Chapter 5

Internal Memory

Semiconductor Memory Types

Today’s technology: 1 Gigabit / sq in

In R&D: 100 Gigabits / sq in

Semiconductor Memory (SRAM)

Semiconductor Memory (DRAM)

16Mbit DRAM

Semiconductor memory (EPROM)

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

Static RAM model

1K X 4 SRAM (Part Number 2114N)

1K X 4 SRAM (Part Number 2114N)

1K X 4 SRAM (Part Number 2114N)

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 (211=2048)– Adding one more pin doubles range of values so x4

capacity

Memory Design – 1K x 4

A[00:09] D[03:00]

Addr Block Select

Memory Design – 1K x 8

A[00:09] D[07:04]

A[00:09] D[03:00]

Addr Block Select => Addr Block Select =>

D[07:04] D[03:00]

Memory Design - 2k x 8

D[07:04] D[03:00]

Block 00

Block 01

Memory Design - 4k x 8 D[07:04] D[03:00]

Block 00

Block 01

Block 10

Block 11

22 x 3 Memory

addressdecoder

word select word WEaddress

writeenable

input bits

output bits

2 BIT Decoder (2 to 4)

2 to 4 Bit Decoder

3 to 8 Bit Decoder

2 to 1 MUX

4 to 1 MUX

8 to 1 MUX

16 to 1 MUX

?

Register

22 x 3 Memory

addressdecoder

word select word WEaddress

writeenable

input bits

output bits

24 x 8 Memory

?

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

Dynamic RAM model

Read Only Memory (ROM)

• Permanent storage—Nonvolatile

• Microprogramming (will address later)

• Library subroutines

• Systems programs (BIOS)

• Function tables• Controllers

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 (All of chip!)

—Flash memory– Whole blocks of memory stored/changed electrically

—EEPROM: Electrically Erasable– Takes much longer to write than read (lower density)

EPROM

Semiconductor Memory

16Mbit DRAM

256kByte Module Organisation (256K x 1)

Typical 16 Mb DRAM (4M x 4)

1MByte Module Organization (1Meg x 8 bits)

Refreshing

• Refresh circuit is included on the chip

• Count through rows

• Read & Write back

• Chip must be disabled during refresh

• Takes time

• Occurs asynchronously

• Slows down apparent performance

Improvements in memory

RAM – continually gets denser.

DRAM – Several improvements: SDRAM – synchronous DRAM DDR-SDRAM - doubles transfer speed RDRAM – asynchronous one transfer per clock cycle

Comparison of improved DRAM

Conventional DRAM – 40 to 100 MB/S transfer rate?

Synchronous DRAM (SDRAM)

• Access is synchronized with an external clock• Address is presented to RAM• RAM finds data (CPU waits in conventional DRAM)• Since SDRAM moves data in time with system

clock, CPU knows when data will be ready• CPU does not have to wait, it can do something

else• Burst mode allows SDRAM to set up stream of

data and fire it out in block• DDR-SDRAM sends data twice per clock cycle

(leading & trailing edge)

SDRAM Read Timing

SDRAM

DDR SDRAM

• SDRAM can only send data once per clock• Double-data-rate SDRAM can send data

twice per clock cycle—Rising edge and falling edge

RAMBUS

• Adopted by Intel for Pentium & Itanium• Main competitor to SDRAM

• Separate bus (hence the name RAMBUS)— maximum 12 centimeter length bus !

• Bus addresses up to 320 RDRAM chips — at 1.6Gbps

• Asynchronous block protocol— Precise control signal timing— 480ns access time

RAMBUS Diagram