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COMPUTER
ORGANIZATION
By: Chandrashekar M A,CSE, SSE
Chapter 2: Machine Instructions
and Programs
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Memory Location and Addresses
 In memory of computer, there are
 Number and character operands
 Instructions
 Memory consists of
 Millions of storage cells,
 Each cell can hold a bit ( 0 or 1 ) of information
 So each bit can hold a very small amount of information
 Memory is organized so that a group of n bits can be
stored or retrieved in a single, basic operation
 Each group of n bits is referred to as word
 n is called as wordlength
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n bits
first word
second word
•
•
•
i th word
•
•
•
last word
Memory words.
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Contd…
• Range of word length of modern computers
• 16 to 64 bits
• Group of 8 bits referred to a byte
• If the word length is 32 bits
• A single word can store a 32-bit 2’s complement number
• Or
• Four ASCII characters
• As shown in the figure
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32 bits
b1
•
•
•
b31 b30
b0
Sign bit: b31= 0 for positive numbers
b31= 1 for negative numbers
(a) A signed integer
8 bits
8 bits
8 bits
8 bits
ASCII
character
ASCII
character
ASCII
character
ASCII
character
(b) Four characters
Examples of encoded information in a 32-bit word.
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Addresses
• Access to memory to store or retrieve a single item of info
( either a byte or a word ) requires distinct names or
addresses for each item location
• normally used addresses => 0 to 2k -1 as addresses of
successive memory locations
• The 2k addresses constitute address space of the computer
• 24-bit address generates an address space of 224 locations = 16M
locations
• 32-bit ?
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Byte Addressability
• A byte is always 8-bit
• Word length ranges from 16 to 64 bits.
• Impractical to assign distinct addresses to individual bit
•
•
•
•
locations
Most practical?
Byte-addressable memory
Byte locations have addresses 0,1,2,..
If the word length of the machine is 32-bits, then
successive words are located at addresses ?
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Big-Endian and Little-Endian Assignments
• 2 ways byte addresses can be assigned across words
• Big-endian when
• Lower byte addresses for more significant bytes (leftmost bytes ) of
the word.
• Little-endian when
• Lower bytes addresses used for less significant bytes (right most
bytes ) of the word.
• In both cases 0,4,8 .. are taken as addresses of
successive words in the memory
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Word
address
Byte address
Byte address
0
0
1
2
3
0
3
2
1
0
4
4
5
6
7
4
7
6
5
4
•
•
•
k
2 -4
k
2 -4
k
2 -3
•
•
•
k
2- 2
k
2 - 1
k
2 - 4
(a) Big-endian assignment
Byte and word addressing.
k
2- 1
k
2 - 2
k
2 -3
k
2 -4
(b) Little-endian assignment
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Word Alignment
• Words are said to be aligned in memory if they begin at a
byte address that is a multiple of the number of bytes in a
word
• Eg : if wordlength is 16bits, aligned words begin at byte addresses
0,2,4,….
• If word length is 32 bits, aligned words begin at 0,4,8 etc…
• If the words don’t begin at byte address that is a multiple
of no of bytes in the word, then words are said to have
unaligned addresses
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Accessing numbers, characters and
strings
 Number
 By its word address as it usually occupies one word
 Character
 By byte address
 Strings
 They are of variable length
 Beginning of the string by giving the beginning byte address which
contains first character
 Successive bytes contains successive characters
 Termination?
 Either by a special control character
 Or a separate memory word location/ register containing a number
indicating the string length
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Problems
Given that a memory has 32-bit address & is byteaddressable, what is the size of the memory(in bytes)?
2. Given that a memory has 24-bit address & is wordaddressable with a word length of 32 bits, what is the
size of the memory(in bytes)?
3. Given that a memory has 16-bit address and is byte
addressable. Word length is 32 bits. How many words
can we store in such a memory?
1.
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Answers
4GB
2. 64MB
3. 16K words
1.
