Editing MOS Technology VIC-II (section)

===Scrolling===
In order to scroll a character screen, the VIC-II is set to 38-column and/or 24-line mode via the registers at {{mono|$D011}} and {{mono|$D016}}. This creates an off-screen buffer where the row of characters to be scrolled is placed. By adjusting the scroll bits in the above-mentioned registers, one row may be moved on-screen after which it repeats unless a new row is put in the buffer. Color RAM is scrolled simultaneous with screen RAM and works the same way.

VIC-II scrolling is a relatively complicated, CPU intensive task, although it is not uncommon for C64 game programmers to cheat by designing graphics so that the color RAM can remain static. Another standard trick is to use a section of the screen (perhaps the bottom or top 4 or 5 character rows) as a game status area to display score, lives, etc. reducing the amount of scrolling that has to be performed. Finally, it is often necessary to use the "double-buffering" technique to prevent screen tearing. Two 1k blocks of screen ram are reserved; one is displayed while another is written to, then during vblank they are quickly swapped through manipulation of the VIC-II registers. Unfortunately this cannot be done with color RAM.

Late in the C64's commercial lifespan, an exploit known as VSP (Variable Screen Positioning) was discovered that involved manipulation of the control bits in {{mono|$D011}} to produce fast scrolling at a much lower CPU cycle cost than the standard scroll registers, however it required careful, cycle-exact coding and did not work reliably on some VIC-II revisions; also it can only be used for horizontal scrolling. It is notably used in ''[[Mayhem in Monsterland]]''.