svspire@sandia.gov (5268 Spires, Shannon V.) (11/13/90)
In article <15571@brahms.udel.edu> don@brahms.udel.edu (Donald R Lloyd) writes: > I've been looking for information on the various type of RAM for a week > or so, (more out of curiosity than out of any real need to know), and so > far have found only what I already knew: > > DRAM is relatively slow & has to be periodically refreshed > static RAM is fast & needs no refresh True, in general. Though there are plenty of exceptions to this, most DRAMs have access times in the 100 to 150 ns range, while most Static RAMs have access times in the 25-50 ns range. All RAMs (DRAM and SRAM) are made with MOS transistors (at least all RAM you'll find in any personal computer. Crays use non-MOS RAMs.) > Why is DRAM slower than SRAM? I can't see how having > to refresh it would cause any tremendous slowdown. You're right. There are many ways to refresh DRAM, and most have no effect on access time. Refresh only has to happen every few milliseconds or so, and then only requires a number of operations equal to the square root of the number of bits in the chip. The big slowdown in DRAM is because it requires clocked addresses, and you have to do two of them for every access. You can see this for yourself: count the number of address pins on a DRAM chip vs. the same size SRAM chip. SRAMs always have exactly the number of address pins they need to cover their whole address space (for example, a 32K SRAM has 15 address pins) while DRAM's always have only half as many pins as they need (a 1M DRAM needs 20 address pins, but it has only 10). The address is multiplexed: First you clock in the upper half of the address using the Row Address Strobe (RAS) signal, then you clock in the lower half of the address using the Column Address Strobe (CAS) signal. Because two operations are required per access and the addresses aren't seen until you clock them, DRAMs take longer to access than SRAM's. (SRAM's aren't clocked. As soon as they see an address come in, they start working.) > What is static column RAM? I've been told it's the > same as static RAM, but that doesn't seem right (otherwise the 3000 > would be a good bit faster and a good bit more expensive). Is it the > equivalent of what Tannenbaum calls pseudo-static? No, static column RAM is not the same as "pseudo-static". S.C. RAM is just DRAM with some enhancements. Referring to my previous paragraph, you'll notice some obvious improvements. What if you need to grab a block of data which differs only in its Column Address and its Row Address stays the same each time? Ah, then you shouldn't have to clock in the same Row Address each time, right? Right. You just need to clock in the differing Column Addresses each time, so access is approximately twice as fast. But we're still not to Static Columns. This scheme is called "Page Mode". Static Column takes this idea one step further and eliminates the need for clocking the new Column Addresses in; it just requires you to supply the new column addresses on the address pins. It effectively allows you to treat each column as though it were Static RAM, hence the name. > What about VRAM? Somehow specially designed for video > memory, or just some marketer's way to make the static RAM on a video > card sound superior? Yes, VRAM is specially designed for video applications. By the way, all VRAM is dynamic, not static. VRAM is basically just DRAM with a shift register added on the chip, and it's also more expensive than DRAM. The shift register effectively lets you read out a single row of memory to your video output circuitry one bit (or 4 bits; VRAMs are usually arranged as 4-bit words) at a time, while still accessing the rest of the memory as if it were DRAM. Thus it's dual-ported; the video circuitry can use it at the same time as the computer does, and you don't have to "cycle steal" and alternate using the memory between the CPU and the video circuitry. > While I'm at it, why are ROMs inherently so slow? There is a tremendous speed variation in ROMs. ROMs are available that are faster than the fastest static RAM, and slower than the slowest DRAM. The slower ones are just a lot cheaper, so they are the ones that usually end up in personal computers. -- Shannon Spires Sandia National Laboratories, Albuquerque, NM Internet: SVSPIRE@SANDIA.GOV