Chapter 4 · 2 4.6 Memory Organization • Computer memory consists of a linear array of...

Chapter 4

MARIE: An Introduction

to a Simple Computer

2

4.6 Memory Organization

• Computer memory consists of a linear array of

addressable storage cells (similar to registers).

• Memory can be byte-addressable, or word-

addressable (word typically 2 or more bytes).

• Memory is constructed of RAM chips, often referred to

in terms of length width:

– Length in words

– Width in bits

Example: The word size of a machine is 16 bits, so we

use a 4M 16 RAM chip.

106 or 220 ?

3


• Computer memory consists of a linear array of

addressable storage cells (similar to registers).

• Memory can be byte-addressable, or word-

addressable (word typically 2 or more bytes).

• Memory is constructed of RAM chips, often referred to

in terms of length width:

– Length in words

– Width in bits

Example: The word size of a machine is 16 bits, so we

use a 4M 16 RAM chip.

This means 222 16-bit words.

4

Trick QUIZ

A machine has 223 Bytes of memory. How many

address bits are needed?

5

Trick QUIZ

A machine has 223 Bytes of memory. How many

address bits are needed?

How big is a word of memory (a.k.a. addressability)?

• If memory is Byte-addressable …

• If memory is 2-Byte-addressable …

• If memory is 4-Byte-addressable …

Do not get confused!

The width of memory chips is always given in bits.

E.g. this chip:

“ 64K x 8, byte-addressable ”

It means “ (64 x 1024)-long x 8 bits-wide,

addressable at the byte level “

6

7

• How does the computer access a memory location

corresponds to a particular address?

• We observe that 4M can be expressed as 2 2 2 20 =

2 22 words.

• The memory locations for this memory are numbered

0 through 2 22 -1.

• Thus, the memory bus of this system requires 22

address lines.

– The address lines “count” from 0 to 222 - 1 in binary. Each

line is either “on” or “off” indicating the location of the

desired memory element.


8

The memory bus of this system requires at least 22

address lines.

– The address lines “count” from 0 to 222 - 1 in binary. Each

line is either “on” or “off” indicating the location of the

desired memory element.

If the addresses are written in hex, what is the range

of addresses available in this machine?

QUIZ

9

• 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


10

We want to build a 32K x 8, byte-addressable

memory out of 2K x 8 RAM chips.

How many chips?

How many bits are needed

for address?

Example

11

We want to build a 32K x 8 byte-addressable memory

out of 2K x 8 RAM chips.


for address?

Example

– Memory is 32K = 25 210 = 215 Bytes

– 15 bits are needed for each address:

–4 bits to select the chip

–11 bits for the offset into the chip to

select the byte.

12

We want to build a 32K x 16 memory out of 2K x 8

RAM chips.

How many chips?


for address?

Example

We call it word-addressable

13


RAM chips.


for address?

Example

– Memory is 32K x2 = 25 210 2= 216

Bytes

– But still only 15 bits are needed for each

address:

– 4 bits to select the row of chips

– 11 bits for the offset into the chip to

select the byte.

Wait a second, aren’t we missing a factor of 2? (One bit?)

14


RAM chips.


for address?

Example

– Memory is 32K x2 = 25 210 2= 216

Bytes

– But still only 15 bits are needed for each

address:

– 4 bits to select the row of chips

– 11 bits for the offset into the chip to

select the byte.

All 16 bits at an address are activated by the same address! The difference is

only in the data bus, which is now 16 bits wide!

15

How exactly do we connect the address lines?

Solution 1: high-order interleaving → the high order

address bits specify the memory bank.

Draw bus picture – decoder

needed!

16

How to address this memory?

Solution 2: low-order interleaving → the low order


Draw bus picture – decoder

needed!

17

Solution 1: high-order interleaving → the high order


18

Solution 2: low-order interleaving → the low order


Low-Order Interleaving

High-Order Interleaving

High- vs. low-order interleaving example: 8

RAM chips, each of size 4x8

High-Order Interleaving

QUIZ: Show the addressing logic (decoder)

and bit values needed to address Byte 25

Low-Order Interleaving

QUIZ: Show the addressing logic (decoder)

and bit values needed to address Byte 14

Same physical position as before!

