Chapter 4
MARIE: An Introduction
to a Simple Computer
Chapter 4 Objectives
Learn the components common to every modern
computer system.
Be able to explain how each component
contributes to program execution.
Understand a simple architecture invented to
illuminate these basic concepts, and how it relates
to some real architectures.
Know how the program assembly process works.
4.1 Introduction
Chapter 1 presented a general overview of
computer systems.
In Chapter 2, we discussed how data is stored
and manipulated by various computer system
components.
Chapter 3 described the fundamental
components of digital circuits.
Having this background, we can now understand
how computer components work, and how they
fit together to create useful computer systems.
4.2 CPU Basics
The computers CPU fetches, decodes, and executes
program instructions.
A program is a sequence of instruction.
An instruction is a command telling CPU what to do.
The two principal parts of the CPU are the datapath
and the control unit.
The datapath consists of an arithmetic-logic unit and storage
units (registers) that are interconnected by a data bus that is
also connected to main memory.
Various CPU components perform sequenced operations
according to signals provided by its control unit.
4
4.2 CPU Basics
4.2 CPU Basics
Registers hold data that can be readily accessed by the
CPU.
They can be implemented using D flip-flops.
General purpose reg.: can be accessed by programmers.
Special purpose reg.: program counter (PC), status,
accumulator.
6
4.2 CPU Basics
The arithmetic-logic unit (ALU) carries out
logical and arithmetic operations as directed by
the control unit.
Addition/subtraction, shift, comparison, bit-wise ops..
4.2 CPU Basics
The control unit determines
which actions to carry out
according to the values in the
program counter register and
the status register.
4.2 CPU Basics
The buses carry data
between CPU and
memory/IO system.
9
4.3 The Bus
The CPU shares data with other system components
by way of a data bus.
A bus is a set of wires that simultaneously convey a single bit
along each line.
Two types of buses are commonly found in computer
systems: point-to-point, and multipoint buses.
These are point-topoint buses:
10
4.3 The Bus
11
4.3 The Bus
Bus protocols are mechanisms to coordinate the
sharing of buses.
Buses consist of data lines, control lines, and address
lines.
While the data lines convey bits from one device to
another, control lines determine the direction of data
flow, and when each device can access the bus.
Address lines determine the location of the source or
destination of the data.
The next slide shows a model bus configuration.
12
4.3 The Bus
13
4.3 The Bus
A multipoint bus is shown below.
Because a multipoint bus is a shared resource,
access to it is controlled through protocols, which
are built into the hardware.
14
4.3 The Bus
In a master-slave configuration, where more than
one device can be the bus master, concurrent
bus master requests must be arbitrated.
Four categories of bus arbitration are:
Daisy chain: Permissions Distributed using self-detection:
are passed from the highestDevices decide which gets the bus
priority device to the
among themselves.
lowest.
Distributed using collision Centralized parallel: Each
detection: Any device can try to
device is directly connected
use the bus. If its data collides
to an arbitration circuit.
with the data of another device,
it tries again.
15
4.4 Clocks
Every computer contains at least one clock that
synchronizes the activities of its components.
A fixed number of clock cycles are required to carry
out each data movement or computational operation.
The clock frequency, measured in megahertz or
gigahertz, determines the speed with which all
operations are carried out.
Clock cycle time is the reciprocal of clock frequency.
An 800 MHz clock has a cycle time of 1.25 ns.
Does a computer with a faster clock run faster?
16
4.4 Clocks
Clock speed should not be confused with CPU
performance.
The CPU time required to run a program is given by
the general performance equation:
We see that we can improve CPU throughput when we
reduce the number of instructions in a program, reduce the
number of cycles per instruction, or reduce the number of
nanoseconds per clock cycle.
We will return to this important equation in later chapters.
17
4.4 Clocks
System performance
18
4.5 The Input/Output Subsystem
A computer communicates with the outside world
through its input/output (I/O) subsystem.
I/O devices connect to the CPU through various
interfaces.
