grim-fandango / megaplot

This MEGA65 program runs from BASIC and adds a text mode plot feature, similar to that found on the ZX81. It uses the PETSCI 'Battenburg' characters to provide each characters space with a 4-pixel plot area, giving a total resolution, in 80 character mode, of 160x50 pixels.

MEGAPLOT

This MEGA65 program runs from BASIC and adds text mode plot and line drawing features, similar to that found on the ZX81, so that users can remain in text mode and draw graphics without having to have a raster screen split. Text and graphics are easily mixed - PRINT, CURSOR and other screen text commands can all be used alongside a 160x50 resolution plot area.

MEGAPLOT uses the PETSCII 'Battenburg' characters to provide each characters space with a 4-pixel plot area.

USAGE

In your own programs, put the MEGAPLOT.PRG file on your disk and initialise it using:

10 BANK 0:BLOAD "MEGAPLOT"

MEGAPLOT will load at the top of BASIC at $7E00. The switch to BANK 0 to is required
to make it visible to BASIC.

The format of the command is:

SYS $7E00,X,Y,C

...where X and Y are the co-ordinates of the point to plot where X is between 0-159 in 80 column mode and 0-79 in 40 column mode. Y is between 0-49 in both modes.

C is the index of the colour that you want to plot, i.e. 0=black, 1=white, 3=red etc.

Overplotting an existing point will not clear it, i.e. there is no XOR feature. The point will be plotted again in the new colour specified.

Negative values and values over 255 will raise an ILLEGAL QUANTITY error.

Available calls are:

• init_and_plot:
  • SYS $7E00,X,Y,C
  • a = x-coord, x = y-coord, y = colour
  • (equivalent to original program's behaviour)
• init:
  • SYS $7E03
  • a 'one-time' init, so that future draw calls don't need to repeatedly init
• plot:
  • SYS $7E06,X,Y,C
  • a = x-coord, x = y-coord, y = colour
• line:
  • SYS $7E09,X1,Y1,X2,Y2
  • a = x1, x = y1, y = x2, z = y2
• set_colour:
  • SYS $7E0C,C
  • a = colour
  • (use this call to set the colour of your next line)

PI DEMO

The Pi demo has an interesting heritage. It was originally written by W TAGG (we think this is Bill Tagg, Head of Maths from Hatfield School in the UK) in November 1979, then converted to an obscure teletext-compatible computer called the ABC800 in November 1980 by A Wallis. In the late 70s and early 80s, Brighton Polytechnic were running an experimental telesoftware service on the UK teletext services CEEFAX and ORACLE. Telesoftware is a method of distributing software to users over broadcast teletext. Those early Brighton experiments used the ABC800 computer to receive off-air broadcasts containing software.

On 22nd February 1981, someone was recording the Charlton Heston film 'Khartoum' on BBC1. The tape survived the intervening 40 years, and the teletext was recovered from that Betamax recording by sampling the video and processing the teletext data dots recorded alongside the film. The teletext pages, including telesoftware, can be reconstructed from these data, and this program can be seen in its original teletext format here: https://twitter.com/grim_fandango/status/1496234608656007171

The telesoftware was extracted from the teletext page and converted to ASCII. From here it was adapted to the MEGA65 using MEGAPLOT to draw the required graphics.

4x SYMMETRY DEMO

This was adapted from the book 'The Art of Programming the 1k ZX81' by M James and M. Gee.

SINE OF THE TIMES DEMO

This is something I knocked up in 10 minutes.

PETLINE

The 'petline.prg' file features several examples that make use of the line drawing capabilities of MEGAPLOT.