22

Back to 32K x 8 example

• In high-order interleaving the high-order

4 bits select the chip.

• In low-order interleaving the low-order

4 bits select the chip.

Why use low-order interleaving?

The vast majority of real-life programs have space-

locality, i.e. they tend to access data that is stored close

together (e.g. arrays)

• With high-order interleaving, reading/writing N Bytes

takes N read cycles, b/c all N are on the same chip

• With low-order interleaving, the reads-writes can be

made to overlap (a.k.a. pipelining) → less than N

cycles!

23 See example on next 2 slides

Space-locality with low-order interleaving: A program

needs to access 4 consecutive addresses in DRAM

Assume:

• Row-access time = 5 ns

• Column-access time = 5 ns

• Bus propagation = 1 ns

When accessing DRAM, the row

addresses need to stabilize

before column addresses can be

provided.

High-order interleaving: (5+5)x4 + 1 = 41 ns

Low-order interleaving: 5 + 5x4 + 1 = 26 ns

Row address only needs to be set once!

See “Async. DRAM access” linked

on our webpage.

Real-life example of low-order

interleaving

25

Triple-channel memory slots, color-coded, on the

eVGA X58 SLI Classified motherboard Source: http://www.guru3d.com/articles_pages/evga_x58_sli_classified_review.html

Not in text

26

We are designing a computer with 2 MB of RAM,

byte-addressable.

The only chips available are 512K x 4.

Draw the memory-addressing diagram with high-

order interleaving.

QUIZ

Image source: http://eent3.lsbu.ac.uk/units/b3embsys/Week11Combinational%20Logic%20Circuits.htm

27

4.7 Interrupts

• The normal execution of a program is altered when

an event of higher-priority occurs. The CPU is

alerted to such an event through an interrupt.

• Interrupts can be triggered by:

– I/O requests

– Arithmetic errors (such as division by zero)

– Memory parity errors

– Invalid instruction

– User-defined breakpoints (debugging)

28

4.7 Interrupts

• Each interrupt is associated with an interrupt-

handling procedure that directs the actions of

the CPU when an interrupt occurs.

• Interrupts can be:

– Maskable → CPU can be instructed to ignore them

under certain circumstances

– Nonmaskable → High-priority interrupts that cannot

be ignored under any circumstances

29

4.8 MARIE

• Our model computer, the Machine Architecture that

is Really Intuitive and Easy, MARIE, was designed

for the singular purpose of illustrating basic computer

system concepts.

• While this system is too simple to do anything useful

in the real world, understanding of its functions will

enable us to comprehend system architectures that

are much more complex.

30

The MARIE architecture has the following

characteristics:• Binary, two's complement data representation.

• Stored program, fixed word length data and

instructions.

• 4K words of word-addressable main memory.

• 16-bit data words.

• 16-bit instructions, 4 for the opcode and 12 for the

address.

• A 16-bit arithmetic logic unit (ALU).

• Seven registers for control and data movement.

4.8 MARIE

31

MARIE’s seven registers are:

• Accumulator, AC, a 16-bit register that holds an

operand:

• The one operand for a conditional operator (e.g., "less than")

• One of the two operands of a two-operand instruction.

• Memory address register, MAR, a 12-bit register that

holds the memory address of an instruction or the

operand of an instruction.

• Memory buffer register, MBR, a 16-bit register that

holds the data after its retrieval from, or before its

placement in memory.

32

MARIE’s seven registers are:

• Program counter, PC, a 12-bit register that holds the

address of the next program instruction to be

executed.

• Instruction register, IR, which holds an instruction

immediately preceding its execution.

• Input register, InREG, an 8-bit register that holds data

read from an input device.

• Output register, OutREG, an 8-bit register, that holds

data that is ready for the output device.

33

MARIE architecture

34

• The registers are interconnected,

and also connected to main memory

through a common data bus.

