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Memory Technology and Interfacing
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Stored Program Model
Memory
Output
Input
CPU
Stored-program computer model
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Types of Memory
Random-Access Memory (RAM)VolatileStore data or programs that can be
conveniently reloaded Read-Only Memory (ROM)
Non-volatileProgramming once store essential
programs
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Random-Access Memory (RAM) Any of its contents may be accessed at
any time. (Why this notation is used?) Two technologies are used:
Static RAM (SRAM)Dynamic RAM (DRAM)
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Static RAM
A static RAM stores a bit of information within a flip-flop.
6-Transistor SRAM Cell
bit bit
word
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Dynamic RAM
A DRAM stores a bit of information as a charge.
row select
bit
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DRAM vs SRAM
DRAM is much higher density (less transistor is used for a cell).
DRAM needs to be refreshed every now and then (to prevent memory loss).Must incorporate with extra (complicated)
circuit for refreshing.
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ROM Technologies
Pure ROM PROM (User-Programmable ROM) EPROM (Erasable PROM) EAROM (Electrically-Alterable ROM)
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Pure ROM
A mask-programmed ROM made from manufacturer.
High cost – need to be produced for large volumes.
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PROM
Can be programmed (once) by user. Using PROM programmer.
It’s a fusible-link PROM each cell is fuse (nichrome or polysilicon). Originally, all rows and columns are connected
(logical 0s). Once a cell is ‘burnt’, it becomes logical 1. “Grow back” problem reliability problem.
Once programmed, it can not be altered ever again.
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EPROM
Can be reprogrammed a number of times using a programmer machine.
Two main types:UV-erasableElectrically-erasable
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EAROM
Can be read from and written to. It actually is non-volatile RAM.
Now, we use Flash Memory
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Accessing Memory
There must be a control bus for the following operations:Read data from memoryWrite data to memoryDo not access memory
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ROM Interfacing
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ROM Read
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RAM Interfacing
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Reading from the Memory (RAM)
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Writing to the Memory
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Memory Interfacing Design & Memory Map
4K
4K
20=121=222=423=824=1625=3226=6427=12828=25629=512210=1024211=2048212=4096213=8192214=16384215=32768216=65536
0000h
0FFFh1000h
1FFFh
4K2000h
2FFFh
4K
4K
E000h
EFFFhF000h
EFFFh
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Use Decoder (i.e. 74138)