19
4.5 The Input/Output Subsystem
I/O can be memory-mapped
where an I/O device behaves like main memory from the
CPUs point of view.
Or I/O can be instruction-based
where the CPU has a specialized I/O instruction set.
We study I/O in detail in chapter 7.
20
4.6 Memory Organization
Computer memory consists
of a linear array of
addressable storage cells
that are similar to registers.
Memory can be byteaddressable, or wordaddressable, where a word
typically consists of two or
more bytes.
Basic memory unit: byte or
word.
21
4.6 Memory Organization
Byte-Addressable
22
Word-Addressable
4.6 Memory Organization
Memory is constructed of
RAM chips, often referred to
in terms of length width
(aspect ratio).
RAM = random-access
memory
If the memory word size of
the machine is 16 bits, then
a 4M 16 RAM chip gives
us 222 of 16-bit memory
locations.
23
4.6 Memory Organization
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.
24
4.6 Memory Organization
Thus, 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.
25
4.6 Memory Organization
Given a 4Mx32 memory,
4M indicates the total number of locations in a memory,
32 means that each memory location is 32 bits wide, which is
four bytes.
This is just one way to specify memory.
On the other hand, byte- and word-addressable memory
are about the organization of memory.
In byte addressable memory, each memory location is one
byte,
while each location is four bytes in word-addressable memory.
26
4.6 Memory Organization
27
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 two slides illustrate these two ideas.
28
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.
29
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.
30
4.6 Memory Organization
31
4.6 Memory Organization
32
4.6 Memory Organization
EXAMPLE 4.1 Suppose we have a 128-word
memory that is 8-way low-order interleaved
which means it uses 8 memory banks; 8 = 23
So we use the low-order 3 bits to identify the bank.
Because we have 128 words, we need 7 bits for
each address (128 = 2 7 ).
33
4.6 Memory Organization
Low-Order Interleaving
High-Order Interleaving
34
4.6 Memory Organization - Summary
Word size is typically 2X bytes.
Byte-addressable vs. word-addressable.
Determines the number of address lines.
The number of address lines is the result of the
number of addressable items.
Memory organization
High-order vs low-order interleaving
The organization of address lines.
Memory addresses are unsigned.
35
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), or when
an invalid instruction is encountered.
Each interrupt is associated with a procedure that
directs the actions of the CPU when an interrupt
occurs.
Nonmaskable interrupts are high-priority interrupts that
cannot be ignored.
36
4.8 MARIE
4.8 MARIE
We can now bring together many of the ideas that
we have discussed to this point using a very simple
model computer.
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, a deep understanding of its
functions will enable you to comprehend system
architectures that are much more complex.
38
4.8.1 The Architecture
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 12
address lines.
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.
39
4.8.2 Registers and Buses
There is also a status register which is not shown
40
4.8.2 Registers and Buses
Accumulator, a 16-bit register that
holds a conditional operator
one operand of a two-operand instruction,
stores the results from ALU.
41
4.8.2 Registers and Buses
Memory address register, a 12-bit
register that holds the memory address
referenced by the current instruction.
42
4.8.2 Registers and Buses
Memory buffer register, a 16-bit register
that holds the data after its retrieval from,
or before its placement in memory.
43
4.8.2 Registers and Buses
Program counter, PC, a 12-bit register
that holds the address of the next
program instruction to be executed.
44
4.8.2 Registers and Buses
Instruction register, IR, which holds an
instruction immediately preceding its
execution
45
4.8.2 Registers and Buses
Input register, InREG, an 8-bit register
that holds data read from an input device.
46
4.8.2 Registers and Buses
Output register, OutREG, an 8-bit
register, that holds data that is ready for
the output device
47
4.8.2 Registers and Buses
The registers are interconnected, and connected
with 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 between the
accumulator and the memory buffer register, and the
ALU and the accumulator and memory buffer
register.
This permits data transfer between these devices
without use of the main data bus.
