Caching
<<Read this section with caution; much more than 48K RAM was available on S/34; the process to squeeze a program into a smaller executable size was called overlaying.>>
A 64K computer program can run on a S/34 when only 48K of RAM is available by using a process called caching. The system uses a cache or workspace on the hard drive to contain portions of the programs currently running. Loading the whole program into the cache area and then moving it piecemeal in and out of storage was a system function performed by the CSP. The MSP performed the instructions in the computer program. Insufficient memory makes the system run slower, since the disk speed was only about as good in the 1980s as PC drives are today, and modern "burst mode" rates were unheard of.
Read more about this topic: IBM System/34