12.18 RAM Disks and Semiconductor Disks

Another pair of interesting mass storage devices you'll find are the RAM and semiconductor disks. A RAM disk is just an application that treats a large block of the computer system's memory as though it were a disk drive, simulating blocks and sectors using memory arrays. A semiconductor disk is a device consisting of RAM memory and a controller that interfaces with the system using a traditional disk interface. Semiconductor disks usually have their own power supply (including a battery backup system) so that they maintain memory integrity when you turn off the PC. The use of a standard disk interface and a separate, uninterruptible, power supply are what differentiate true semiconductor disks from software-based RAM disks.

The advantage of memory-based disks is that they are very high performance. RAM disks and semiconductor disks do not exhibit the time delays associated with head seek time and rotational latency that you find on hard, optical, and floppy drives . Their interface to the CPU is also much faster, so data transfer times are very high, often running at the maximum bus speed. It is hard to imagine a faster storage technology than a RAM or semiconductor disk.

RAM and semiconductor disks, however, have two disadvantages: cost and volatility. The cost per byte of storage in a semiconductor disk system is very high. Indeed, byte-for-byte, semiconductor storage is as much as 1,000 times more expensive than magnetic hard disk storage. Because of the high cost of semiconductor storage, semiconductor disks usually have low storage capacities , typically no more than a couple of gigabytes. And, semiconductor disks are volatile - they lose their memory unless they are powered at all times. A battery- backed , uninterruptible power supply can help prevent memory loss during power failures, but you cannot disconnect a semiconductor disk from the power line for an extended period of time and expect the data to persist. This generally means that semiconductor disks are great for storing temporary files and files you'll copy back to some permanent storage device before shutting down the system. Because of their low-latency, high data transfer rates, and relatively low storage capacity, semiconductor disks are excellent for use as swap storage for a virtual memory subsystem. They are not particularly well suited for maintaining important information over long periods of time.

The popularity of semiconductor disks tends to rise and fall with motherboard and CPU designs. Semiconductor disks tend to be more popular when it is physically impossible to extend the amount of memory in a given computer system. Semiconductor disks tend to be less popular when a computer system allows memory expansion. The reason for this is simple: It is far less expensive to increase the RAM in a typical computer system and use a software-based RAM disk than it is to add a semiconductor disk to the system. A software-based RAM disk is usually faster than a semiconductor disk because the system can access the RAM disk at memory bus speeds rather than at disk controller speeds. In fact, there are only two disadvantages to RAM disks: their memory is volatile, and every byte you allocate to a RAM disk is one less byte available for your applications. In a few systems, these two disadvantages prevent the use of RAM disks. For most uses, however, if there is a little extra unused RAM in the system, and the user is careful to copy important data from the RAM disk to nonvolatile storage before shutting off the system, a software-based RAM disk can be a very cost-effective solution.

The problems with software-based RAM disk solutions begin when you have added all the RAM your system can support, and your applications require most of the memory in the system. Back when CPUs had a 16-bit address space, users quickly reached the point where they had installed as much as 64 KB of memory on their machines (2 ¹⁶ bytes is 64 KB). When the 8088/8086 rolled around with a 20-bit address bus, it wasn't long before users had installed the maximum amount of memory in those machines too. Ditto for CPUs with a 24-bit address bus, allowing a maximum of 16 MB of memory. Once CPUs started supporting 32-bit address buses, it seemed like the amount of memory one could install in the system had hit infinity, but today we're once again bumping up against that limit. It's not uncommon now to find machines with the maximum amount of memory already installed, particularly since motherboards often limit the amount of RAM that can be installed on a system even though the system CPU can address a much larger amount of RAM.

Semiconductor disks become practical when you've installed the maximum amount of RAM in your system and the applications or OS are making use of that memory, so that there isn't a large block of memory lying around that you can use for a RAM disk. Because the semiconductor disk's memory exists outside the CPU's address space, it does not impact the memory limits that apply to motherboard designs.