48
4.8.2 Registers and Buses
This is the MARIE data
path shown graphically.
49
4.8.3 Instruction Set Architecture
A computers instruction set architecture (ISA)
specifies the format of its instructions and the
primitive operations that the machine can perform.
The ISA is an interface between a computers
hardware and its software.
Some ISAs include hundreds of different instructions
for processing data and controlling program
execution.
The MARIE ISA consists of only thirteen instructions.
50
4.8.3 Instruction Set Architecture
This is the format
of a MARIE instruction:
The fundamental MARIE instructions are:
51
4.8.3 Instruction Set Architecture
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 memory address
from which the data is loaded is 3.
52
4.8.3 Instruction Set Architecture
This is a bit pattern for a SKIPCOND instruction as it
would appear in the IR:
Bits 11 and 10 are define the condition in AC for
skipping the next instruction.
00 skip if the value in the AC is negative.
01 skip if the value in the AC is 0.
10 skip if the value in the AC is greater than zero.
53
4.8.3 Register Transfer Notation
An instruction can be written in its binary format, or
using its name.
An instruction in binary format is called machine
instruction/code.
An instruction used by its name is called assembly
language instruction.
An assembly program is a sequence of assembly
language instructions.
An assembly program needs to be translated to a
machine program of machine instructions by an
assembler.
54
4.8.4 Register Transfer Notation
Each of our instructions actually consists of a
sequence of smaller instructions called microoperations.
The exact sequence of micro-operations 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.
55
4.8.4 Register Transfer Notation
The RTL for the Load X instruction is:
MAR X
MBR M[MAR]
AC MBR
Similarly, the RTL for the Store X instruction is:
MAR X
MBR AC
M[MAR] MBR
56
4.8.4 Register Transfer Notation
The RTL for the Add X instruction is:
MAR X
MBR M[MAR]
AC AC + MBR
Similarly, the RTL for the Subt X instruction is:
MAR X
MBR M[MAR]
AC AC - MBR
57
4.8.4 Register Transfer Notation
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 our instruction set:
If IR[11 - 10] = 00
If AC < 0 then PC
else If IR[11 - 10]
If AC = 0 then PC
else If IR[11 - 10]
If AC > 0 then PC
58
then
PC + 1
= 01 then
PC + 1
= 11 then
PC + 1
4.9 Instruction Processing
59
4.9.1 The Fetch-Decode-Execution Cycle
The fetch-decode-execute cycle is the series of
steps that a computer carries out when it runs a
program.
We first have to fetch an instruction from memory,
and place it into the IR.
Once in the IR, it is decoded to determine what
needs to be done next.
If a memory value (operand) is involved in the
operation, it is retrieved and placed into the MBR.
With everything in place, the instruction is executed.
The next slide shows a flowchart of this process.
60
4.9.1 The Fetch-Decode-Execution Cycle
61
4.9.2 Interrupts and the Instruction Cycle
All computers provide a way of interrupting the
fetch-decode-execute cycle.
Interrupts occur when:
A user break (e.,g., Control+C) is issued
I/O is requested by the user or a program
A critical error occurs
Interrupts can be caused by hardware or
software.
Software interrupts are also called traps.
62
4.9.2 Interrupts and the Instruction Cycle
Interrupt processing involves adding another step to
the fetch-decode-execute cycle as shown below.
The next slide shows a flowchart of Process the interrupt.
63
4.9.2 Interrupts and the Instruction Cycle
64
4.9.2 Interrupts and the Instruction Cycle
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.
65
4.9.2 Interrupts and the Instruction Cycle
Interrupts are very useful in processing I/O.
However, interrupt-driven I/O is complicated, and is
beyond the scope of our present discussion.
We will look into this idea in greater detail in Chapter 7.
MARIE, being the simplest of simple systems, uses a
modified form of programmed I/O.
All output is placed in an output register, OutREG,
and the CPU polls the input register, InREG, until
input is sensed, at which time the value is copied into
the accumulator.
