Random access memory - EduTechLearners

161 Pages·2015·5.44 MB·English

by  Amit Agarwal

Random access memory
••
SSequenttiiall ciircuiittss allll ddeppendd uppon tthhe ppressence off mmemmory.
–
A flip-flop can store one bit of information.
–
A register can store a single “word,” typically 32-64 bits.
••
RRanddom access memory, or RRAAMM, allllows us tto sttore even llarger amountts
of data. Today we’ll see:
–
The basic interface to memory.
––
HHooww yyoouu ccaann iimmpplleemmeenntt ssttaattiicc RRAAMM cchhiippss hhiieerraarrcchhiiccaallllyy.
•
This is the last piece we need to put together a computer!
Random Access Memory 1
Introduction to RAM
••
RRanddomm-accessss mmemmory, or RRAAMM, pproviiddess llarge quanttiittiiess off ttemmpporary
storage in a computer system.
•
Remember the basic capabilities of a memory:
–
IItt sshhoouulldd bbee aabbllee ttoo ssttoorree aa vvaalluuee.
–
You should be able to read the value that was saved.
–
You should be able to change the stored value.
••
AA RRAAMM iiss ssiimmiillaarr, eexxcceepptt tthhaatt iitt ccaann ssttoorree mmaannyyvvaalluueess.
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An address will specify which memory value we’re interested in.
–
Each value can be a multiple-bit word (e.g., 32 bits).
••
WWee’llll rreeffiinnee tthhee mmeemmoorryy pprrooppeerrttiieess aass ffoolllloowwss::
A RAM should be able to:
- SSttorree mmaannyy wworrddss, onnee ppeerr aaddddrreessss
- Read the word that was saved at a particular address
- Change the word that’s saved at a particular address
Random Access Memory 2
Picture of memory
•• YYou can tthhiinkk off commpputter mmemmory ass bbeiing one AAddddrreessss DDaattaa
big array of data. 00000000
– The address serves as an array index. 00000001
– 0000000000000022
EEaacchh aaddddrreessss rreeffeerrss ttoo oonnee wwoorrdd ooff ddaattaa.
• .
You can read or modify the data at any given
.
memory address, just like you can read or
modifyy the contents of an arrayy at anyy ggiven .
index. .
• If you’ve worked with pointers in C or C++, then .
you’ve already worked with memory addresses. .
.
.
.
.
FFFFFFFD
FFFFFFFE
FFFFFFFFFFFFFFFF
Random Access Memory 3
Block diagram of RAM
2k x n memory
CS WR Memory operation
k n
ADDRRSS OOUUT 0 x None
n
DATA 1 0 Read selected word
CS 1 1 Write selected word
WR
•
This block diagram introduces the main interface to RAM.
–
A Chip Select, CS, enables or disables the RAM.
–
AADDRRSS speciiffiies tthhe addddress or llocattiion tto readd ffrom or wriitte tto.
–
WR selects between reading from or writing to the memory.
To read from memory, WR should be set to 0.
OOUUTT wwiillll bbee tthhee nn-bbiitt vvaalluuee ssttoorreedd aatt AADDRRSS.
To write to memory, we set WR = 1.
DATA is the n-bit value to save in memory.
•
TThhiiss iinntteerrffaaccee mmaakkeess iitt eeaassyy ttoo ccoommbbiinnee RRAAMMss ttooggeetthheerr, aass wwee’llll sseeee.
Random Access Memory 4
Memory sizes
•• WWe reffer tto tthhiiss ass a 22kk xx n mmemmory.
– There are k address lines, which can specify one of 2k addresses.
–
Each address contains an n-bit word.
2k x n memory
kk nn
AADDRRSS OOUUTT
n
DATA
CS
WWRR
• For example, a 224 x 16 RAM contains 224 = 16M words, each 16 bits long.
–
TThhe RRAAMM woulldd needd 2244 addddress lliines.
– The total storage capacity is 224 x 16 = 228 bits.
Random Access Memory 5
Size matters!
••
MMemmory ssiizess are ussualllly ssppeciiffiiedd iin nummbberss off bbyttess ((88 bbiittss)).
• The 228-bit memory on the previous page translates into:
228 bits / 8 bits pper byyte = 225 byytes
•
With the abbreviations below, this is equivalent to 32 megabytes.
Prefix Base 2 Base 10
10 3
K Kilo 2 = 1,024 10 = 1,000
20 6
M Mega 2 = 1,048,576 10 = 1,000,000
30 9
GG GGiiggaa 22 == 11,007733,774411,882244 1100 == 11,000000,000000,000000
•
To confuse you, RAM size is measured in base 2 units, while hard drive
ssiizzee iiss mmeeaassuurreedd iinn bbaassee 1100 uunniittss.
–
In this class, we’ll only concern ourselves with the base 2 units.
Random Access Memory 6
Typical memory sizes
•• SSomme ttyppiicall mmemmory cappaciittiiess:: AAddddrreessss DDaattaa
– 00000000
PCs usually come with 512MB – 2GB RAM.
– 00000001
PDAs have 16-64MB of memory.
0000000000000022
–
DDiigiittall cameras andd MMPP33 pllayers can hhave
.
32MB-8GB or more of onboard storage.
.
•
Many operating systems implement virtual
.
mmeemmoorryy, wwhhiicchh mmaakkeess tthhee mmeemmoorryy sseeeemm llaarrggeerr
.
than it really is.
– .
Most systems allow up to 32-bit addresses.
This works out to 232, or about four billion, .
different possible addresses. .
– .
With a data size of one byte, the result is
apparently a 4GB memory! .
– .
TThhe operattiing systtem uses hhardd ddiiskk space
as a substitute for “real” memory. FFFFFFFD
FFFFFFFE
FFFFFFFFFFFFFFFF
Random Access Memory 7
Reading RAM
••
TTo readdffromm tthhiiss RRAAMM, tthhe conttrolllliing ciircuiitt mmusstt::
–
Enable the chip by ensuring CS = 1.
–
Select the read operation, by setting WR = 0.
–
SSendd tthhe ddesiiredd addddress tto tthhe AADDRRSS iinputt.
–
The contents of that address appear on OUT after a little while.
•
Notice that the DATA input is unused for read operations.
2k x n memory
k n
ADRS OUT
n
DATA
CCSS
WR
Random Access Memory 8
Writing RAM
••
TTo wriittetto tthhiiss RRAAMM, you needd tto::
–
Enable the chip by setting CS = 1.
–
Select the write operation, by setting WR = 1.
–
SSendd tthhe ddesiiredd addddress tto tthhe AADDRRSS iinputt.
–
Send the word to store to the DATA input.
•
The output OUT is not needed for memory write operations.
2k x n memory
k n
ADRS OUT
n
DATA
CCSS
WR
Random Access Memory 9
Static memory
••
HHow can you iimmppllemmentt tthhe mmemmory chhiipp??
•
There are many different kinds of RAM.
–
We’ll start off discussing static memory, which is most commonly
uusseedd iinn ccaacchheess aanndd vviiddeeoo ccaarrddss.
–
Later we mention a little about dynamic memory, which forms the
bulk of a computer’s main memory.
•
SSttaattiicc mmeemmoorryy iiss mmooddeelleedd uussiinngg oonnee llaattcchhffoorr eeaacchh bbiitt ooff ssttoorraaggee.
•
Why use latches instead of flip flops?
–
A latch can be made with only two NAND or two NOR gates, but a
fflipp-fflopp reqquires at least twice that much hardware.
–
In general, smaller is faster, cheaper and requires less power.
–
The tradeoff is that getting the timing exactly right is a pain.
Random Access Memory 10