In vintage Macintosh system software (System 1 through System 6), screenBits and BitMap management were the foundational elements of the 2D graphics engine known as QuickDraw. Created by Bill Atkinson, these systems defined how the early Mac rendered its groundbreaking black-and-white graphical user interface (GUI) using a Motorola 68000 CPU without any dedicated graphics hardware acceleration. What is a BitMap in Vintage Mac Software?
In basic QuickDraw, a BitMap is a specific Pascal data structure that maps a collection of 1-bit black-and-white pixels to a physical or virtual memory space. Unlike modern formats, it holds no color data—a bit value of 1 represents a black pixel, and 0 represents a white pixel.
The standard BitMap structure contains exactly three fields:
Type BitMap = RECORD baseAddr: Ptr; {Pointer to the actual bit image in memory} rowBytes: Integer; {Number of bytes in a horizontal row} bounds: Rect; {A bounding box defining the coordinate system} END; Use code with caution. Basic QuickDraw – Apple Developer