66
4.10 A Simple Program
Consider the simple MARIE program given below.
We show a set of mnemonic instructions stored at
addresses 0x100 0x106 (hex):
67
4.10 A Simple Program
Lets look at what happens inside the computer when
our program runs.
This is the LOAD 104 instruction:
68
4.10 A Simple Program
Our second instruction is ADD 105:
69
4.10 A Simple Program
Lets simulate the execution of the program.
100
106
70
4.11 A Discussion on Assemblers
Mnemonic instructions, such as LOAD 104, are
easy for humans to write and understand.
They are impossible for computers to understand.
Assemblers translate instructions that are
comprehensible to humans into the machine
language that is comprehensible to computers
We note the 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.
71
4.11 A Discussion on Assemblers
Address labels
makes assembly
programming simpler
and more portable.
Assembly code using address labels
72
4.11 A Discussion on Assemblers
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.
73
4.11 A Discussion on Assemblers
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 as shown.
74
4.11 A Discussion on Assemblers
After the second pass, the
assembly is complete.
Question: what operation does this program implement?
75
4.12 Extending Our Instruction Set
76
4.12 Extending Our Instruction Set
Example 4.3
100
101
102
103
104
105
106
107
108
109
10A
10B
10C
10D
77
|If,
LOAD X
|
Subt Y
|
SKIPCOND 400
|
JUMP Else
|Then,
LOAD X
|
ADD X
|
STORE X
|
JUMP Endif
|Else,
LOAD Y
|
Subt X
|
STORE Y
|Endif, HALT
|X,
DEC 12
|Y,
DEC 20
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 program, you are
already familiar with indirect addressing.
78
4.12 Extending Our Instruction Set
We have included three indirect addressing mode
instructions in the MARIE instruction set.
The first two are LOADI X and STOREI X where
specifies the address of the operand to be loaded
or stored.
In RTL :
MAR X
MBR M[MAR]
MAR MBR
MBR M[MAR]
AC MBR
LOADI X
79
MAR
MBR
MAR
MBR
M[MAR]
X
M[MAR]
MBR
AC
MBR
STOREI X
4.12 Extending Our Instruction Set
The ADDI instruction is a combination of LOADI X
and ADD X:
In RTL:
MAR X
MBR M[MAR]
MAR MBR
MBR M[MAR]
AC AC + MBR
ADDI X
80
4.12 Extending Our Instruction Set
Another helpful programming tool is the use of
subroutines.
The jump-and-store instruction, JNS X, gives us
limited subroutine functionality. The details of the
JNS X instruction are given by the following RTL:
MBR PC
MAR X
M[MAR] MBR
MBR X
AC 1
AC AC + MBR
AC PC
81
Does JNS permit
recursive calls?
4.12 Extending Our Instruction Set
Our first new instruction is the CLEAR instruction.
All it does is set the contents of the accumulator to
all zeroes.
This is the RTL for CLEAR:
AC 0
We put our new instructions to work in the program
on the following slide.
