Operating systems like the original MS-DOS were not capable of multitasking–they could only run one program at a time. But you could fake multitasking by taking advantage of the “terminate and stay resident” (TSR) system call in DOS. Essentially, a program that used this call was not cleared from RAM when a new program loaded and remained available to the system until the next reboot.

Such “RAM resident software” was the subject of this next Computer Chronicles episode from November 1986. Stewart Cheifet opened the program by telling Gary Kildall that when Borland first released Sidekick, it was a big deal that you could pop-up a simple calculator on your screen using a RAM resident program. Cheifet then showed a physical calculator–an HP-12C financial calculator–and noted there was now a computer version of the device, which was also available as a TSR program.