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diff --git a/scroll/README.md b/scroll/README.md index e29186a..45c96e8 100644 --- a/scroll/README.md +++ b/scroll/README.md @@ -8,11 +8,30 @@ try to understand the concept of scrolling in NES/Famicom programming. The very basic concepts of scrolling can be seen in [toggle.s](./toggle.s), which gives you this as a result: -TBD +<div align="center"> + <img src="../docs/toggle.gif" alt="toggle.gif" /> +</div> That is, we only have filled the two nametable available, and we are modifying the [PPU scroll register](https://www.nesdev.org/wiki/PPU_registers#PPUSCROLL) -to move between one or the other. +to move between one or the other. Another important note, easily missed when +programming scrolling on the NES/Famicom for the first time, is that whenever +the PPU scroll "wraps around", you should also update the base nametable address +from the [PPU control +register](https://www.nesdev.org/wiki/PPU_registers#PPUCTRL). That happens in +two situations: + +1. If you are scrolling right and PPU scroll turns into a `$00` value, then it + means that there's nothing else to show from the origin nametable, and that + `$00` on the scroll means it's `$00` in respect to a new nametable. +2. If you are scrolling left and the PPU scroll turns into `$FF` (i.e. the first + step on scrolling left), then the base nametable address has to be updated + because it's `$FF` from the point of view of the nametable from the left. + +It's easy to miss these points, but from a PPU perspective (and hence from the +perspective of a programmer interfacing with the PPU), it really makes sense. +All in all, the scroll register is relative to whatever base nametable is set on +the control register. This looks rather simplistic but some games used this technique. For example, in Dropzone it was used to perform some effects on the title screen. Hence, |