The other main issue was that early Macs lack a memory management unit (MMU), which precludes the possibility of several fundamental modern features. An MMU provides memory protection to ensure that programs cannot accidentally overwrite other program's memory, and provisions shared memory that allows data to be easily passed among libraries. Lacking shared memory, the API was instead written so the operating system and application shares all memory, which is what allows QuickDraw to examine the application's memory for settings like the line drawing mode or color.