Some systems store the page table in main memory. This works but is slow. It also detracts from memory that programs might use for useful work. If the system has a separate MMU chip, the page table or tables (if there is more than one program running at the same time) may be stored in high speed memory on that chip. When the MMU is just a part of the main processing chip, as is common nowadays, the page tables might be kept in very high speed RAM on the chip. Such RAM is called a cache (pronounced "cash") because its existence is kept secret, or hidden, from the general programmer. The French word for "to hide" is cacher (pronounced "cash-AY"). We will study more about cache memories later, since they are used in other ways. The main drawback is that when the page table gets to be very big, it is no longer practical to keep it in these special high speed and expensive memory. So we are again forced to resort to main memory.