main memory, DRAM

"the one single development that put computers on their feet was the invention of a reliable form of memory, namely, the core memory. ... cost was reasonable, it was reliable and, because it was reliable, it could ... be made large."
--Maurice Wilkes

2 kinds of RAM:

capacity of DRAM is 4 to 8 times that of comparable SRAM, but SRAM has cycle times 8 to 16 times faster than DRAM, and is 8 to 16 times more costly.

main memory (main store) is almost always DRAM, while caches are almost always SRAM.

DRAM Technology

principle of operation of dynamic RAM (reading)

principle of operation of dynamic RAM (writing)

principle of operation of dynamic RAM (single cell)

One transistor and capacitor per cell

RAS and CAS timing diagram

Fast Page Mode DRAM (FPM DRAM)

Prior to newer forms of DRAM, Fast Page Mode DRAM (FPM DRAM) was the most common kind of DRAM in personal computers. Page mode DRAM essentially accesses a row of RAM without having to continually respecify the row. A row access strobe (RAS) signal is held active while the column access strobe (CAS) signal changes to read a sequence of contiguous cells. This reduces access time and lowers power requirements. Clock timings for FPM DRAM are typically 6-3-3-3 (meaning 3 clock cycles for access setup, and 3 clock cycles for the first and each of three successive accesses based on the initial setup).


fast page: once assert RAS you can access any column addr.
therefore clock through column addresses.
typically row widths are integer number of cache blocks: work well with cache.

static column DRAM

can change col. addr. without having to re-assert CAS.

compare with SRAM Technology (will talk about in later lectures)

-Four to six transistors per cell
 -no need to refresh (hence "static")
 -access time and cycle time identical (no need to write back a row after read)
 -emphasis of design on speed and not capacity
   -DRAM 4-8 times as dense as SRAM
   -SRAM 8-16 times as fast as DRAM
   -SRAM 8-16 times as expensive

Strategies for Improving Main Memory (DRAM) Performance