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Memory Operations
• Both program instructions and data operands are stored
in memory
• To execute an instruction
• Processor control circuits must cause the word(s) containing the
instruction to be transferred from the memory to the processor
• Operands and results must also be moved between memory and
the processor
• Two basic operations involving the memory
• Load ( or Read or Fetch )
• Store ( or Write )
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Memory Operations –
LOAD operation
• Load transfers a copy of the contents of a specific
memory location to the processor
• The memory contents remain unchanged
• To start a load operation
• Processes sends the address of the desired location to memory
• Request that its contents be read
• The memory reads the data and sends to the processor
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Memory operations
Store operation
-
• Store operation transfers an item of information from the
processor to a specific memory location
• Destroys the former contents of that memory location
• Processor needs to send the address of the desired
memory location and also the data to be written into that
location
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Instructions and Instruction Sequencing
• Tasks that are carried out by a computer consists of a
sequence of small steps
• Eg., add two numbers, test for a particular condition, read a
character from keyboard, display a character on screen
• A computer must have instructions capable of performing
four types of operations
• Data transfers between the memory and the processor registers
• Arithmetic and logic operations on data
• Program sequencing and control
• I/O transfers
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Register transfer notation
 Used to describe transfer of information from one location in
the computer to another
 Possible locations are memory locations, processor registers, and
registers in the I/O subsystem
 To identify a location
 Symbolic name standing for its hardware binary address
 Eg., name of memory locations – LOC,A,VAR2 etc
 Name of registers – R0,R5
 Name of I/O registers – DATAIN
 Contents of a location denoted by placing square brackets
around the name
 R1  [LOC] means contents of memory location LOC are transferred
into processor register R1
 R3  [R1] + [R2] ?
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Assembly language notation
• Used to represent machine instructions and programs
• Example
• To perform the data transfer R1 [LOC] the statement is Move
LOC,R1
• Old contents of register R1 are overwritten but contents of LOC
unchanged
• R3  [R1] + [R2] is denoted by statement ?
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Basic Instruction Types
• A high level language program command C = A + B adds
the contents of variables A and B and stores the result in
variable C
• After compilation, the three variable A,B and C are assigned
distinct locations in memory
• The contents of these locations represent the value of the three
variables
• The action is C  [A] + [B]
• Contents of A and B locations are fetched from memory and
transferred into processor where the computation is performed
• Result is then sent back to the location C
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Accomplish C=A+B using Single Machine
Instruction
 The instruction contains memory addresses of 3
operands – A,B and C
 Add A,B,C
 A and B are source operands
 C is destination operand
 Add is the operation to be performed
 Operation Source1,Source2, Destination
 If k bits are needed to specify memory address of each
operand
 In addition to the bits needed to denote Add operation we need 3k
bits more
 For a modern processor with 32-bit address space,a 3-address
instruction is too large to fit in one word for a reasonable
wrodlength
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Accomplish C = A + B using two-address
instructions
 Operation Source, Destination
 Eg., Add A,B which performs the operation B  [A] + [B]
 When sum is calculated result is sent to memory replacing
original contents of location B
 We cannot use a single two-address instruction as we don’t
want to destroy the contents of A or B
 Solution?
 Use a instruction which copies contents of one memory location to
another
 Move B,C performs the operation C  [B] leaving contents of B
unchanged
 Actually it only “copies” not “moves”
 Final solution is
 Move B,C
 Add A,C
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Accomplish C = A + B using one-address
instructions
 Even two-address instructions will not fit into one single
word of memory
 Use one-address instruction
 Second operand , whenever required, is present in a unique
location
 Eg., usage of accumulator, a processor register
 Add A
 means Add the contents of memory location A to the contents of
accumulator register and place the sum back into the accumulator
 Load A
 Copies contents of memory location A into accumulator
 Store A
 Copies contents of accumulator to memory location A
 Solution?
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Contd…
• Load A
• Add B
• Store C
• Operand specified may be source or destination depending on the
instruction
• For load it is source,
• For store it is destination
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Accomplish C = A + B using one-address
instructions, and General purpose
registers
• Most modern computers have 32 General Purpose
Registers or more
• Only 5 bits sufficient to address 32 GPRs. How?
• Registers are used to store data temporarily
• Since access time to registers is much less than memory
frequent access to memory is reduced hence enhancing
speed
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Accomplish C = A + B using one-address
instructions, and General purpose
registers
• If Ri is a GPR
• Load A,Ri
• Store Ri,A
• Add A,Ri are generalizations for the single-accumulator case
• Data transfer instruction - Move
• When we want to move from one place to another.
