Nascom News­let­ter

won’t know what to do next! The section of memory just paged in must, in this situation, contain the next program instruction and it must be located at the next consecutive address to that in the program counter at the time of changing page.

I have found this facility particularly useful whilst developing programs destined to reside in ROM in my paged EPROM board. Experience has shown that it is all too easy to make mistakes in shifting control from one page to another, and re-programming EPROMs is a tedious business! The paged RAM in the AVC provides an easy way of simulating the paged EPROM, allowing easy testing and modification without the need to re-program an EPROM every time.

Another possible use for the AVC memory is to hold large blocks of data to which the main program (in normal RAM) refers only when required. This could arise, for example, in database programs, word-processor or financial applications … in fact, wherever memory capacity is being strained by storage of large amounts of data, look up tables, etc.

I hope this article will stimulate other owners of the AVC to suggest more ‘unadvertised’ uses for this excellent, but poorly promoted, board.

Incidentally, reverting momentarily to the graphics application of the AVC, I am having trouble patching into the DUMP command due, I believe, to an error in the Lucas software or manual. If anyone has dealt with this problem I would be pleased to hear from them.

Listing 1.

Machine code program to page-in the required 16K page of memory and return contral to Nas-Sys.

xx=00 for normal system RAM
xx=01 for AVC ‘RED’ page
xx=02 for AVC ‘GREEN’ page
xx=03 for AVC ‘BLUE’ page

Listing 2.

Basic instruction to select the various pages.

OUT 178,xx

see above for values of xx.