Memory Hierarchy Design - Computer Architecture: A Quantitative Approach - page 101

FIGURE 2.15 Power consumption for a DDR3 SDRAM operating under three condi-
tions: low power (shutdown) mode, typical system mode (DRAM is active 30% of the
time for reads and 15% for writes), and fully active mode, where the DRAM is continu-
ously reading or writing when not in precharge . Reads and writes assume bursts of 8
transfers. These data are based on a Micron 1.5V 2Gb DDR3-1066.
Flash Memory
Flash memory is a type of EEPROM (Electronically Erasable Programmable Read-Only
Memory), which is normally read-only but can be erased. The other key property of Flash
memory is that it holds it contents without any power.
Flash is used as the backup storage in PMDs in the same manner that a disk functions in a
laptop or server. In addition, because most PMDs have a limited amount of DRAM, Flash may
also act as a level of the memory hierarchy, to a much larger extent than it might have to do so
in the desktop or server with a main memory that might be 10 to 100 times larger.
Flash uses a very different architecture and has different properties than standard DRAM.
The most important differences are
1. Flash memory must be erased (hence the name Flash for the “flash” erase process) before it
is overwriten, and it is erased in blocks (in high-density Flash, called NAND Flash, which
is what is used in most computer applications) rather than individual bytes or words. This
means when data must be writen to Flash, an entire block must be assembled, either as
new data or by merging the data to be writen and the rest of the block's contents.
2. Flash memory is static (i.e., it keeps its contents even when power is not applied) and
draws significantly less power when not reading or writing (from less than half in standby
mode to zero when completely inactive).
3. Flash memory has a limited number of write cycles for any block, typically at least 100,000.
By ensuring uniform distribution of writen blocks throughout the memory, a system can
maximize the lifetime of a Flash memory system.
4. High-density Flash is cheaper than SDRAM but more expensive than disks: roughly $2/GB
for Flash, $20 to $40/GB for SDRAM, and $0.09/GB for magnetic disks.
5. Flash is much slower than SDRAM but much faster than disk. For example, a transfer of
256 bytes from a typical high-density Flash memory takes about 6.5 μs (using burst mode
transfer similar to but slower than that used in SDRAM). A comparable transfer from a
DDR SDRAM takes about one- quarter as long, and for a disk about 1000 times longer. For