Chapter 5 Internal Memory

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Chapter 5Internal Memory
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Semiconductor Memory Types
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Static RAM (SRAM)

• Desired for main memory
• Used in cache
• Basically an array of flip-flops
• Simple to interface and control
• Fast
• Relatively low density - complex
• Relatively expensive

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Static RAM model
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Semiconductor Memory
What is it ?
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2 BIT Decoder (2 to 4)

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3 Bit Decoder (3 to 8)
?

4 Bit Decoder (4 to 16)
?
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2 to 4 Bit Decoder
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3 to 8 Bit Decoder
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2 to 1 MUX

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4 to 1 MUX

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8 to 1 MUX
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16 to 1 MUX
?
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Register
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22 x 3 Memory
word select
word WE
input bits
address
write enable
address decoder
output bits
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24 x 8 Memory
?
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Dynamic RAM (DRAM)

• Used in main memory
• Bits stored as charge in capacitors
• Essentially analog device
• Charges leak
• Need refreshing even when powered
• Need refresh circuits
• Higher density (more bits per chip)
• Slower than Static RAM
• Less expensive

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Dynamic RAM model
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Read Only Memory (ROM)

• Permanent storage
• Nonvolatile
• Microprogramming (see later)
• Library subroutines
• Systems programs (BIOS)
• Function tables

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Types of ROM

• ROM Written during manufacture
• Very expensive for small runs
• PROM Programmable (once)
• Needs special equipment to program
• Read mostly
• EPROM Erasable Programmable
• Erased by UV
• EEPROM Electrically Erasable
• Takes much longer to write than read
• Flash memory
• Erase whole memory (block) electrically

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EPROM
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Memory Organization

• A 16Mbit chip can be organised as 1M of 16 bit
  words
• A bit per chip system has 16 lots of 1Mbit chip
  with bit 1 of each word in chip 1 and so on
• A 16Mbit chip can be organised as a 2048 x 2048 x
  4bit array
• Reduces number of address pins
• Multiplex row address and column address
• 11 pins to address (2112048)
• Adding one more pin doubles range of values so x4
  capacity

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Typical 16 Mb DRAM (4M x 4)
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Semiconductor Memory
16Mbit DRAM
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Refreshing

• Refresh circuit is included on the chip
• Count through rows
• Read Write back
• Chip must be disabled during refresh
• Takes time
• Slows down apparent performance

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256kByte Module Organisation (256K x 1)
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1MByte Module Organization (1Meg x 8 bits)