Memories and the Memory Subsystem; The Memory Hierarchy; Caching; ROM

Memory: Some embedded systems require large amounts; others have small memory requirements Often must use a hierarchy of memory devices Memory allocation may be static or dynamic Main concerns [in embedded systems]: make sure allocation is safe minimize overhead

Memory types: RAM DRAM—asynchronous; needs refreshing SRAM—asynchronous; no refreshing Semistatic RAM SDRAM—synchronous DRAM ROM—read only memory PROM—one time EPROM—reprogram (uv light) EEPROM—electrical reprogramming FLASH—reprogram without removing from circuit

fig_04_01 Standard memory configuration: Memory a “virtual array” Address decoder Signals: address, data, control

fig_04_02 ROM—usually read-only (some are programmable-”firmware”) transistor (0 or 1)

fig_04_04 SRAM—similar to ROM In this example—6 transistors per cell (compare to flipflop?)

fig_04_05 SRAM read: precharge bi and not bi to a value halfway between 0 and 1; word line drives bi’s to stored value SRAM write: R/W line is driven low

fig_04_06 Dynamic RAM: only 1 transistor per cell READ causes transistor to discharge; it must be restored each time refresh cycle time determined by part specification

fig_04_08 DRAM read and write timing:

fig_04_08 Comparison—SRAM / DRAM

fig_04_09 Memory: typical organization (SRAM or DRAM):

fig_04_11 Two important time intervals: access time and cycle time

fig_04_12 Terminology: Block: logical unit of transfer Block size Page—logical unit; a collection of blocks Bandwidth—word transition rate on the I/O bus (memory can be organized in bits, bytes, words) Latency—time to access first word in a sequence Block access time—time to access entire block

Memory interface: Restrictions which must be dealt with: Size of RAM or ROM width of address and data I/O lines

fig_04_14 Memory example: 4K x 16 SRAM Uses 2 8-bit SRAMs (to achieve desired word size) Uses 4 1K blocks (to achieve desired number of words) Address: 10 bits within a block 2 bits to specify block— CS (chip select)

fig_04_16 Write: 8-bit bus, two cycles per word

fig_04_17 Read: choose upper or lower bits to put on the bus

If insufficient I/O lines: must multiplex signals and store in registers until data is accumulated (common in embedded system applications) Requires MAR / MDR configuration typically

DRAM: Variations available: EDO, SDRAM, FPM— basically DRAMs trying to accommodate ever faster processors Techniques: --synchronize DRAM to system clock --improve block accessing --allow pipelining As with SRAM, typically there are insufficient I/O pins and multiplexing must be used

fig_04_19 Terminology: RAS—row address strobe CAS—column address strobe note either leading or trailing edge can capture address RAS cycle time RAS to CAS delay Refresh period

fig_04_20 Example: EDO—extended data output: one row, multiple columns

fig_04_21 Refreshing the DRAM: overhead; must refresh all rows, not just those accessed; Will this be controlled internally or externally? Example: refresh one row at a time, external refresh 4M memory: 4K rows, 1K columns: 22 address bits 12 I/O pins, 10 shared between row and column Refresh each row every 64 ms 2-phase clock (for greater flexibility), 50 MHz source

fig_04_22 Refresh timing: must refresh one row every 16 musec Use a 9-bit counter incremented from either phase of the 25 MHz clock, refresh at count 384 (15.36musec)—this provides some timing margin

fig_04_23 Refresh address—12 bit binary counter—increment following the completion of each row refresh

fig_04_24 Address selection: Read / write refresh

fig_04_25 Refresh arbitration: avoid R/W conflicts 1.If normal R/W operation starts, allow it to complete 2.If refresh operation has started, remember R/W operation 3.For a tie, normal R/W operation has priority Required signals: R/W—has been initiated Refresh interval—has elapsed Normal request—by arbitration logic Refresh request—by arbitration logic Refresh grant—by arbitration logic Normal active—R/W has started Refresh active—refresh has started

fig_04_25 Row and column addresses (column only 10 bits): Generate row, column addresses on phase 1, RAS, CAS on phase 2:

fig_04_27 Arbitration circuit: request portion followed by grant portion

fig_04_28 Complete system:

fig_04_29 R/W cycles:

fig_04_30 Memory organization: Typical “Memory map” For power loss

fig_04_31 Memory hierarchy

fig_04_32 Paging / Caching Why it typically works: locality of reference (spatial/temporal) “working set” Note: in real-time embedded systems, behavior may be atypical; but caching may still be a useful technique

fig_04_33 Typical memory system with cache: hit rate (miss rate) important

Basic caching strategies: Direct-mapped Associative Block-set associativequestions: what is “associative memory”? what is overhead? what is efficiency (hit rate)? is bigger cache better?