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At this place I present different hints concerning the screen access of the PCW.
For the sake of completeness I represent here all routines known to me – they may be similar or even equal.
Although the PCW is designed as a text machine it is also useable for grafic applications.
The screen memory stores characters not as bytes but as bits.
That allows to attache individual pixels on the screen – to set or reset.
But the acccess of them is a bit special.
The addresses of the single lines will be held within an address table, the so called Roller RAM.
Each entry points to a line in the screen memory.
The benefit of this technique is the fast scrolling by changing addresses only.
It is faster than moving the entire screen memory.
In the past – 1985 – I requested AMSTRAD how the access to screen memory takes place. The response was a very interesting paper found here. Later on many articles referring grafics were published, among others regarding the Roller RAM (1), (2). |