Playfield objects are difficult to move smoothly. Memory map playfield can be moved by rewriting sections of memory. However, this is extremely time-consuming if large sections of the screen must be moved smoothly. Character playfield objects can be moved easily in a jerky fashion by changing the memory scan counter.
However, this results in a large position jump from one character position to another, not a smooth motion. For this reason hardware registers (VSCROL and HSCROL) and counters are provided to allow smooth horizontal or vertical motion, up to one character width horizontally and up to one character height vertically. After this much smooth motion has been done by increasing the value in these registers, memory is rewritten or the memory scan counter is modified and smooth motion is resumed for another character distance.
is described under HSCROL in section III.