Text on TV

Display Data

This section covers the data that is used.

A 'Display' array:

A 'Row Attributes' array:

Cursor position:

A 'Font' array:

Structure of Font Data

Structure of Font Data

The Display Line Renderer reads these variables/arrays to display the screen.
The Serial processing routines modify some of these variables/arrays.

The 'Row Attributes' array is an important speed-up. Without it, clearing the screen (either wholly or partially) would take an exorbitant amount of time - possibly over 4000 clock cycles. That equates to several scanlines of disruption to the video signal. Using the array means that the screen can be 'cleared' by simply setting 25 bytes to 0 - a much faster proposition, especially as it makes loop-unrolling feasible (see tricks and tips).

The downside to the 'row attributes' speed-up is that at some point, the characters in the display array will actually have to be cleared before that row can be displayed normally again. Clearing the characters within a 'not displayed' row so that it can be marked for normal display has been called 'normalising'.

Normalising is carried out when no character (or a NUL character) has been received. To guarantee that the row containing the cursor will be normalised before the next character is received, the serial data rate is limited (purely by specification) to a maximum of one character every-other scanline. This means that normalisation can always be carried out in-between characters being received.