80-Bus News


March–April 1984, Volume 3, Issue 2

Page 46 of 51

Odd features of the IVC stem from the Nascom background. e.g. the alternative screen format being 48 characters per line. The block graphics are those of the Nascom 2, rather than the teletext standard. (Same symbols, just controlled by different bits within the byte.)

The IVC accepts 8-bit characters from the host system. All the standard printable ASCII characters (whose codes are in the range 20-7F), and all characters for the alternate character generator (whose codes are in the range 80H-FFH), are placed directly into the display at the current cursor position. Characters in the range 00-1FH are interpreted as control characters, and are used to control the extensive features of the IVC.

One reason why I have been looking for a review of the IVC is to see if anbody feels that important features are lacking from the escape sequences. (Do you all want protected fields? Do you want a “page” mode as well as a “Scroll” mode?) My feeling, echoed recently in these pages by David Hunt, is that 97% of users do not use anything other than the few basic functions like ‘cursor addressing’ that are used by programs such as Wordstar and dBase II.


The IVC supports a single flexible attribute. This is the programmable character generator (pcg), sometimes refered to in the IVC documentation as the alternate or upper character generator. On power-up the contents of the standard character generator (an EPROM) are copied to the alternate character generator and are inverted on the way. This means that by using the alternate character generator, (either by setting the msb of a character before it is sent to the IVC, or using the sequence <ESC> A to make the IVC do it for you), video-inverse characters can be displayed. (e.g. to highlight headings and menus within Wordstar and other such programs.) Note that inverse video is the default setting, there is nothing to stop you programming other bit combinations into the character boxes. For example you could set up a Greek alphabet or a series of specialist mathematical symbols for use by a custom wordprocessing package. The Henry‘s package HI-RES uses the pcg to offer a pseudo high resolution graphics mode on the IVC.

Transparent access

As I mentioned earlier, the absence of visual interference on the screen when the on-board processor accesses the screen memory is essential. There are various ways that this can be achieved, but this to some extent depends upon the choice of controlling microprocessor. If a Motorola 6800 had been used for the IVC processor, then a very neat arrangement could have been implemented where the processor and display accesses could have been interleaved, leaving each controller in total ignorance of the other. However history dictated that a Z80 was used. The Z80 has asynchronous memory cycles, (asynchronous to the extent that its memory cycles are not all the same length), and so it can not easily be arranged to access the display memory between each displayed character. With the IVC design the easy way out was taken, and the Z80 was given priority access to the display memory. (i.e. it can barge in when ever it likes.) However the horizontal and vertical sync pulses and the display blanking signal from the display controller are routed to a port so that the Z80 can monitor their state. Thus, by monitoring this port, it knows when it can safely “barge in” and update the screen memory without causing any interference on the display.

Page 46 of 51