• A single instruction Move can be used in place of Load
and Store
• Move Source, Destination
• Move A, Ri is same as Load A,Ri
• Move Ri,A is same as Store Ri,A
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Contd
• If artihmetic operations are allowed only on operands in
processor registers then to achieve C = A + B
• Move A,Ri
• Move B,Rj
• Add Ri,Rj
• Move Rj,C
• If one operand can be in memory but other must be in
register then
• Move A,Ri
• Add B,Rj
• Move Rj,C
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Zero-Address instructions
• Locations of all operands defined implicitly
• Machines which store operands in a structure called
pushdown stack
• Instructions does not specify any memory address
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Instruction execution and straight line
sequencing
• Assumptions
Begin execution here
Address
i
i +4
i +8
Contents
Move
Add
Move
A,R0
B,R0
R0,C
3-instruction
program
segment
• GPRs
• One addr insts
• Wordlength 32bits
A
Data for
the program
B
• Byte addressable
• Full address in
C
• Single word instruction
Figure 2.8.
• The three insts at
• i, i + 4, i + 8
A program for C  [A] + [B].
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Instruction execution and straight line
sequencing
• PC holds address of instruction to be executed next
• To begin execution
• place the address of first inst to be executed in PC
• Processor control circuits use info in PC to fetch and execute
instructions, one at a time, in order of increasing addresses
• This is called as Straight-line sequencing
• During the execution of each instruction, PC is incremented by 4
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Instruction execution and straight line
sequencing
• Execution is two phase
• Instruction Fetch
• Instruction is fetched from memory location whose address in PC
• This instruction is placed in IR
• Instruction Execute
• IR is examined to determine which operation is to be performed
• Specified operation is then performed by processor
• Involves fetching operands from memory or processor registers,
performing arithmetic or logic operations, storing results in destination
• At some point during this two-phase, the contents of PC is
advanced to point to next instruction
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Branching
• Branch instruction load a new value to PC than the
address of next immediate instruction following the branch
instruction
• This new address is called branch target
• A conditional branch instruction causes branch only if a
specified condition is satisfied
Eg., Branch>0 LOOP
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i
i + 4
i + 8
i + 4n - 4
i + 4n
SUM
NUM1
NUM2
NUM n
Figure 2.9.
Move
Add
Add
Add
Move
NUM1,R0
NUM2,R0
NUM3,R0
•
•
•
NUM n ,R0
R0,SUM
•
•
•
•
•
•
A straight-line program for adding n numbers.
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Move
Clear
LOOP
Program
loop
N,R1
R0
Determine address of
"Next" number and add
"Next" number to R0
Decrement
Branch>0
Move
SUM
N
NUM1
NUM2
NUM n
Figure 2.10.
R1
LOOP
R0,SUM
•
•
•
n
•
•
•
Using a loop to add n numbers.
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Assignment to be submitted on Thursday
 Express the following signed numbers in 2’s
complement notation and perform addition and
subtraction. State whether overflow occurs or not
 4-bit notation
 2&3, 5 & -6, -7 & 6, -8 & -3, 7 & 4
 5-bit notation
 12 & 3, 7 & -7, -6 & 14, -10 & -4, 12 & 8
 Represent the following numbers in 32-bit Big-endian
and Little-endian memory organization
 81234561
 -81234561
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Indirection and pointers
• Indirect mode – the Effective address of the operand is
the contents of a register or memory location whose
address appears in the instruction
• The register of memory location that contains the address
of an operand is called a pointer
• Analogy of treasure hunt
• Instead of finding the treasure, we find the address where we find
the treasure
• By changing the contents of register R1 or location A in the
following figure, the same add instruction fetches different
operands to add to register R0.
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Add
(R1),R0
Add
(A),R0
Main
memory
B
R1
Operand
B
Register
(a) Through a general-purpose register
A
B
B
Operand
(b) Through a memory location
Figure 2.11. Indirect addressing.
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Indirect addressing logic for the program of adding
n numbers using loop
Address
LOOP
Contents
Move
Move
Clear
Add
Add
Decrement
Branch>0
Move
N,R1
#NUM1,R2
R0
(R2),R0
#4,R2
R1
LOOP
R0,SUM
Initialization
Figure 2.12. Use of indirect addressing in the program of Figure
2.10.
