The disturbed incremental magnetization changes that accompany the read and write operations of coincident-current memories are the cause of a serious noise problem in the attainment of high-speed memory performance. This paper examines the noise problem. Relationships which express the memory driver tolerances in terms of the core characteristics are presented; the operating schmoo is evolved and from it the various schmoo responses are obtained and utilized for selection of optimum operating currents and sense amplifier threshold bias. A unique system for automatically exercising coincident-current magnetic core memory systems and visually displaying their performance characteristics is included. The instrumentation includes a versatile memory exerciser that not only subjects the memory under test to simulated conditions of computer operation, but also varies the driving currents into the stack over wide extremes and compares the resultant output signals to the information stored in the data register. Suitable synchronizing and display signals are provided to an oscilloscope which records each error as a momentary intensification of the electron beam as it sweeps the scope face. The resultant pattern is called a schmoo plot and as such provides a visual measure of performance of memory systems.