80-Bus News

Disadvantages

1.   For application programs chaining could be a slower method of program management than overlays.

2.   Variables used in one program cannot (usually) be passed across to the following program.

3.   The chain function that you use may error if it does not process the command tail in the same way as the CCP. For example the command:

COMP PROG1.COM PROG2.COM

when issued normally would call the comparison program COMP to compare the two specified files. Normally the CCP would take this command and load COMP.COM into memory at 100H. The rest of the command tail is placed in memory at 81H with the command tail length at 80H. The command tail is then scrutinized and if it contains filenames then they are put at 5CH and 6CH respectively. The program is then executed.

When run COMP will take a look at the two File control blocks (FCB’s) at 5CH and 6CH in order to get the names of the files that it is to compare.

So you can see from this that the chain command will cause an error if it does not perform all the above tasks when invoking a program that requires information in the command tail.

Using a simple chain module that simply read in the COM file and executed it reaked havoc when I chained Wordstar once. Of course the name WS.COM was still in the default FCB at 5C from the reading operation, and so when Wordstar took over it tried to open WS.COM on the disk as a document file, which is no use to anyone. I had forgotten to fill the FCB’s with spaces before invoking Wordstar.

Methods of program chaining

Of course the simplest method of all is to use a language that has a chain command incorporated in it. Beware here, though, for the very reasons that are mentioned above, they may not work in all cases.

Referring back to Dr. Dark’s bit on chaining in Hisoft Pascal (80-BUS News, Vol 3. Iss 4), he gave us a method by which programs can run each other via the use of the $$$.SUB file (Read it to find out how) and also suggested that any variables could be passed across by first storing them in a disk file. This is a nice one as it lets the CCP do all the work, but will only work on drive A:.

I had a peep at the chain function in my ECO C library just to get some ideas, and here are the basics of how it works in English.

chain(filename)

      open chain file using default FCB at 5CH.

if unable to open then error and return.

read loading routine into area just below BDOS.

set stack pointer below loading routine.

jump to loading routine.

read in chain file.

set stack pointer to BDOS -1.

Jump to program at 100H.

{p38}

You should be able to find a diagram called ‘Principles of the CHAIN function’ somewhere, if you take a look at that you will see that it represents a memory map type illustration of what is going on inside the computers memory.

Program 1 is an assembler routine, conceived from the above, that will allow you to chain another program. It is only a simple demonstration and will only chain programs that do not expect any information to be contained in the command tail. You will notice that the name of the next program is held in the routine under the label ‘fcbh:’, this implies that the name of the next program is known. It is possible of course to change this so that any program can be chained from this standard routine. This routine is deliberately simple to demonstrate the basics, if a more complex/​universal routine is required then you will have to add the code required to process the command tail and put any valid filenames into the default FCB’s. I’m willingly leaving that up to you!!

You will notice that the default FCB at 5CH is filled with spaces after the chain file has been read, this means that Wordstar or Pen can be run without confusing them with odd command tails.

Program 1 will run on its own for testing or may be included in a larger program.

In my search for more information on this subject I took a look at the Gemini Utility program KEYCHAIN.COM written by David Parkinson, and later modified by Richard Beal. This program generates a COM file that, when run, will set up the Gemini keyboard function keys and then optionally run a named program.