• Each device on the bus is identified

by a unique number that is set on

the control lines whenever that

device is required to carry out an

operation.

• Separate connections are also

provided: AC to MBR, ALU to AC

and MBR to ALU.

MARIE Datapath

They allow data transfer

w/o use of the main bus!

35

This is the diagram from the 4th ed. Neither

are correct!!

AC to MBR must be a separate input to

MBR, not direct connection! (3rd ed.

Diagram is correct). It’s used in STORE, in

order to perform two transfers at the same

time for speed:

MBR to AC is actually not needed, b/c it’s

only used in LOADI, in which it’s a

separate step, so it can be implemented

over the data bus:

To do for next time:

• Read pp.224-233 of Ch.4

• Answer Review Questions 16 through 25

• Solve Exercises 4, 7

37

EOL 1

QUIZ

Exercise 15

38

QUIZ Exercise 18

39

40

Why do high-performance computers use low-

order interleaving for their memory chips?

QUIZ

41

We are designing a computer with 2 MB of RAM,

byte-addressable.

The only chips available are 512K x 4.

Draw the memory-addressing diagram with high-

order interleaving.

QUIZ

Image source: http://eent3.lsbu.ac.uk/units/b3embsys/Week11Combinational%20Logic%20Circuits.htm

42

What are interrupts?

Why are they needed in the operation of a

computer?

QUIZ

43

What are MARIE’s 7 registers?

What are their functions?

Can data go straight from

memory into AC?

QUIZ

44




memory into AC?

Since the answer to the last

questions is negative, what is

connection 4 for?

QUIZ

45




memory into AC?

Since the answer to the last

questions is negative, what is

connection 4 for?

A: To transfer data from other

registers (InReg, IR) into AC.

QUIZ

46

• A computer’s instruction set architecture (ISA)

specifies the format of its instructions and the

primitive operations that the hardware can perform.

• The ISA is an interface between a computer’s

hardware and its software.

• The concept of an ISA was invented at IBM in the

1980s, for their System/360 family of computers (see

links on webpage).

4.8.3 MARIE ISA

47

• Some ISAs include hundreds of different instructions

for processing data and controlling program

execution.

• The MARIE ISA consists of only 13 “fundamental”

instructions.

• Real-life ISAs often have hundreds of instructions.

4.8.3 MARIE ISA

48

Format of a

MARIE instruction:

The fundamental MARIE instructions are:

49

• This is a bit pattern for a LOAD instruction as it would

appear in the IR:

• We see that the opcode is 1 and the address from

which to load the data is 3.

• Where does the data go? Always into the AC

(implicit, i.e. hard-wired)

Instruction example

50

This is a bit pattern for a SKIPCOND instruction as it

would appear in the IR:

Instruction example

These two bits specify which condition is tested:

• 00 → skip if AC < 0

• 01 → skip is AC = 0

• 10 → skip if AC > 0

• 11 → ???

51

SKIPCOND

• The opcode is 8 and bits 11 and 10 are 10, meaning

that the next instruction will be skipped if the value

in the AC is greater than zero.

How is skipping accomplished in the hardware?

Increment PC.

Instruction example

52

What is the difference between SKIPCOND and

JUMP X?

Instruction example

How is jumping accomplished in the hardware?

Load X into PC (PC must be a loadable register!)

53

• Each of our instructions actually consists of a

sequence of smaller instructions called

microoperations.

• The exact sequence of microoperations that are

carried out by an instruction can be specified using

register transfer language (RTL).

In the MARIE RTL, we use the notation M[X] to indicate

the actual data value stored in memory location X,

and to indicate the transfer of bytes to a register or

memory location.

Microops and RTL

54

• The RTL for the LOAD instruction is:

• The RTL for the ADD instruction is:

MAR X

MBR M[MAR]

AC AC + MBR

MAR X

MBR M[MAR]

AC MBR

RTL examples

55

• The RTL for the INPUT instruction is:

AC InREG

RTL examples

56

Write the RTL for the following instructions:

• OUTPUT

• JUMP X

• SUBT X

QUIZ: RTL

57

• Recall that SKIPCOND skips the next instruction

according to the value of the AC.

