Disk access is slow compared to RAM access. Potentially, using a combination of real memory and disk memory to implement the address space could greatly slow down program execution. However, with clever electronics and a good operating system, virtual memory is only slightly slower than physical memory.
Computer systems also have cache memory. Cache memory is very fast RAM that is inside (or close to) the processor. It duplicates sections of main storage that are heavily used by the currently running programs. The processor does not have to use the system bus to get or store data in cache memory. Access to cache memory is much faster than to normal main memory.
Like virtual memory, cache memory is invisible to most programs. It is an electronic detail below the level of abstraction provided by assembly language. Hardware keeps cache up to date and in synch with main storage. Your programs are unaware that there is cache memory and virtual memory. They just see "main memory". Application programs don't contain instructions that say "store this in cache memory", or say "get this from virtual memory". They only refer to the contents of main memory at a particular address. The hardware makes sure that the program gets or stores the correct byte, no matter where it really is.
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