Inside Macintosh: Macintosh Toolbox Essentials
The type KeyMap is used in the GetKeys function GetKeys to return the current state of the keyboard, including the keypad, if any. The KeyMap type is interpreted as an array of 128 elements, each having a Boolean value. Each key on the keyboard or keypad corresponds to an element in the KeyMap array. The index for a particular key is the same as the key's virtual key code minus 1. For example, the key with virtual key code 38 (the "J" key on the Apple Keyboard II) is the 38th element in the returned array. A KeyMap element is true if the corresponding key is down and false if it isn't. The maximum number of keys that can be down simultaneously is two character keys plus any combination of the five modifier keys.
typedef long KeyMap[4]; /* records state of keyboard */