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Technical information for Teletext Elite

Details of how Elite was converted to use teletext

Teletext Elite rear space view

Under the hood, Teletext Elite is identical to the disc version of BBC Micro Elite, but instead of setting up a mode 4/5 split-screen mode and poking pixels into screen memory, we stay in mode 7 and poke sixels and text characters into screen memory.

To make this process easier, we can scale from the original mode 4/5 screen pixels to mode 7 sixels as follows:

  • The original Elite screen mode is 256x248 pixels, with 256x192 pixels for the space view and 256x56 pixels for the dashboard (see the deep dive on the split-screen mode for details).
  • Mode 7 is 40x25 characters, with columns 0 to 39 and rows 0 to 24. Each character is three sixels high by two sixels wide, so that gives us a resolution of 80x75 sixels.
  • A quick divide-by-4 converts the 256x192 pixels of the space view into 64x48 sixels (i.e. 16 mode 7 character rows of 32 characters each).
  • This leaves nine mode 7 character rows, which I split between the title, message bar and dashboard. This gives seven character rows for the dashboard, which works out at exactly one character row for each of the seven controls.
  • The space view is not a full screen width - it's 32 characters out of 40 - but because it is sandwiched between two full-width blue title bars, it feels a lot wider than 64 sixels.
  • The space view in the original game is 192 pixels high, which equates to 24 standard character rows in mode 4, so to accommodate screens like the Status Mode screen and Market Prices screen, we have to use the whole mode 7 screen height.

Here's a summary of the mode 7 screen structure in the space view, during flight:

Character rowContents
0Title row showing the space view name and hyperspace countdown
1-1616 rows for the space view (48 vertical sixels mapped to 192 pixels via a factor of 4)
17Message row showing current in-flight message
18-247 rows for the dashboard (one row per control)

And here's the structure of the trade screens and charts:

Character rowContents
0Galfax header
2Title row showing the current screen name
3-24Contents (text and graphics)

Next, here's a full list of routines that poke into screen memory; these are the core routines that have been modified in Teletext Elite to poke sixels and text instead of pixels. The links will take you to the source code for the original disc version, without the Teletext modifications (to see the latter, see below for information on exploring the Teletext Elite source code).

LOINDraw a line
HLOINDraw a horizontal line
PX3Plot a single pixel at (X, Y) within a character block
PIXELDraw a 1-pixel dot, 2-pixel dash or 4-pixel square
CHPRPrint a character at the text cursor (docked)
TT26Print a character at the text cursor (flight)
CLYNS/LYNClear the bottom three text rows of the mode 4 screen
CPIX2Draw a single-height dash on the dashboard
DILXUpdate a bar-based indicator on the dashboard
DIL2Update the roll or pitch indicator on the dashboard
MSBARDraw a specific indicator in the dashboard's missile bar
HANGERDisplay the ship hangar
HAS2Draw a hangar background line from left to right
HAS3Draw a hangar background line from right to left
SCANDisplay the current ship on the scanner
SHPPTDraw a distant ship as a point rather than a full wireframe

Finally, in addition to the above, the loader has been updated in a few important places, to disable the split-screen mode and plot sixels for the Saturn loading screen instead of pixels:

B%VDU commands for setting the square mode 4 screen
IRQ1The main screen-mode interrupt handler
PIXDraw a single pixel at a specific coordinate

You can see every single modification made in Teletext Elite in the source code (see the next section).

Exploring the Teletext Elite source code

The source code for Teletext Elite is available for you to explore. It is fully documented and fully buildable on modern computers, and includes labelled modifications in the main game's source code so you can see exactly how I modified the original disc version of Elite to bring it into the world of sixels.

To see the source and learn how to build Teletext Elite on your own machine, visit the project's GitHub repository.