82
4.12 Extending Our Instruction Set
Example 4.2
100
101
102
103
104
105
106
107
108
109
10A
10B
10C
10D
83
| LOAD Addr
| STORE Next
| LOAD Num
| SUBT One
| STORE Ctr
|Loop LOAD Sum
| ADDI Next
| STORE Sum
| LOAD Next
| ADD One
| STORE Next
| LOAD Ctr
| SUBT One
| STORE Ctr
10E
10F
110
111
112
113
114
115
116
117
118
119
11A
11B
| SKIPCOND 000
| JUMP Loop
| HALT
|Addr HEX 117
|Next HEX 0
|Num
DEC 5
|Sum
DEC 0
|Ctr
HEX 0
|One
DEC 1
| DEC 10
| DEC 15
| DEC 2
| DEC 25
| DEC 30
4.12 Extending Our Instruction Set
PC =
100
101
102
103
104
105
106
107
108
109
10A
10B
10C
10D
, AC =
| LOAD Addr
| STORE Next
| LOAD Num
| SUBT One
| STORE Ctr
|Loop LOAD Sum
| ADDI Next
| STORE Sum
| LOAD Next
| ADD One
| STORE Next
| LOAD Ctr
| SUBT One
| STORE Ctr
Example 4.2
84
, MAR =
, MBR =
10E
10F
110
111
112
113
114
115
116
117
118
119
11A
11B
| SKIPCOND 000
| JUMP Loop
| HALT
|Addr HEX 117
|Next HEX 0
|Num
DEC 5
|Sum
DEC 0
|Ctr
HEX 0
|One
DEC 1
| DEC 10
| DEC 15
| DEC 2
| DEC 25
| DEC 30
4.12 Extending Our Instruction Set
85
4.12 Extending Our Instruction Set
Example 4.5
100
101
102
103
104
105
106
107
108
109
10A
10B
10C
10D
10E
10F
86
|
Load
X
|
Store temp
|
JnS Subr
|
Store X
|
Load
Y
|
Store temp
|
JnS subr
|
Store Y
|
Halt
|X,
Dec 20
|Y,
Dec 48
|temp,
Dec 0
|subr,
Hex 0
|
Load
temp
|
Add temp
|
JumpI subr
PC =
AC =
Computer Programs
int x = 10;
while (y >= 10) {
if (x % 5 == 0)
x = x / 5;
else
x = x + 1;
y = y x;
}
Compiler
87
Assembler
Chapter 4 Conclusion
The major components of a computer system are its
control unit, registers, memory, ALU, and data path.
A built-in clock keeps everything synchronized.
Computers run programs through iterative fetch-decodeexecute cycles.
Computers can only run programs that are in machine
language.
An assembler converts mnemonic code to machine
language.
ISA for programmer to control the CPUs.
88
4.13 A Discussion on Decoding
The textbook provides a complete list of the register
transfer language for each of MARIEs instructions.
The micro-operations given by each RTL define the
operation of MARIEs control unit.
Each micro-operation consists of a distinctive signal
pattern that is interpreted by the control unit and
results in the execution of an instruction.
Recall, the RTL for the Add X instruction is:
MAR X
MBR M[MAR]
AC AC + MBR
89
4.13 A Discussion on Decoding
A computers control unit keeps things
synchronized, making sure that bits flow to the
correct components as the components are needed.
There are two general ways in which a control unit
can be implemented: hardwired control and
microprogrammed control.
With microprogrammed control, a small program is placed
into read-only memory in the microcontroller.
Hardwired controllers implement this program using digital
logic components.
90
4.13 A Discussion on Decoding
Each of MARIEs registers
and main memory have a
unique address along the
datapath.
The addresses take the
form of signals issued by
the control unit.
How many signal lines does
MARIEs control unit need?
91
4.13 A Discussion on Decoding
In microprogrammed control, instruction microcode
produces control signal changes.
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, which is
also called the control store.
A microcode instruction is retrieved during each clock
cycle.
92
4.13 A Discussion on Decoding
This is how a generic
microprogrammed
control unit might look.
93
4.13 A Discussion on Decoding
If MARIE were microprogrammed, the
microinstruction format might look like this:
MicroOp1 and MicroOp2 contain binary codes for
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.
94
4.13 A Discussion on Decoding
The table below contains MARIEs microoperation
codes along with the corresponding RTL:
95
4.13 A Discussion on Decoding
The first nine lines of MARIEs microprogram are given
below (using RTL for clarity):
96
4.13 A Discussion on Decoding
97
4.13 A Discussion on Decoding
Its important to remember that a microprogrammed
control unit works like a system-in-miniature.
Microinstructions are fetched, decoded, and executed
in the same manner as regular instructions.
This extra level of instruction interpretation is what
makes microprogrammed control slower than
hardwired control.
The advantages of microprogrammed control are that
it can support very complicated instructions and only
the microprogram needs to be changed if the
instruction set changes (or an error is found).
98
