Memory Taken from Digital Design and Computer Architecture by Harris and Harris and Computer...

Memory

Taken from Digital Design and Computer Architecture by Harris and

Harris and Computer Organization and Architecture by Null and Lobur

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Copyright © 2007 Elsevier 7-<2>

Single-Cycle Processor

SignImm

CLK

A RD

InstructionMemory

+

4

A1

A3

WD3

RD2

RD1WE3

A2

CLK

Sign Extend

RegisterFile

0

1

0

1

A RD

DataMemory

WD

WE0

1

PC0

1PC' Instr

25:21

20:16

15:0

5:0

SrcB

20:16

15:11

<<2

+

ALUResult ReadData

WriteData

SrcA

PCPlus4

PCBranch

WriteReg4:0

Result

31:26

RegDst

Branch

MemWrite

MemtoReg

ALUSrc

RegWrite

Op

Funct

ControlUnit

Zero

PCSrc

CLK

ALUControl2:0

ALU


Memory Arrays

Address

Data

ArrayN

M

• Efficiently store large amounts of data

• Three common types:– Dynamic random access memory (DRAM)

– Static random access memory (SRAM)

– Read only memory (ROM)

• An M-bit data value can be read or written at each unique N-bit address.


• Two-dimensional array of bit cells

• Each bit cell stores one bit

• An array with N address bits and M data bits:– 2N rows and M columns

– Depth: number of rows (number of words)

– Width: number of columns (size of word)

– Array size: depth × width = 2N × M

Memory Arrays

Address

Data

ArrayN

M

Address Data

11

10

01

00

depth

0 1 0

1 0 0

1 1 0

0 1 1

width

Address

Data

Array2

3


• 22 × 3-bit array

• Number of words: 4

• Word size: 3-bits

• For example, the 3-bit word stored at address 10 is 100

Memory Array: Example

Example:Address Data

11

10

01

00

depth

0 1 0

1 0 0

1 1 0

0 1 1

width

Address

Data

Array2

3


Memory Arrays

Address

Data

1024-word x32-bitArray

10

32


Memory Array Bit Cells

stored bit

wordline

bitline

Example:

stored bit = 0

wordline = 1

stored bit = 1

stored bit = 0

stored bit = 1

bitline =

(a) (b)

wordline = 1

wordline = 0

wordline = 0

bitline =

bitline =

bitline =


Memory Array Bit Cells

stored bit

wordline

bitline

Example:

stored bit = 0

wordline = 1

stored bit = 1

stored bit = 0

stored bit = 1

bitline =

(a) (b)

wordline = 1

wordline = 0

wordline = 0

bitline =

bitline =

bitline =0

1

Z

Z


• Wordline: – similar to an enable

– allows a single row in the memory array to be read or written

– corresponds to a unique address

– only one wordline is HIGH at any given time

Memory Array

wordline311

10

2:4Decoder

Address

01

00

storedbit = 0

wordline2

wordline1

wordline0

storedbit = 1

storedbit = 0

storedbit = 1

storedbit = 0

storedbit = 0

storedbit = 1

storedbit = 1

storedbit = 0

storedbit = 0

storedbit = 1

storedbit = 1

bitline2 bitline1 bitline0

Data2 Data1 Data0

2


Types of Memory

• Random access memory (RAM): volatile• Read only memory (ROM): nonvolatile


RAM: Random Access Memory

• Volatile: loses its data when the power is turned off

• Read and written quickly

• Main memory in your computer is RAM (DRAM)

Historically called random access memory because any data word can be accessed as easily as any other (in contrast to sequential access memories such as a tape recorder)


ROM: Read Only Memory

• Nonvolatile: retains data when power is turned off

• Read quickly, but writing is impossible or slow

• Flash memory in cameras, thumb drives, and digital cameras are all ROMs

Historically called read only memory because ROMs were written at manufacturing time or by burning fuses. Once ROM was configured, it could not be written again. This is no longer the case for Flash memory and other types of ROMs.


Types of RAM

• Two main types of RAM:

– Dynamic random access memory (DRAM)

– Static random access memory (SRAM)

• Differ in how they store data:

– DRAM uses a capacitor

– SRAM uses cross-coupled inverters


Robert Dennard, 1932 -• Invented DRAM in 1966 at IBM

• Others were skeptical that the idea would work

• By the mid-1970’s DRAM was in virtually all computers


• Data bits stored on a capacitor

• Called dynamic because the value needs to be refreshed (rewritten) periodically and after being read:– Charge leakage from the capacitor degrades the value

– Reading destroys the stored value

DRAM

stored bit

wordline

bitline

wordline

bitline

storedbit


DRAM

wordline

bitline

wordline

bitline

+ +storedbit = 1

storedbit = 0


SRAM

stored bit

wordline

bitline

wordline

bitline bitline


wordline311

10

2:4Decoder

Address

01

00

storedbit = 0

wordline2

wordline1

wordline0

storedbit = 1

storedbit = 0

storedbit = 1

storedbit = 0

storedbit = 0

storedbit = 1

storedbit = 1

storedbit = 0

storedbit = 0

storedbit = 1

storedbit = 1

bitline2 bitline1 bitline0

Data2 Data1 Data0

2

Memory Arrays

wordline

bitline bitlinewordline

bitline

DRAM bit cell: SRAM bit cell:


ROMs: Dot Notation

11

10

2:4 Decoder

Address

Data0Data1Data2

01

00

2

wordline

bitline

wordline

bitline

bit cellcontaining 0

bit cellcontaining 1

To read the cell the bitline is weakly pulled high.