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INDEXED ADDRESSING
Add
20(R1),R2
1000
1000
R1
20
R1
20 = offset
1020
Operand
(a) Offset is given as a constant
Add
1000(R1),R2
1000
20 = offset
1020
Operand
(b) Offset is in the index register
Figure 2.13. Indexed addressing.
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n
Student ID
Test 1
Test 2
N
LIST
LIST + 4
LIST + 8
LIST + 12
LIST + 16
Test 3
Student ID
Test 1
Test 2
Test 3
•
•
•
Figure 2.14. A list of students' marks.
Student 1
Student 2
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LOOP
Move
#LIST,R0
Clear
R1
Clear
R2
Clear
R3
Move
N,R4
Add
4(R0),R1
Add
8(R0),R2
A dd
12(R0),R3
Add
#16,R0
Decrement
R4
Branch>0
LOOP
Move
R1,SUM1
Move
Move
R2,SUM2
Note: contents of R0,
which is used as
indexed register, are not
changed when it is used
in indexed addressing
mode to access test
scores.
Contents of R0 change
only in last Add
instruction, to move
from one student
record to the next
R3,SUM3
Figure 2.15. Indexed addressing used in accessing test scores in the list in Figure
2.14.
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Indexed addressing contd…
• Several variations of the basic form of indexed addressing
provide efficient access to memory operands
• A second register may be used to contain the offset X, called as
•
•
•
•
•
•
based indexed addressing mode
Denoted as ( Ri, Rj )
Effective address is the sum of contents of Ri and Rj
Second register is called base register.
Provides more flexibility to the user
Eg., suppose, in the previous example, instead of only 3 items,
each student record contain a large no of items, say k
We can replace the three Add instructions by a single instruction
inside a second loop
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Indexed addressing contd…
• Problem
• The list of student marks shown in prev figure 2.14, is changed to
contain j test scores for each student. Assume that there are n
students. Write an assembly language program for computing the
sums of the scores on each test and store these sums in the
memory word locations at addresses SUM, SUM + 4, SUM + 8,….
The number of tests, j, is larger than the number of registers in the
processor. Use two nested loops, the inner loop should accumulate
the sum for a particular test, and the outer loop should run over the
number of tests, j. assume that j is stored in memory location J.
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Solution
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Indexed addressing mode contd..
• based indexed addressing mode with offset
• Uses two registers plus a constant
• X( Ri, Rj ) - Effective address is the sum of the constant X and the
contents of registers Ri and Rj
• This mode implements 3-Dimensional array
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Relative addressing
• Till now for index mode, registers we used are general purpose
•
•
•
•
registers
A useful variation of this is to use Program Counter PC instead of a
general purpose register
X(PC) – to address a memory location that is X bytes away from the
location presently pointed to by the program counter
Relative mode- the effective address is determined by the index mode
using the program counter in place of the general-purpose register Ri.
MOST common use is to specify the target address in branch
instructions
• Eg., Branch>0 LOOP
• If branch condition is true, the program execution goes to branch
target location identified by name LOOP
• Can compute this location by specifying it as an offset from the
current value of the program counter.
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Example for relative addressing
mode
Address
LOOP


Contents
Move
Move
Clear
Add
Add
Decrement
Branch>0
Move
N,R1
#NUM1,R2
R0
(R2),R0
#4,R2
R1
LOOP
R0,SUM
Initialization
 Assume 4 instructions starting from LOOP are located at
memory locations 1000,1004,1008 & 1012.
 Updated contents at the time branch target address is
generated is 1016. to branch to location LOOP(1000), the
offset value needed is X = -16
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Additional modes
• Autoincrement and autodecrement mode – useful for
accessing data items in successive locations in the
memory
• Autoincrement mode –
• The effective address of the operand is the contents of a register
specified in the instruction. After accessing the operand, the
contents of this register are automatically incremented to point to
next element in the list
• (Ri)+
• The increment is 1 for byte-sized operands, 2 for 16-bit operands and 4
for 32-bit operands. Usually size of operand is specified in the
instruction
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Autoincrement addressing logic for the
program of adding n numbers using loop
LOOP
Move
Move
Clear
Add
Decrement
Branch>0
Move
N,R1
#NUM1,R2
R0
(R2)+,R0
R1
LOOP
R0,SUM
Initialization
Figure 2.16. The Autoincrement addressing mode used in the program of Figure 2.12.
