feed is found in the message, the controlling software stops and awaits a key press before continuing. As with the program overlays I wrote a short program to generate the help file. It simply reads in a Wordstar text file and breaks it every time it finds a comment line (one which starts with two dots). The comment becomes the title and all that follows is the message up to the next comment (or end of file). At the moment the message numbers have to be assigned from the console. because I wanted them to be non-contiguous and I could not be bothered to devise a way to declare them in the source.