Accurate millisecond timing on Apple’s Macintosh using Drexel’s MilliTimer

Many of the timed functions that concern psychologists, such as perceptual presentations and reaction time, are sensitive to a maximum variability in display timing caused by screen-refresh characteristics. For the Apple Macintosh, the screen operating speed is 60 Hz, which translates to an average of 8.33-msec variability. For microcomputers other than the Macintosh, a variety of hardware and software modifications to generate millisecond timing have become standard (e.g., Reed, 1979). Other than Reed College’s (1985) implementation in Rascal, which requires the Rascal development language, there has been no method of which we were aware to synchronize experimental timing with display presentation on the Macintosh. This limitation in the usefulness of the Macintosh as an otherwise excellent research tool can be overcome using Drexel University’s MilliTimer. The assembler code which follows should be considered in the public domain and can be readily adapted to any of the Macintosh-based languages.