• The RTL for the this instruction is the most complex

in MARIE’s instruction set:

RTL examples

58

4.9 Instruction Processing

Remember the fetch-execute cycle:

• fetch an instruction from memory and place it into IR

• decode IR to determine what needs to do next

• if a memory value (operand) is involved in the

operation, get it (address in MAR, value in MBR)

• with everything in place, execute the instruction

• if the result needs to go into memory, store it (result

in MBR, address in MAR)

The next slide shows a flowchart of this process.

59

Fetch-decode-execute

60

All computers provide a way of interrupting the

fetch-decode-get-execute cycle.

• Sources of interrupts:

– User break (e.g. Control+C) is issued

– I/O request

– Critical error (divide by 0, illegal opcode)

• Interrupts can be caused by hardware or

software.

– Software interrupts are also called traps

61

Interrupt processing involves adding another step to the

fetch-decode-execute cycle:

ISR = Interrupt Service Routine

The starting addresses of all ISRs are stored in an Interrupt

Vector Table

62

Processing an interrupt

Normally stored

in the low-

memory area

63

• For general-purpose systems, it is common to

disable all interrupts during the time in which an

interrupt is being processed.

– Typically, this is achieved by setting a bit in the flags

register.

• Interrupts that are ignored in this case are called

maskable.

• Nonmaskable interrupts are those interrupts that

must be processed in order to keep the system in

a stable condition.

Processing an interrupt

64

Interrupts are very useful in processing I/O.

However, interrupt-driven I/O is complicated

(see Ch.7)

MARIE uses polled I/O instead:

– All output is placed in OutREG

– The CPU polls InREG, until input is sensed, at

which time the value is copied into AC.

Interrupts vs. polling

65

• Consider the simple MARIE program given below.

We show a set of mnemonic instructions stored at

addresses 100 - 106 (hex):

4.10 A Simple Program

66

This is the LOAD 104 instruction:


67

ADD 105


68

4.11 Discussion on Assemblers

Assemblers translate mnemonics (instructions that are

comprehensible to humans) into the machine

language that is comprehensible to computers

Distinction between an assembler and a compiler:

• In assembly language, there is a one-to-one

correspondence between a mnemonic instruction and

its machine code.

• With compilers, this is not usually the case.

69

• Assemblers create an object program file from

mnemonic source code in two passes.

• During the first pass, the assembler assembles as

much of the program as it can, while it builds a

symbol table that contains memory references for

all symbols in the program.

• During the second pass, the instructions are

completed using the values from the symbol table.

4.11 Assemblers

70

• Consider our example

program at the right.

– Note that we have included two directives HEX and

DEC that specify the radix

of the constants.

• The first pass, creates

a symbol table and the

partially-assembled

instructions:

4.11 Assemblers

71

• After the second pass, the

assembly is complete.

4.11 Assemblers

72

Remark on our Instruction Set

• So far, all of the MARIE instructions that we have

discussed use a direct addressing mode.

• This means that the address of the operand is

explicitly stated in the instruction.

• It is often useful to employ a indirect addressing,

where the address of the address of the operand

is given in the instruction.

– If you have used pointers in programming, you are

already familiar with indirect addressing.

Individual work recommended:

3, 5, 6

73

EOL 2

74

4.12 Extending Our Instruction Set

• So far, all of the MARIE instructions that we

have discussed use a direct addressing mode.

• This means that the address of the operand is

explicitly stated in the instruction.

• It is often useful to employ a indirect

addressing, where the address of the address

of the operand is given in the instruction.

– If you have ever used pointers in a C/C++

program, you are already familiar with indirect

addressing!

75

• We have 4 indirect addressing mode instructions in

the MARIE ISA.

• The first two are LOADI X and STOREI X

– X specifies the address of the address of the operand to

be loaded or stored.

