On CRTCs 0/1/2, if a Write Only register is read from, "0" is returned. The register accessing scheme on CRTCs 3/4 makes it impossible to happen.
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=== Default values ===
We can calculate the screen size in words by multiplying R1 x R6 x (R9+1). And then multiplying the result by 2 to have the screen size in bytes.
With the default CRTC values, we obtain 40x25x8x2 = 16000 bytes.
So we can observe that only part of the 16384 bytes (=16KB) page of VRAM is actually displayed on screen.
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