1.1 Memory technology

Almost all fast memories used today are semiconductor-based. Magnetic core memory is still used in applications where radiation “hardness” — resistance to changes caused by ionizing radiation — is important. They come in two flavors: dynamic random access memory (DRAM) and static random access memory (SRAM). The term random means that you can address memory locations in any order. This is to distinguish random access from serial memories, where you have to step through all intervening locations to get to the particular one you are interested in. An example of a storage medium that is not random is magnetic tape. The terms dynamic and static have to do with the technology used in the design of the memory cells. DRAMs are charge-based devices, where each bit is represented by an electrical charge stored in a very small capacitor. The charge can leak away in a short amount of time, so the system has to be continually refreshed to prevent data from being lost. The act of reading a bit in DRAM also discharges the bit, requiring that it be refreshed. It’s not possible to read the memory bit in the DRAM while it’s being refreshed.

SRAM is based on gates, and each bit is stored in four to six connected transistors. SRAM memories retain their data as long as they have power, without the need for any form of data refresh.

DRAM offers the best price/performance, as well as highest density of memory cells per chip. This means lower cost, less board space, less power, and less heat. On the other hand, some applications such as cache and video memory require higher speed, to which SRAM is better suited. Currently, you can choose between SRAM and DRAM at slower speeds — down to about 50 nanoseconds (ns). SRAM has access times down to about 7 ns at higher cost, heat, power, and board space.

In addition to the basic technology to store a single bit of data, memory performance is limited by the practical considerations of the on-chip wiring layout and the external pins on the chip that communicate the address and data information between the memory and the processor.

Access time

The amount of time it takes to read or write a memory location is called the memory access time . A related quantity is the memory cycle time . Whereas the access time says how quickly you can reference a memory location, cycle time describes how often you can repeat references. They sound like the same thing, but they’re not. For instance, if you ask for data from DRAM chips with a 50-ns access time, it may be 100 ns before you can ask for more data from the same chips. This is because the chips must internally recover from the previous access. Also, when you are retrieving data sequentially from DRAM chips, some technologies have improved performance. On these chips, data immediately following the previously accessed data may be accessed as quickly as 10 ns.

Access and cycle times for commodity DRAMs are shorter than they were just a few years ago, meaning that it is possible to build faster memory systems. But CPU clock speeds have increased too. The home computer market makes a good study. In the early 1980s, the access time of commodity DRAM (200 ns) was shorter than the clock cycle (4.77 MHz = 210 ns) of the IBM PC XT. This meant that DRAM could be connected directly to the CPU without worrying about over running the memory system. Faster XT and AT models were introduced in the mid-1980s with CPUs that clocked more quickly than the access times of available commodity memory. Faster memory was available for a price, but vendors punted by selling computers with wait states added to the memory access cycle. Wait states are artificial delays that slow down references so that memory appears to match the speed of a faster CPU — at a penalty. However, the technique of adding wait states begins to significantly impact performance around 25?33MHz. Today, CPU speeds are even farther ahead of DRAM speeds.

The clock time for commodity home computers has gone from 210 ns for the XT to around 3 ns for a 300-MHz Pentium-II, but the access time for commodity DRAM has decreased disproportionately less — from 200 ns to around 50 ns. Processor performance doubles every 18 months, while memory performance doubles roughly every seven years.

The CPU/memory speed gap is even larger in workstations. Some models clock at intervals as short as 1.6 ns. How do vendors make up the difference between CPU speeds and memory speeds? The memory in the Cray-1 supercomputer used SRAM that was capable of keeping up with the 12.5-ns clock cycle. Using SRAM for its main memory system was one of the reasons that most Cray systems needed liquid cooling.

Unfortunately, it’s not practical for a moderately priced system to rely exclusively on SRAM for storage. It’s also not practical to manufacture inexpensive systems with enough storage using exclusively SRAM.

The solution is a hierarchy of memories using processor registers, one to three levels of SRAM cache, DRAM main memory, and virtual memory stored on media such as disk. At each point in the memory hierarchy, tricks are employed to make the best use of the available technology. For the remainder of this chapter, we will examine the memory hierarchy and its impact on performance.

In a sense, with today’s high performance microprocessor performing computations so quickly, the task of the high performance programmer becomes the careful management of the memory hierarchy. In some sense it’s a useful intellectual exercise to view the simple computations such as addition and multiplication as “infinitely fast” in order to get the programmer to focus on the impact of memory operations on the overall performance of the program.