• In RTL :

MAR IR[11..0]

MBR M[MAR]

MAR MBR

MBR M[MAR]

AC MBR

LOADI X

This is where X is

76

Your turn: Write the RTL code for STOREI

MAR IR[11..0]

MBR M[MAR]

MAR MBR

MBR M[MAR]

AC MBR

?STOREI XLOADI X

77

MAR IR[11..0]

MBR M[MAR]

MAR MBR

MBR AC

M[MAR] MBR

STOREI XLOADI X

78

• The ADDI X instruction is a

combination of LOADI X and ADD X:

• In RTL: MAR IR[11..0]

MBR M[MAR]

MAR MBR

MBR M[MAR]

AC AC + MBR

79

• The JUMPI X instruction is similar to JUMP

• In RTL: MAR IR[11..0]

MBR M[MAR]

PC MBR

80

What is indirect addressing used for?

• Do you remember the Interrupt Vector Table?

• When the interrupt occurs, the hardware places in the

IR the code of a JUMPI, with the appropriate address

X!

Extra-credit

81

84

• The jump-and-store instruction, JnS X, gives us

subroutine functionality.

• In RTL:

MBR PC

MAR X

M[MAR] MBR

MBR X

AC 1

AC AC + MBR

PC AC

Another programming tool is the

use of subroutines.

Explain this in

your own words!

Make a memory

diagram!

85

• The jump-and-store instruction, JnS X, gives us

subroutine functionality.

• In RTL:

MBR PC

MAR X

M[MAR] MBR

MBR X

AC 1

AC AC + MBR

PC AC

Does JnS permit

recursive calls?

Example 4.5: subroutine that doubles

Load X

Store Temp

JnS Subr

Store X

Load Y

Store Temp

JnS Subr

Store Y

Halt

X, Dec 20

Y, Dec 42

Temp, Dec 0

Subr, Hex 0

LoadTemp

Add Temp

JumpI

Subr

END

Assembler directive

QUIZ: Based on this model, write a

subroutine that subtracts two numbers

Load X

Store Temp

JnS Subr

Store X

Load Y

Store Temp

JnS Subr

Store Y

Halt

X, Dec 20

Y, Dec 42

Temp, Dec 0

Subr, Hex 0

LoadTemp

Add Temp

JumpI

Subr

END

Assembler directive

88

• Our last instruction is CLEAR.

• It resets the contents of AC to all zeroes.

• In RTL:

AC 0


89

• Our last instruction is CLEAR.

• It resets the contents of AC to all zeroes.

• In RTL:

AC 0


Trick question: What addressing mode is this?

The new instructions

90

91

Table 4.7 on p.252

See handout!

93

RTL.

Implementing loops in assembly

94

EOL 3

Discussion of exam problems

95

QUIZ:

How/where do MARIE subroutines preserve

the return address?

96

QUIZ:

How/where do MARIE subroutines preserve

the return address?

97

QUIZ:

Can MARIE subroutines be reentrant (recursive)?

Explain!

98

QUIZ:

Can MARIE subroutines be reentrant (recursive)?

Explain!

99

No, b/c a second call to the subroutine

would overwrite the return address of

the first.

QUIZ:

How does a MARIE program pass parameters to

subroutines?

100

QUIZ:

How does a MARIE program pass parameters to

subroutines?

101

In this example, the parameter is passed

in the memory address Temp.

It’s also possible to pass it in the AC

register, but that approach fails if there

are multiple parameters.

QUIZ:

How does a MARIE subroutine pass a return value

back to the main program?

102

QUIZ:

How does a MARIE subroutine pass a return value

back to the main program?

103

In this example, the return value is

passed in the AC register.

It’s also possible to pass it in a memory

location.

QUIZ: Write a subroutine that outputs ‘P’ if

the number in AC is >0 and ‘NP’ otherwise

Hints: Use the “block” approach for the IF statement explained

before. Make sure the blocks don’t run sequentially!

104

105

4.13 Decoding

• A computer’s control unit keeps things

synchronized, making sure that bits flow to the

correct components as the components are needed.

