80-Bus News

Nuts & Bolts.

I will now try and explain the technicalities of working with GSX. These will be clarified with the aid of diagrams (a picture’s worth etc), as is does seem rather complex at first. It is worth mentioning that once you, as the applications or device driver writer, have created a routine that allows you to easily reference the data arrays concerned, the task is not quite so daunting as it first seems.

As calls to the BDOS involve the use of the BC and DE register to inform of your intentions, the problem is how do you manage to pass sometimes large amounts of data over using only one 16 bit value. Of course the answer is with the use of pointers as usual. Don’t forget that the C register contains 115 on all calls to GDOS regardless and therefore cannot be used for pointer work.

The GSX standard expects the application programmer to have set up 5 arrays, and to give them their proper names, these are:

Parameter Block

This 5x16 bit array contains the start addresses of the other data arrays described below. On a call to GDOS the DE register pair must contain the start address of this array.

PB pointed to by DE.

Fig 1. On call to BDOS requiring a GSX function.