Chapter 6 Flashcards
Internal Memory
List
The two technologies that RAM technology is divided into
Dynamic RAM
Static RAM
Define
DRAM
Made with cells that store data as charge on capacitors.
Presence or absence of charge interpreted as 1 or 0.
Requires periodic charge refreshing to maintain data storage.
The term dynamic refers to tendency of stored charge to leak away, even with power continuosly applied.
dynamic RAM
considered analog
Define
SRAM
Digital device that uses the same logic elements used in the processor.
Binary values are stored using traditional flip-flop logic gate configurations.
Will hold data as long as power is supplied to it.
static RAM
Considered Digital
Define
SRAM vs. DRAM
both volatile
DRAM: simpler to build, smaller, more dense, less expensive, requires the supporting refresh circuitry, tend to be favored for large memory requirements, used for main memory.
SRAM: faster, used for cache memory (both on and off chip)
Define
Read Only Memory (ROM)
Contains a permanent pattern of data that cannot be changed or added to.
No power source required (non-volatile)
Data or program is permanent in main memory and never needs to be loaded from a secondary storage device.
Data is actually wired into the chip as part of the fabrication process.
Define
Disadvantages of ROM
No room for error, if 1 bit is wrong, the whole batch of ROMs must be thrown out.
Data insertion step includes a relatively large fixed cost.
Define
Programmable ROM (PROM)
Less expensive alternative to ROM.
Nonvolatile and may be written into only once.
Writing process is performed electrically and may be performed by supplier or customer at a later time than the original chip fabrication.
Special equipment is required for the writing process.
Provides flexibility and convenience.
Attractive for high volume production runs.
Define
Erasable PROM (EPROM)
programmable ROM
Erasure process can be performed repeatedly.
More expensive than PROM
Define
Electrically EPROM (EEPROM)
erasable programmable ROM
Can be written into at any time without erasing prior contents.
Combines the advantages of non-volatility with the flexability of being updateable in place.
More expensive than EPROM.
Define
Flash memory
Intermediate between EPROM and EEPROM in both cost and functionality.
Uses an electrical erasing technique that does not provide byte level erasure.
Microchip is organized so a section of memory cells are erased in a single action or a “flash.”
Define
Interleaved memory
Composed of collection of DRAM chips grouped together to form a memory bank.
Each bank is independently able to service a memory read or write request.
K banks can service K requests simultaneously, increasing memory read/write by a factor of K.
If consecutive words in m are stored in different banks, the transition of a block of m is sped up.
list
Different memory errors to correct
Hard failure
Soft failure
Define
Hard failure
memory
Permanent or physical defect.
M cells or cells affected can’t reliably store data but become stuck at 0 or 1 or switch erratically.
Can be caused by: harsh environmental abuse, manufacturing defects, wear.
Define
Soft error
memory
Random, non-destructive event that alters the contents of 1 or more memory cells.
No permanent damage to memory.
Can be caused by: power supply problems, alpha particles.
Define need for
Advanced DRAM organization
One of the most critical system bottlenecks when using high-performance processors is the interface to main internal memory.
The traditional DRAM chip is constrained both by its internal architecture and by its interface to the processor’s memory bus.
A number of enhancements to the basic DRAM have been explored. Current market dominators are SDRAM and DDR_DRAM.
Define
Synchronous DRAM (SDRAM)
Exchanges data with the processor synched to an external clock signal and running at the full speed of the processor/memory bus without imposing wait states.
With synchronous access the DRAM moves data in and out under control of the system clock.
The processor or other master issues the instruction and address info which is latched by the DRAM.
The DRAM then responds after a set number of clock cycles.
Meanwhile, the master can safely do other tasks while the DRAM is processing.
Define
Double Data Rate SDRAM (DDR SDRAM)
Developed by the JEDEC.
Numerous companies make DDR chips.
Achieves higher data rates in three ways:
1. data transfer is synched to both the rising and falling edge of the clock, rather than just the rising edge.
2. DDR uses higher clock rate on the bus to increase the transfer rate
3. A buffering scheme is used.
Define
JEDEC Solid State Technology Organization
Electronic Industry Alliance’s semiconductor engineering standardization body
Define
Embedded DRAM (eDRAM)
A DRAM integrated on the same chip as or MCM of an app-specific integrated circuit (ASIC) or microprocessor.
For a number of metrics, eDRAM is intermediate between on-chip SRAM and offchip DRAM.
Better than SRAM in memory size, and cost-per-bit, but DRAM still better.
Worse than SRAM in access time, but better than DRAM.
Define
How flash memory works
Starts with typical transistor and adds a second gate called floating gate. Initially the floating gate doesn’t interfere, this state represents 1. But applying a large voltage across the oxidizing layer that makes it ‘floating’ causes electrons to tunnel through it and become trapped, even if power is disconnected. This represents 0. Large voltage from the opposite direction returns state to 1.
Define
NOR flash memory
Basic access unit is a bit, referred to as a memory cell. Cells are connected in parallel to the bit lines so each cell can be read/write/erased individually. If any memory cell of the device is turned on by the corresponding word line, the bit line goes low.
Traditionally preferred for internal memory
Define
NAND flash memory
Organized in transistor arrays with 16 or 32 transistors in series. The bit line goes low only if all the transistors in the correspnding word lines are turned on.
Traditionally preferred for external memory
List
New memory technologies coming soon
STT-RAM
PCRAM
ReRAM
Define
STT-RAM
Spin-transfer torque RAM.
New type of magnetic RAM (MRAM).
Employs new write method called polarization-current-induced magnetic switching.
Features non-volatility, fast r/w speed, high program endurance, and zero standby power.
good candidate for cache or main memory
Define
PCRAM
Phase-changing.
Something with chalcogenide-based material.
good candidate to replace or supplement DRAM for main memory
Define
ReRAM
aka RRAM
Resistive RAM.
Creates resistance rather than directly storing charge.
Good candidate to replace or supplement secondary storage and main memory