HP Visualize J5000 HP Visualize B1000, C3000, J5000 Workstation Memory Subsyst - Page 3

latency of the HP VISUALIZE UNIX® Workstation memory subsystem, and contributes to an improved price/performance ratio. The new memory architecture was designed so that it operates with a minimum of one DIMM loaded in the system, in contrast to the HP PA-8200 memory subsystem that requires a minimum of 2 DIMMs loaded in the system. This allows the HP VISUALIZE UNIX® Workstation systems to reach maximum memory bandwidth with just 1 DIMM in the system, allowing customers to buy a minimum of memory and still achieve maximum memory bandwidth. New Synchronous DRAMs A key feature in this new subsystem is the use of synchronous DRAMs (SDRAMs), a new DRAM architecture. Changes from asynchronous DRAMs, which were used in the HP PA-8200 workstations and are currently used in Sun's Ultrasparc5, Ultrasparc10 and Ultrasparc60 systems, include a synchronous interface between the DRAM and the outside world and multiple banks internal to the DRAM. One advantage of the synchronous interface of SDRAMs is that it latches address and control signals into the chip on a given clock edge. This frees up the address and control bus for other activities, such as issuing precharge and activate commands to other banks in the subsystem, while the SDRAMs decode the latched signals. With asynchronous DRAMs, the memory controller has to sit idle while these internal DRAM operations are performed. This extra address and control bandwidth provided by SDRAMs increases with higher frequencies and contributes to lower busy system memory latency in the HP VISUALIZE UNIX® Workstation systems. The synchronous data interface of SDRAMs is another big advantage of SDRAMs over asynchronous DRAMs. With SDRAMs, data is clocked in or out of the SDRAM with the SDRAM or system clock, making the data rate of SDRAMs equal to the clock rate. With asynchronous DRAMs, other timing constraints come into play that limit the data rate of these devices. Comparing the SDRAMs used in the HP VISUALIZE UNIX® Workstation systems with the asynchronous DRAMs used in the HP PA-8200 systems shows that the per device bandwidth of the SDRAMs is close to 5 times higher than that of the asynchronous DRAMs. This synchronous data interface significantly contributes to the higher memory bandwidth of the HP VISUALIZE UNIX® Workstation systems. Another advantage of SDRAMs is that they have multiple independent banks on a single chip. This allows memory interleaving to take place at the chip level. HP VISUALIZE UNIX® Workstation systems can now take advantage of interleaving performance gains with only one DIMM in the system, as opposed to needing multiple DIMMs in a system like the HP PA-8200 systems. This interleaving is advantageous in that with multiple internal banks, one page in a bank of an SDRAM can be accessed while a page in a second bank of the same SDRAM is activated, allowing the page in the second bank to be accessed as soon as possible after access to the first bank. This process hides the activate time of the page in the second bank in the access time of the page in the first bank. This feature of SDRAMs contributes to improved busy system memory latency. Memory in the HP VISUALIZE UNIX® Workstations uses varying SDRAM densities- 64Mb, 128Mb and, in the future, 256Mb- to provide customers with a range of memory DIMM sizes. 08/23/99 HP VISUALIZE WORKSTATIONS 2