SOFTWARE FOR PROGRAMMABLE GRAPHICS
A character generator is merely a memory device which stores data defining
which points within the character area are ‘on’. In the Nascom each character
consists of 16 lines of 8 dots and the data is stored as 16 consecutive bytes.
When the V.D.U. circuitry is displaying a particular character it looks at the data
stored in the generator, selecting the byte appropriate to the current line. This byte
is passed to a device which looks at each bit in turn; if the bit is a ‘1’ the intensity of
the beam scanning the T.V. screen is intensified and a dot appears.
In the Nascom 2 the data in the character generator is displayed with the
least significant bit on the right of the character, while the system fitted to my
Nascom 1, which is a combination of Steven Hope’s P.C.G. and an early Bits and
P.C.s graphics board, displays the bits in the opposite order. In the software
discussed below I shall stick to the Nascom 2 display order, but indicate how to
change the programs for non-standard machines. A further incompatibilty is caused
by the fact that the Nascom 2 only displays 14 lines per character. However, a
simple modification has been published in the Liverpool Software Gazette which
produces the full 16 line display.
The simplest way to use a programmable character generator is to store
sets of characters on tape and load each set as it is required, but this soon becomes
tedious. Special characters can be incorporated into the program that uses them
and written to the P.C.G. either by a copy routine, for a machine code program, or
by a READ, POKE loop in Basic. However, character A0 and the pixel set (C0 –
FF) merit individual attention. The Zeap assembler uses character A0 to mark
the end of a line. In the standard Nascom 2 graphics set this character is identical
to the space character and so is not noticed on the screen; if A0 is not clear, as it will
not be if you have Just switched on or have been using special characters, the
Zeap display is very untidy. A similar problem is found with Nas Debug, which uses
character C0 as a separator. Of course, you can always clear these characters
directly by a modify command, but a better solution is to have a short routine in
EPROM which clears A0 and writes the standard pixel set from C0 to FF. A listing
of such a routine is given below.
It is followed by an even simpler program which writes the
into the Nascom P.C.G. This is quite useful if you are trying to adapt a program
which uses TRS80 graphics to run on your Nascom. If your machine displays the
generator bits in inverse order you will have to change line 270 to LD A 15 and line
320 to ADD A £F0; similarly, the values in lines 540 and 580 should be interchanged.
On a Nascom 1 the upper 4 pixels consist of 5 lines of 4 dots, while the bottom 2
contain 6 lines. As an unmodified Nascom 2 misses out the two bottom lines, in this
case the lowest pixels are 4 x 4.
You will soon want to invent your own characters, either to be used
singly or in blocks – you can get very impressive high-resolution pictures with 128
graphics characters in a 16x8 block. The difficulty is working out what data to put in
the P.C.G. RAM. One method is to draw the characters on graph paper and then
convert the diagrams to bytes. The third program below can be used to draw
characters directly from the keyboard. The program is executed by entering E1000
AAAA, where AAAA is the address of your P.C.G. RAM or, if your P.C.G.
address is coincident with an area of EPROM as in Steve Hope’s design, an area of