If one is running on a desktop where power is not an issue, leaving all the chips in standby makes sense. But on a laptop where power consumption is an issue, one would only keep active the chips that are actually being used.
In a system, there are always multiple applications running at a time. Each application only uses a part of the memory and the memory that a given application is using will often fit in one bank. While the banks that are active will be controlled dynamically, generally these changes will be correlated more with process switches than with individual memory access as you suggest. Taking the 90ns hit on a process switch is not a big deal.
Secondly, Laptop usage is bursty. The CPU will be used intensely for a short while and then sit idle. For example, if one is typing to Word, when the character is typed, Word has to do a lot of work to properly update the display. It then sits in the idle loop. When one fetches a web page, one wants that page to display quickly, then one reads it and the CPU is in the idle loop.
With DDR ram, all banks must be kept active during these short idle periods. With RDRam, only one bank needs to be kept active while in the idle loop. This saves a lot of power,
John is actually being very conservative in assuming that 4 chips will be active at any given time. It will usually be much less, often just one. This makes the RDRam power savings more like a 3 to 1 ratio.
-- Alan |
