Page:CTSS programmer's guide.djvu/52

 To ensure insensitivity to timing, proper interlocking of I/O commands and the CPU program must be guaranteed, usually with  instructions. When this is not done, it is sometimes possible to get erratic program operation due to the probabilistic nature of other program interrupts (and consequent program delays) occurring during the operation time of I/O commands which change the working data area of the interrupted program. This also applies to control-card coding. 38