cachememorysdramsram
DRAM is slower to write than read because it takes time to either charge or discharge a DRAM memory cell. But what about the SRAM in my processor's L1 and L2 caches? It's slower to write as well but AFAIK, SRAM is latched memory based on gates.
So why the slower SRAM writing speed?
Thanks.
Yep, it's still the case. The principle behind DRAM is that of a capacitor (simply, 1 small transistor and one large one), while that behind SRAM is a latch, made from a few (more) transistors. DRAM has more capacitance. If you assume they have equal* input impedance, then it's straightforward to see that the DRAM RC-delay is slower than that of SRAM.
DRAM is cheap, though, so we still love it. SRAM is awesome, so we love it, too.
DRAM:
(source: emrl.de)
SRAM:
*(They likely won't -- it's really matched to its reading circuitry, but who cares.)
The main advantage of synchronous design is that it's behavior is easy to predict, model, and validate because everything happens on a predefined schedule. However, waiting for a specified time to perform an action makes synchronous design slower than a comparable asynchronous design. And even when the circuit is not responding to its logic inputs, it is still drawing power since it is responding to the clock signal.
An asynchronous circuit can be much faster because it responds to its inputs as they change. No waiting around for a clock signal before processing can take place. They also can take less power since they don't have anything to do when the inputs are inactive and have better EMI performance since there isn't a constant digital signal floating around. But the design of such systems is much more difficult because all combinations of inputs over time need to be taken into consideration to ensure proper operation of the circuit. When two inputs change at almost the same time, this is called a race condition and the circuit can have undefined behavior if the designer didn't plan for every combination of inputs at every combination of time.
Comparing and contrasting synchronous to asynchronous design, you're probably thinking that big companies like Samsung can spend billions on the research and design to fully model a DRAM circuit so that its operation is really stable and then we would have really fast, really low power memory. So why is SDRAM so much more popular?
While asynchronous design is faster than synchronous in sequential operations, it is much much easier to design a circuit to perform parallel or simulations operations if the operations are synchronous. And when many operations can be performed at the same time, the speed advantage of asynchronous design disappears.
So three main things to consider when designing a RAM circuit are speed, power, and ease of design. SDRAM wins over plain DRAM on two out of three of those and by a very large margin.
Wikipedia quotes:
Dynamic random-access memory -
The most significant change, and the primary reason that SDRAM has supplanted asynchronous RAM, is the support for multiple internal banks inside the DRAM chip. Using a few bits of "bank address" which accompany each command, a second bank can be activated and begin reading data while a read from the first bank is in progress. By alternating banks, an SDRAM device can keep the data bus continuously busy, in a way that asynchronous DRAM cannot.
Synchronous dynamic random-access memory -
Classic DRAM has an asynchronous interface, which means that it responds as quickly as possible to changes in control inputs. SDRAM has a synchronous interface, meaning that it waits for a clock signal before responding to control inputs and is therefore synchronized with the computer's system bus. The clock is used to drive an internal finite state machine that pipelines incoming commands. The data storage area is divided into several banks, allowing the chip to work on several memory access commands at a time, interleaved among the separate banks. This allows higher data access rates than an asynchronous DRAM.
Pipelining means that the chip can accept a new command before it has finished processing the previous one. In a pipelined write, the write command can be immediately followed by another command, without waiting for the data to be written to the memory array. In a pipelined read, the requested data appears after a fixed number of clock cycles after the read command (latency), clock cycles during which additional commands can be sent.
Best Answer
Are you really sure the caches are slower to write than to read? I suspect what you heard was the overall operation, not just the SRAM read or write time. Writing to a cache is a more expensive operation because eventually the slower memory backing the cache must be written to. It also means a cache block can't simply be reused without flushing it first if it's been written to and that write hasn't been propagated back to the main memory.
The actual write operation is probably the same speed, but the overall process of writing, and all the stuff that ultimately puts in motion, is longer.