COMP203/NWEN Memory Technologies 0 Plan for Memory Technologies Topic Static RAM (SRAM) Dynamic RAM (DRAM) Memory Hierarchy DRAM Accelerating Techniques –Reading B.5

COMP203/NWEN Memory Technologies 1 Static Random Access Memory (SRAM) Registers are not cost-effective for main memory –Too much logic per bit –SRAM and DRAM used instead SRAM –Static Random access memory Made of D-latches Contents persists as long as power is on Read and write times are constant –Described by height (number of cells) × width (bits per cell) –e.g. 256K x 1 or even 1G x 1 18 address inputs or 30 address inputs 1 output (1 bit of data per cell)

COMP203/NWEN Memory Technologies 2 A Simplified Layout of a Memory Matrix wordlineswordlines memory cell (physical word) memory element (1 bit) 2 bit line SRAM 1 bit line DRAM bit lines (input/output)

COMP203/NWEN Memory Technologies 3 Example: A Typical SRAM Chip Chip select is used to turn this particular chip on - required prior to R or W Output enable allows the selected cell to assert its contents on Dout Write enable makes the selected cell update itself Typical spec – ns read time –4M x 1 (1997) –512Mx1 (2003) –1G X 1 (2008) Address Chip select Output enable Write enable Din7 - Din SRAM 32K x 8 Dout7 - Dout0 8

COMP203/NWEN Memory Technologies 4 Three-State (or Tri-State) Buffers register 0 register 1 32-to-1 multiplexor Dout The register file used a 32- to-1 multiplexor to ensure only one register drives Dout For a 64Kx1 SRAM a ‘centralised’ multiplexor is not practical Instead we let multiple memory elements drive each output via a three- state buffer As long as only one tri-state buffer is enabled all others will have a high impedance output Dout Select 0 Data 0 elem 0 Select 1 Data 1 elem 1 32 register Tri-state buffer

COMP203/NWEN Memory Technologies 5 Decoding Addresses in SRAMs This design is okay for small SRAM But for large SRAMs the decoder becomes the limiting factor Solution: two-level decoding (next slide)

COMP203/NWEN Memory Technologies 6 Example: Two-Level Decoding 32 KB In the first level each SRAM enables one 64-bit cell out of 512 cells –9 address lines needed In second level each multiplexor chooses one bit out of 64 –6 address lines needed Net effect is a large savings in PLAs size

COMP203/NWEN Memory Technologies 7 Dynamic Random Access Memory SRAMs use two inverters to store bits –bits can be stored indefinitely DRAMs use one transistor and one capacitor –much less hardware –but charge leaks away over time –bits need to be refreshed i.e. read out and written back in Capacitor Pass transistor Word line Bit line SRAM latch DRAM

COMP203/NWEN Memory Technologies 8 Refreshing Mechanisms Can be done by processor –but to refresh megabytes several times a second becomes costly More often done by memory chips themselves –entire rows can be refreshed at once –approximately 1% of memory cycles are lost to refreshing

COMP203/NWEN Memory Technologies 9 Two-Level DRAM In the first level the decoder enables one row out of 2048 –11 address lines needed –Each row has 2048 bits In the second level the multiplexor selects 1 bit out of 2048 –11 address lines needed Refresh involves writing a row from the column latches into the array row decoder x 2048 DRAM array 2048 column latches multiplexor Dout Address 10 to Address 0 Address 21 to Address 11 RefreshRefresh bits 1

COMP203/NWEN Memory Technologies 10 SRAM Performance in Early Sixties IC memory chips appeared in early sixties: –Capacity (size): 128 bits –Time to read (speed): ~ 1 microsecond –Power consumption: ~ 1 mW The same time Morris Mini was very popular: –Capacity: 4 persons –Speed: 100 km/hour –Consumption: 5 liters/100 km

COMP203/NWEN Memory Technologies 11 SRAM Performance After Fifty Years Capacity (size): 1.0 Gbits Time to read (speed): ~ 5 nanoseconds Power consumption: ~ 1 microWatt The net result of the development: –Capacity increase: 10 million times –Speed increase: 500 times –Consumption decrease: 1000 times

COMP203/NWEN Memory Technologies 12 What about Morris Mini? Morris Mini is still running on our streets What if it had undergone the same neck breaking development? –Capacity: persons (10 times the whole NZ) –Speed: km/hour –Consumption: 5 mill liters/100 km

COMP203/NWEN Memory Technologies 13 Example Memory Architecture CPU Registers (Flip-Flops) L1 Cache (SRAM) Main Memory (DRAM) Fastest, most expensive, smallest Slower, less expensive, greater Hard Disk Even slower, even less expensive, even greater

COMP203/NWEN Memory Technologies 14 Modern DRAM To access 1 bit: must set row and then column address –Fast page-mode DRAMS Provide faster access to subsequent bits on same row First access sets row and column addresses Subsequent accesses only change column address –Nibble-mode DRAMs go better Automatically generate next 3 column addresses So, each row access reads 4 bits sequentially –No need to wait for new column either –Extended Data Out (EDO) DRAM Similar to Fast page-mode But, can start loading new row address while still reading from previous row Provides ~50% speedup of page-mode –Bursting – fast delivering several consecutive words

COMP203/NWEN Memory Technologies 15 Summary SRAM –D-latches as memory elements –Separate word, bit input and bit output lines –Address decoding, 2-level decoding –Tri-state buffers –Very fast, but not very dense DRAM –Transistor/capacitor elements, need for refreshing –Word lines and common bit input/output lines –Slower but very dense –FPM, EDO, SDRAM Bursting