Memory
Taken from Digital Design and
Computer Architecture by Harris and
Harris and Computer Organization and
Architecture by Null and Lobur
Single-Cycle Processor
31:26
5:0
MemtoReg
Control
MemWrite
Unit
Branch
ALUControl2:0
Op
ALUSrc
Funct
RegDst
PCSrc
RegWrite
CLK
0
PC'
PC
1
A
RD
Instr
Instruction
Memory
25:21
20:16
A1
A2
A3
WD3
CLK
WE3
RD2
0 SrcB
1
Register
File
20:16
+
WriteReg4:0
Sign Extend
RD
Data
Memory
WD
0
1
<<2
+
15:0
WriteData
A
ReadData
1
SignImm
4
ALUResult
WE
0
15:11
PCPlus4
Zero
SrcA
RD1
ALU
CLK
PCBranch
Result
Copyright © 2007 Elsevier
7-<2>
Memory Arrays
• 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 Nbit address.
Address
N
Array
M
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5-<3>
Data
Memory Arrays
• 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
Address Data
Address
N
Array
Address
2
3
M
Data
Data
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Array
5-<4>
11
0 1 0
10
1 0 0
01
1 1 0
00
0 1 1
width
depth
Memory Array: Example
•
•
•
•
22 × 3-bit array
Number of words: 4
Word size: 3-bits
For example, the 3-bit word stored at address 10 is 100
Example:
Address Data
Address
2
Array
3
Data
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11
0 1 0
10
1 0 0
01
1 1 0
00
0 1 1
width
5-<5>
depth
Memory Arrays
Address
10
1024-word x
32-bit
Array
32
Data
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5-<6>
Memory Array Bit Cells
bitline
wordline
stored
bit
Example:
bitline =
wordline = 1
bitline =
wordline = 0
stored
bit = 0
stored
bit = 0
bitline =
wordline = 1
bitline =
wordline = 0
stored
bit = 1
stored
bit = 1
(a)
Copyright © 2007 Elsevier
(b)
5-<7>
Memory Array Bit Cells
bitline
wordline
stored
bit
Example:
bitline = Z
bitline = 0
wordline = 1
wordline = 0
stored
bit = 0
stored
bit = 0
bitline = Z
bitline = 1
wordline = 1
wordline = 0
stored
bit = 1
stored
bit = 1
(a)
Copyright © 2007 Elsevier
(b)
5-<8>
Memory Array
• 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
2:4
Decoder
11
Address
wordline3
2
10
01
00
Copyright © 2007 Elsevier
bitline2
wordline2
wordline1
wordline0
bitline1
stored
bit = 0
stored
bit = 1
stored
bit = 0
stored
bit = 1
stored
bit = 0
stored
bit = 0
stored
bit = 1
stored
bit = 1
stored
bit = 0
stored
bit = 0
stored
bit = 1
stored
bit = 1
5-<9>
Data2
Data1
bitline0
Data0
Types of Memory
• Random access memory (RAM): volatile
• Read only memory (ROM): nonvolatile
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5-<10>
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)
Copyright © 2007 Elsevier
5-<11>
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.
Copyright © 2007 Elsevier
5-<12>
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
Copyright © 2007 Elsevier
5-<13>
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
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5-<14>
DRAM
• 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
bitline
bitline
wordline
wordline
stored
bit
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stored
bit
5-<15>
DRAM
bitline
bitline
wordline
wordline
stored + +
bit = 1
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stored
bit = 0
5-<16>
SRAM
bitline
wordline
stored
bit
bitline
bitline
wordline
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5-<17>
Memory Arrays
2:4
Decoder
11
Address
wordline3
2
10
01
00
bitline2
wordline2
wordline1
wordline0
bitline1
stored
bit = 0
stored
bit = 1
stored
bit = 0
stored
bit = 1
stored
bit = 0
stored
bit = 0
stored
bit = 1
stored
bit = 1
stored
bit = 0
stored
bit = 0
stored
bit = 1
stored
bit = 1
Data2
Data1
bitline0
Data0
SRAM bit cell:
DRAM bit cell:
bitline
wordline
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bitline
bitline
wordline
5-<18>
ROMs: Dot Notation
bitline
wordline
2:4
Decoder
11
Address
bit cell
containing 0
2
10
bitline
wordline
01
bit cell
containing 1
00
Data2 Data1 Data0
To read the cell the bitline is weakly pulled high.
Copyright © 2007 Elsevier
5-<19>
ROM Storage
2:4
Decoder
11
Address
Address Data
11
0 1 0
10
1 0 0
01
01
1 1 0
00
00
0 1 1
2
10
Data2 Data1 Data0
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5-<20>
width
depth
ROM Logic
2:4
Decoder
11
Address
Data2 = A1  A0
Data1 = A1 + A0
2
10
Data0 = A1A0
01
00
Data2 Data1 Data0
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5-<21>
Example: Logic with ROMs
• Implement the following logic functions using a 22 × 3-bit
ROM:
– X = AB
– Y=A+B
– Z=AB
2:4
Decoder
11
Address
A, B
2
10
01
00
Data2 Data1 Data0
X
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5-<22>
Y
z
Example: Logic with ROMs
• Implement the following logic functions using a 22 × 3-bit
ROM:
– X = AB
– Y=A+B
– Z=AB
2:4
Decoder
11
A, B
2
10
01
00
X
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5-<23>
Y
Z
Logic with Memory Arrays
• Called lookup tables (LUTs): look up output at each input
combination (address)
4-word x 1-bit Array
2:4
Decoder
00
Truth
Table
A
0
0
1
1
B
0
1
0
1
Y
0
0
0
1
A
A1
B
A0
bitline
stored
bit = 0
01
stored
bit = 0
10
stored
bit = 0
11
stored
bit = 1
Y
Copyright © 2007 Elsevier
5-<24>
Fujio Masuoka, 1944• Developed memories and high
speed circuits at Toshiba from 19711994.
• 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.
Copyright © 2007 Elsevier
5-<25>
Reprogrammable ROM
Electronically erasable PROM (EEPROM)
Flash memory is similar
Programmable ROM (PROM)
Multi-ported Memories
• 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
CLK
N
N
N
M
Copyright © 2007 Elsevier
A1
WE3
A2
A3
RD1
RD2
Array
WD3
5-<27>
M
M
Verilog Memory Arrays
// 256 x 3 memory module with one read/write port
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;
endmodule
Copyright © 2007 Elsevier
5-<28>
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);
end
endmodule
Parameterized memory module
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
Copyright © 2007 Elsevier
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
5-<30>
Memory Organization
• Memory can be byte-addressable, or wordaddressable, where a word typically consists of two or
more bytes.
• Memory is constructed of RAM chips, often referred to
in terms of length  width.
31
4.6 Memory Organization
• 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.
32
4.6 Memory Organization
Low-Order Interleaving
High-Order Interleaving
33
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.
34
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.
35