This document describes how the CRTC can be programmed so that it can display graphics from a range of 32k without. Once initialised, this effect does not require maintenance.
The CPC display hardware generates a memory address by using the CRTC MA (MA0-MA13) and RA (RA0-RA5) outputs in the following way:
|Memory address Signal||Signal Source||Signal name|
The CRTC has a internal MA register which has the same value as the MA signal outputs. This register is incremented for each CRTC character and reloaded at the start of a CRTC frame from R12 and R13.
Using R12 and R13 we can define the initial state of the internal MA register in the following way:
|CRTC Register||Register Bit||CRTC signal|
If we want to display data from a 32k range, then MA12 output MUST increment from the initial setting defined by R12.
Depending on the initial setting of R12 and R13, the internal MA register must increment between:
Using R12 we can set MA11 and MA10, and from this we can create a overscan screen (a screen which fills the entire monitor display), which does not require any maintenance. Once this effect has been initialised, no furthur CPU time is require to maintain the display.
Properties of the overscan screen:
The screen has 1440 characters, and the internal MA counter of the CRTC will increment by 1440 from the initial MA settings defined by R12 and R13.
The following table shows the settings of MA11 and MA10 for the maximum (&3ff) and minimum (&000) scroll settings. In this example, the scroll setting is defined by MA9-MA0.
|MA11||MA10||Number of characters for MA12 to change|
From the table above we can see that:
Therefore the best settings for our overscan screen are MA11="1" and MA10="1".
Example: [ highlighted | original ]
Can we scroll this screen?
Yes, but the range is limited.
If MA11="1" and MA10="1" we are limited to scrolling 1024 characters. For a full scroll through the entire 32k range we would need to be able to scroll 4096 characters.
Scrolling 4096 characters requires using MA11 and MA10 for the scroll offset, and as we have already seen, when MA11 and MA10 are not "1", then at some point MA12 will not change, and the effect will be lost, and we will only see repeating graphics from a 16k range.