• There are two ways in which a control unit can be

implemented:

– Hardwired → a hardware controller creates all

signals with combinational logic

– Microprogrammed → a small program is placed

into read-only memory (ROM) and used to create

those signals

106

• The microoperations given by each RTL instruction

define the operation of MARIE’s control unit.

• Each microop. consists of a distinctive signal pattern

that is interpreted by the control unit and results in the

execution of an instruction.

Decoding

Example: RTL for ADDI X instruction:

MAR X

MBR M[MAR]

MAR MBR

MBR M[MAR]

AC AC + MBR

107

MARIE Datapath

Note the datapath

addresses of the

components connected

to the data bus!

How many datapath address

lines (or bits) are needed?

Don’t confuse datapath addresses

with memory addresses!

108

If you answered three,

you’re on the right track!

In fact, we need two sets of

three signals:

• P2, P1, P0, control

reading from memory or

a register

• P5, P4, P3, controls

writing to memory or a

register.

MBR example

109

Read/Write control for MBRMBR is enabled for reading when P0 and P1 are high, and it is enabled

for writing when P3 and P4 are high.

(Note: The authors consider that the unmentioned signals are

automatically zero.)

Remember from

Digital Circuits:

Why are tri-state

buffers needed?

110

QUIZ: What are the values of P5 through P0. in order to:

• Read from InReg?

• Write to InReg? Hint: what is the bus

address of InReg?

111

Inspection of MARIE’s instructions’ RTL reveals that the

ALU has only three operations: add, subtract, and

clear.

We also define a fourth

“do nothing” state:

Decoding

112



clear.



Decoding

Which of MARIE’s

instructions do not

use the ALU?

113



clear.



Decoding

The entire set of MARIE’s

control signals consists of:

– Register controls: P5 through P0.

– ALU controls: A0 and A1

– Timing: T0 through T7 and counter reset Cr Explained

later

114

• The RTL for MARIE’s Add instruction is:MAR X

MBR M[MAR]

AC AC + MBR

• After an Add instruction is fetched from memory,

the address X is in the rightmost 12 bits of the IR,

which has a datapath address of 7.

• The first RTL instruction above says that X is

copied to the MAR (so it can be used next to

address the memory).

EXAMPLE: Decoding the ADD instruction

115

P3 P2 P1 P0 T0: MAR X

P4 P3 T1: MBR M[MAR]

A0 P5 P1 P0 T2: AC AC + MBR

Cr T3: [Reset counter]

Complete signal sequence for

Add instruction

Explain the values!

116






Add instruction

Explain the values!

Solution:

• We’re reading from IR (where the address X is). IR has datapath

address 7, so P2 P1 P0 must be 111.

• We’re writing to MAR which has datapath address 1, so P5 P4 P3

must be 001.

EOL 4

117






Add instruction

Explain the values!

118






Add instruction

Explain the values!

119






Add instruction

Why do we need to reset?

120

What is the maximum nr. of

microops. needed for a MARIE

instruction?

Table 4.7 on p.252

121

The counter will therefore count

cyclically through 7 states

S0 … S6,

but for some (most) instructions, we

need to cut the cycle short.

Note: Fetching the instruction from memory

and incrementing PC are not represented in

this table!

122






Add instruction

How exactly are the timing signals used?

123

Timing diagram for

ADD





The instruction bits in IR are constant

until the end of the current Fetch-

Execute cycle, but the Pi signals

need to change from one microop. to

the next!

QUIZ

124

Hint: How many clock cycles are needed?

Based on this, draw the timing signals Ti

QUIZ

125

Now figure out the control signals for each cycle …

QUIZ

126

127

• The signal pattern described so far is the same

whether the CPU uses hardwired or

microprogrammed control.

• In hardwired control, the bit pattern of machine

instruction in the IR is decoded by combinational

logic.

• The decoder output works with the control signals

of the current system state to produce a new set

of control signals.

4.3.12 Decoding

128

Hardwired Decoding

(Flip-Flops)

(Combinational)

129

The Instruction Decoder

130

QUIZ: Based on the instruction opcodes,

create the decoder outputs for Subt and Input

131

The cycle counter

A cycle/ring

counter with 5

states

What are the states?

