Today i an going to share something interesting about MEMORY SYSTEM DESIGN. The development of high density,high speed semiconductor memory has been the prime cause of the accelerated use of microprocessor.in this section we will discuss about the details of designing the memory subsystem and interfacing it to microprocessor's bus lines.
If we see this topic, from the system designer's point of view some interesting are of interest :
1)Word length and Capacity : How does one organize memory chip so as to realize a target memory of some desired word length and capacity ?
2)Arranging Chip on the memory board : How are these chips arranged on a memory board as an array of rows and colums ?
3)Memory allocation: What area of the addressing space of the microprocessor should these larger memory cover ?
4)Memory interface: How the target memory interfaced to the microprocessor external bus lines in a way that meets these assignments ?
5)Memory system Speed: How can someone design faster memory chips?
Before discussing in detail, it is necessary to clarify that only increasing the processor's input clock frequency alone does not guarantee an increase in the overall system performance .The fast processor will be forced to slow down of the data it receives comes from a slower memory. So properly matching the processor and memory bandwidths is the best way of eliminating this performance degradation. The simplest way to achieve this is the faster memory chips. we can also use different techniques like utilization of new accessing modes available on recent large DRAMs and increasing the memory bandwidth by designing interleaved memories.
For detail information of different parameters of memory and bandwidth click here.
Here we focus on the following topics
a)Read Only Memory
(alternatices ) 1- PROM
2-EPROM
3-EEPROM
b)Read and Write Memory
(types ) 1-Static RAM
2-Dynamic RAM
ROMs : The basic types of permanent storage is the ROM, also called masked ROM , which permits only reading of its contents and not writing new information into it. Technologies used for ROM manufacture include bipolar TTL and MOS (PMOS ,NMOS ,CMOS and FAMOS). ROM memories are programmed or burned initially during manufacture by using special "masks". They are very fast.
PROMs: A variation of ROM is the "programmable ROM" or "write once memory".The time required to write information in a PROM is measured in milliseconds.PROM memories can be programmed by the customer PROM memories can be programmed by the customer only once and are usually manufactured in bipolar technology.PROM are programmed using special equipment call "PROM burners".Each PROM usually has a pic compatible counterpart in the form of ROM.How ever correcting error is not possible with PROM.
EPROMs : The last disadvantage of PROMs can be alleviated by using another type of ROM : the EPROM (erasable PROM). These memories are programmable more than once. The basic difference from RAM memories is that their write time is significantly longer.The contents of an EPROM memory can be erased by exposing the chip to ultraviolet light and and the memories can be programmed again .The EPROM are more expensive and slower than bipolar PROMs.
EEPROMS : The electrically erasable PROM is a type of PROM which can have selected part of its Memory electrically charged. Electrically erasability implies the potential for programming the device without removing it from circuit and without the need of ultraviolet.There are two type of EEPROMs :
1) metal nitride oxide semiconductor EEPROM
2)floating gate EEPROMs
The memory part can be erased in a few millisecond and rewritten at the rate of few hundres microseconds. Erasing and writing require threshold voltages of a few volts DC.These electrically erasable PROMs are generally too slow to be used as memories in the final product and are expensive
.................. to be continued
If we see this topic, from the system designer's point of view some interesting are of interest :
1)Word length and Capacity : How does one organize memory chip so as to realize a target memory of some desired word length and capacity ?
2)Arranging Chip on the memory board : How are these chips arranged on a memory board as an array of rows and colums ?
3)Memory allocation: What area of the addressing space of the microprocessor should these larger memory cover ?
4)Memory interface: How the target memory interfaced to the microprocessor external bus lines in a way that meets these assignments ?
5)Memory system Speed: How can someone design faster memory chips?
Before discussing in detail, it is necessary to clarify that only increasing the processor's input clock frequency alone does not guarantee an increase in the overall system performance .The fast processor will be forced to slow down of the data it receives comes from a slower memory. So properly matching the processor and memory bandwidths is the best way of eliminating this performance degradation. The simplest way to achieve this is the faster memory chips. we can also use different techniques like utilization of new accessing modes available on recent large DRAMs and increasing the memory bandwidth by designing interleaved memories.
For detail information of different parameters of memory and bandwidth click here.
Here we focus on the following topics
a)Read Only Memory
(alternatices ) 1- PROM
2-EPROM
3-EEPROM
b)Read and Write Memory
(types ) 1-Static RAM
2-Dynamic RAM
ROMs : The basic types of permanent storage is the ROM, also called masked ROM , which permits only reading of its contents and not writing new information into it. Technologies used for ROM manufacture include bipolar TTL and MOS (PMOS ,NMOS ,CMOS and FAMOS). ROM memories are programmed or burned initially during manufacture by using special "masks". They are very fast.
PROMs: A variation of ROM is the "programmable ROM" or "write once memory".The time required to write information in a PROM is measured in milliseconds.PROM memories can be programmed by the customer PROM memories can be programmed by the customer only once and are usually manufactured in bipolar technology.PROM are programmed using special equipment call "PROM burners".Each PROM usually has a pic compatible counterpart in the form of ROM.How ever correcting error is not possible with PROM.
EPROMs : The last disadvantage of PROMs can be alleviated by using another type of ROM : the EPROM (erasable PROM). These memories are programmable more than once. The basic difference from RAM memories is that their write time is significantly longer.The contents of an EPROM memory can be erased by exposing the chip to ultraviolet light and and the memories can be programmed again .The EPROM are more expensive and slower than bipolar PROMs.
EEPROMS : The electrically erasable PROM is a type of PROM which can have selected part of its Memory electrically charged. Electrically erasability implies the potential for programming the device without removing it from circuit and without the need of ultraviolet.There are two type of EEPROMs :
1) metal nitride oxide semiconductor EEPROM
2)floating gate EEPROMs
The memory part can be erased in a few millisecond and rewritten at the rate of few hundres microseconds. Erasing and writing require threshold voltages of a few volts DC.These electrically erasable PROMs are generally too slow to be used as memories in the final product and are expensive
.................. to be continued
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