It is very weird.. this code is is full of the duplicative, hard-coded constants, something which I would avoid in any software I write.

For example, take a look at the display module [0] -- this 190-line program contains over 20 instances of constant "128", in lines like those:

    sa := base + u0*4 + sy*128; da := base + v0*4 + dy*128;

What does this mean? See, there is only one display supported by this OS -- an 1024 x 768 monochrome one. Since this is monochrome, you only need one bit per pixel, and you can put 8 of them in the byte. If the pixels are sent out serially and lines follow each other in the buffer, then start of each line is (1024 / 8) = 128 bytes apart.

But instead of writing something like "const LineStride = Width / 8", the code just hardcodes the value all over the place.

[0] https://people.inf.ethz.ch/wirth/ProjectOberon/Sources/Displ...