slopspace is unused space at the right side of a screen. If your desktop has a resolution that’s not dividable by the engine granularity, such a desktop is created ‘oversized’ (horizontally) to make it possible for the engine, but also for CRTC’s to do their ‘burst’ like accesses.
The DAC output (or these days digital link) does send other timing than the desktop is configured in, so for the user it seems the requested resolution is set, while in fact, it is not.
Different parts of graphics cards have different restrictions:
- CRTC often (horizontally): 8 pixels, but often more
- 2D engine
- 3d engine (could be same engine)
- backend scaler
- hardware colorspace converters
The trick is to find the largest granularity for each of the used components and use that to set the mode, otherwise things will fail. (for instance VLC did this wrong long time ago: I had to send a patch especially for nvidia gfx cards since these had a larger granularity than anything they saw before)