ROM Storage

11

10

2:4 Decoder

Address

Data0Data1Data2

01

00

2

Address Data

11

10

01

00

depth

0 1 0

1 0 0

1 1 0

0 1 1

width


ROM Logic

11

10

2:4 Decoder

Address

Data0Data1Data2

01

00

2

Data2 = A1 A0

Data1 = A1 + A0

Data0 = A1A0


Example: Logic with ROMs

11

10

2:4 Decoder

Address

Data0Data1Data2

01

00

2

• Implement the following logic functions using a 22 × 3-bit ROM:– X = AB

– Y = A + B

– Z = A B

zYX

A, B


Example: Logic with ROMs• Implement the following logic functions using a 22 × 3-bit

ROM:– X = AB

– Y = A + B

– Z = A B 11

10

2:4Decoder

A, B

ZYX

01

00

2


Logic with Memory Arrays• Called lookup tables (LUTs): look up output at each input

combination (address)

storedbit = 1

storedbit = 0

00

01

2:4Decoder

A

storedbit = 0

bitline

storedbit = 0

Y

B

10

11

4-word x 1-bit Array

A B Y0 00 11 01 1

0001

TruthTable

A1

A0


Fujio Masuoka, 1944-• Developed memories and high

speed circuits at Toshiba from 1971-1994.

• Invented Flash memory as an unauthorized project pursued during nights and weekends in the late 1970’s.

• The process of erasing the memory reminded him of the flash of a camera

• Toshiba slow to commercialize the idea; Intel was first to market in 1988

• Flash has grown into a $25 billion per year market.

Reprogrammable ROM

Programmable ROM (PROM)

Electronically erasable PROM (EEPROM)Flash memory is similar

http://www.techquark.com/2009/12/inside-flash-memory-and-drives.html


Multi-ported Memories

A1

A3

WD3

WE3

A2

CLK

Array

RD2

RD1M

M

N

N

N

M

• Port: address/data pair

• 3-ported memory– 2 read ports (A1/RD1, A2/RD2)

– 1 write port (A3/WD3, WE3 enables writing)

• Small multi-ported memories are called register files


// 256 x 3 memory module with one read/write portmodule mem( input clk, we, input [7:0] a

input [2:0] wd, output [2:0] rd);

reg [2:0] RAM[255:0];

assign rd = RAM[a];

always @(posedge clk) if (we) RAM[a] <= wd;endmodule

Verilog Memory Arrays

Initializing Memory

module mem(input clk, we, input [7:0] a

input [2:0] wd, output [2:0] rd);

reg [2:0] RAM[255:0]; assign rd = RAM[a];

always @(posedge clk) if (we) RAM[a] <= wd; initial begin $readmemb("memVectors.txt", RAM); endendmodule


module ram # (parameter N = 6, M = 32) ( input clk, we,

input [N-1:0] adr, input [M-1:0] din,

output [M-1:0] dout);

reg [M-1:0] mem[2**N-1:0];

always @(posedge clk) if (we) mem[adr] <= din;

assign dout = mem[adr];endmodule

Parameterized memory module

module bigRam (input clk, we, input [9:0] adr,

input [31:0] din, output [31:0] dout);

// instantiate the ram ram #(10,32) memory(clk, we, adr, din, dout);

endmodule

31

Memory Organization

• Memory can be byte-addressable, or word-addressable, where a word typically consists of two or more bytes.

• Memory is constructed of RAM chips, often referred to in terms of length width.

32

• Physical memory usually consists of more than one RAM chip.

• Access is more efficient when memory is organized into banks of chips with the addresses interleaved across the chips

• With low-order interleaving, the low order bits of the address specify which memory bank contains the address of interest.

• Accordingly, in high-order interleaving, the high order address bits specify the memory bank.

The next slide illustrates these two ideas.

4.6 Memory Organization

33

Low-Order Interleaving

High-Order Interleaving

4.6 Memory Organization

34

4.6 Memory Organization• Example: Suppose we have a memory consisting of

16 2K x 8 bit chips.

– Memory is 32K = 25 210 = 215

– 15 bits are needed for each address.

– We need 4 bits to select the chip, and 11 bits for the offset into the chip that selects the byte.

35

4.6 Memory Organization• In high-order interleaving the high-order

4 bits select the chip.

• In low-order interleaving the low-order 4 bits select the chip.

Memory Taken from Digital Design and Computer Architecture by Harris and Harris and Computer...

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