Merging processing and memory into a single DRAM chip could revolutionize the semiconductor industry . A CASE FOR INTELLIGENT RAM

wo trends call into question the current practice of fabricating microprocessors and DRAMs as different chips on different fabrication lines. The gap between processor and DRAM speed is growing at 50% per year; and the size and organization of memory on a single DRAM chip is becoming awkward to use, yet size is growing at 60% per year. Intelligent RAM, or IRAM, merges processing and memory into a single chip to lower memory latency, increase memory bandwidth, and improve energy efficiency. It also allows more flexible selection of memory size and organization, and promises savings in board area. This article reviews the state of microprocessors and DRAMs today, explores some of the opportunities and challenges for IRAMs, and finally estimates performance and energy efficiency of three IRAM designs. The division of the semiconductor industry into microprocessor and memory camps has many advantages. First and foremost, manufacturers can tailor a fabrication line to suit the device. Microprocessor fab lines offer fast transistors to make fast logic and many metal layers to accelerate communication and simplify power distribution. DRAM fabs, on the other hand, offer many polysilicon layers to achieve both small DRAM cells and low leakage current to reduce the DRAM refresh rate. Separate chips also mean separate packages. So microprocessors can use expensive packages that dissipate high power (5 to 70 W) and provide hundreds of pins to make wide connections to external memory. And DRAMs can use inexpensive packages that dissipate low power (1 W) and use only a few dozen pins. Separate packages, in turn, mean computer designers can scale the number of memory chips independently of the number of processors. Most desktop systems have one processor and 4 to 32 DRAM chips, but most server systems have 2 to 16 processors and 32 to 256 DRAMs. Memory systems have standardized on single in-line memory module (SIMM) or dual in-line memory module (DIMM) packaging, which allow the end user to scale the amount of memory in a system. Quantitative evidence of the industry's success is its size: In 1995, DRAMs were a $37-billion industry, and microprocessors were a $20-billion industry. In addition to financial success, the technologies of these industries have improved at unparalleled rates. DRAM capacity has quadrupled on average every three years since 1976, while microprocessor speed has done the same since 1986. However, the split into two camps has its disadvantages as well. Figure 1 …