• The "1" travels from one flip-flop to the next, until

it reaches the final flip-flop. At the next clock

cycle, it shifts out and all flip-flops are zero!

• That makes the NOR gate inject a new "1" into the

first flip-flop

132

The cycle counter

A cycle/ring

counter with 5

states

What size counter is

needed for MARIE?

133

Hardwired control matrix for

Add (0011) instruction

Add

To do for next time:

Read and take notes: pp.255-62

Solve Exercise 55.

134

EOL 5

135

QUIZ

We noted that fetching the

instruction from memory and

incrementing PC are not

represented in this table - how

many extra steps are needed to

achieve that?

Write the RTL code!

136

QUIZ

We noted that fetching the

instruction from memory and

incrementing PC are not

represented in this table - how

many extra steps are needed to

achieve that?

Write the RTL code!

T0: MAR ← PC

T1: IR ← M[MAR]

T2: PC ← PC + 1

137

Extra-credit quiz

138

Second extra-credit quiz

139

QUIZ

How can the hardware know which input

is the correct one?

140

QUIZ

How can the hardware know which input

is the correct one? P.258 of our text:

141

Advantages:

• Speed

• Minimum hardware

Disadvantages:

• Low-level design is more error-prone

• Lack of flexibility

Hardwired Decoding -

CONCLUSION

142

• The instruction microcode produces the changes

in the control signals.

• Machine instructions are the input for a

microprogram that converts the 1s and 0s of an

instruction into control signals.

• The microprogram is stored in firmware ROM,

a.k.a. the control store.

• A microcode instruction is retrieved during each

clock cycle.

4.13.2 Microprogrammed Decoding

143

Microprogrammed Control Unit

144

• MicroOp1 and MicroOp2 contain binary codes for

each instruction.

MARIE microinstruction format

145


each microop. needed to execute MARIE’s ops.

• QUIZ: How many bits are needed to code microops.?

MARIE microinstruction format

146

Table 4.7 on p.252

We count 22 different microops. …

147

We count 22 different microops. …

So we need 5 bits!

148

MARIE’s microoperation codes

Actually, for microprogramming purposes, we need 2 more microops.

149

MARIE’s microoperation codeshandout

150

QUIZ: Microoperation codes

Where in Table 4.7 are the microops. circled?

151

QUIZ: Microoperation codes

A: They are not shown in Table 4.7, since they are part of

the fetch step!

152


each instruction.

• Jump is a single bit indicating that the value in the

Dest field is a valid address and should be placed in

the microsequencer.

• MARIE’s entire microprogram has < 128 lines, so 7

address bits are needed for Dest

Microprogrammed Decoding

153

Microprogram

• The first four lines implement the fetch step (the same

for all instructions).

18 bits for each microop.

154

• The next lines implement a jump table, using the extra if

microop.

155

• After the jump table, there are sections of

microinstructions for each MARIE instruction – do you

recognize these?

156

• A microprogrammed control unit works like a

computer system-in-miniature.

• Microinstructions are fetched, decoded, and

executed in the same manner as regular instructions.

• Disadvantage: the extra level of instruction

interpretation makes microprogrammed control slower

than hardwired control.

• Advantages:– it makes it easier to design complicated instructions

– it makes it easier to fix bugs

– it allows microcode reuse: and only the microprogram

needs to be changed if the instruction set changes

Microprogrammed Decoding

• Use the “Microprogramming” handout, so

we have easy access to all relevant figures

157

Explain in detail the execution of

the microprogram when we

execute instruction ADD One

from Example 4.1:

158

Microprogrammed

Decoding EXAMPLE

Correct the microcodes that are

wrong!

QUIZ

Explain in detail the execution

of the microprogram for the

instruction LOAD Addr

from Example 4.1:

159

4.14 Real World Architectures

• Read an take notes

• Check out the links on our webpage, under

Real-World ISAs

160

Homework for Ch.4:

9, 16, 21, 22, 45, 56, 57, 62

Due next Tuesday, Nov. 8

161

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