A lesson from the Computer Memory module of the bootcamp. About a 12 minute read.
Main memory is not one technology but two. SRAM holds a bit in a small loop of transistors and is fast and expensive. DRAM holds it as charge in a capacitor and is dense, cheap and slow. Why both exist, and why processors have outrun memory by a factor of hundreds, is the reason every later lesson in this module exists.
STATIC RAM stores each bit in a cell of six transistors wired as two inverters feeding each other. Whatever value the loop holds, it keeps reinforcing, so the bit stays put as long as the power is on. It is fast, reachable in about a nanosecond, and it takes a lot of silicon per bit, so it is used for the processor's caches. DYNAMIC RAM stores each bit as charge on a tiny capacitor behind a single transistor. That makes it several times denser and far cheaper, so the gigabytes in your machine are DRAM. The price is speed: about 100 nanoseconds to get an answer back. Fast and small, or big and slow. Every design decision in the rest of this module is a way of getting the first while paying for the second.
The rest of this lesson continues with 6 further sections. See the full curriculum.
Browse all 576 practice puzzles