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Autodecrement mode
• The contents of a register specified in the instruction are
first automatically decremented and are then used as the
effective address of the operand
• Denoted by –(Ri)
• Operands are accessed in descending address order.
• The way the autoincrement and autodecrement modes
are specified in very useful when implementing an
important data structure called a stack.
• Always we can use two instructions to perform the functionality of
autoincrement and autodecrement mode
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ASSEMBLY LANGUAGE
• Machine instructions are represented by patterns of 0s
and 1s
• Such patterns are awkward to deal when discussing or preparing
programs
• Hence, usage of symbolic names to represent the patterns
• Such symbolic acronyms( short forms) is called as mnemonics
• A complete set of symbolic names and rules for their use
constitute a programming language, referred to as
assembly language
• The set of rules for using the symbolic names is called as
syntax of the language
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ASSEMBLY LANGUAGE
• Programs written in an assembly language can be
automatically translated into a sequence of machine
instructions by a program called an assembler.
• The user program in its original form is called as source program
• The assembled machine language program is called object
program
• The assembly language for a given computer may or may
not be case sensitive
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ASSEMBLY LANGUAGE
 A move instruction is written as
 MOVE R0,SUM
 The mnemonic MOVE represents the binary pattern, or OP
code, for the operation performed by the instruction
 The assembler translates this mnemonic into the binary OP code that
the computer understands
 OP-code followed by at least one blank space character
 Then the information that specifies the operands is given
 In above example , source operand is in register R0
 Followed by specification of the destination operand – separated from
source operand by a comma, with no intervening blanks
 In above example destination operand is in memory location that has
its binary address represented by name SUM
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ASSEMBLY LANGUAGE
• Since there are several possible addressing modes for
specifying operand locations, the assembly language
must indicate which mode is being used.
• # sign indicates immediate operand ( eg., ADD #5,R3)
• In some assembly languages the immediate addr mode is intended
in the OP-code mnemonic( eg., ADDI 5,R3)
• Indirect addressing mode is usually specified by putting
parentheses around the name denoting pointed operand.
• If number 5 is to be placed in a memory location whose address is
held in register R2, it is specified as MOVE #5,(R2)
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ASSEMBLER DIRECTIVES
 In addition to providing mechanisms for representing instructions in a
program, the assembly language allows programmer to specify other
information needed to translate the source program to object program
 Eg., if name SUM is used to represent value 200, it can be conveyed to
assembler as
 SUM EQU 200
 This statement will not appear in the object program.. Rather informs the




assembler that the name SUM is to be replaced by the value 200
Such statements are called as assembler directives
Eg., ORIGIN – where in the memory should the data/instruction block
that follows be placed
DATAWORD – load the location with specified value
RETURN – identifies the point at which execution of the program must
be terminated. Assembler returns control to OS by inserting an
appropriate machine instruction
LOOP
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100
104
Move
Move
N,R1
#NUM1,R2
108
112
116
Clear
Add
Add
R0
(R2),R0
#4,R2
120
124
128
132
Decrement
Branch>0
Move
R1
LOOP
R0,SUM
SUM
N
200
204
NUM1
NUM2
208
212
NUM n
Memory
address
label
Assembler
directives
SUM
N
NUM1
Statements
that generate
machine
instructions
100
Assembler
directives
START
LOOP
Operation
Addressing
or data
information
EQU
200
ORIGIN
204
DATAWORD 100
RESERVE
400
ORIGIN
100
MOVE
N,R1
MOVE
#NUM1,R2
CLR
R0
ADD
(R2),R0
ADD
#4,R2
DEC
R1
BGTZ
LOOP
MOVE
R0,SUM
RETURN
END
START
604
Fig2.17.Memory arrangement for program in Fig 2.12.
Figure 2.18. Assembly language representation
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Any statement
that results in
instructions or
data being placed
in memory
location may be
given a memory
address label.
Label is assigned
the value equal
to the address of
that location
Memory
address
label
Assembler directives
SUM
N
NUM1
Statements that
generate
machine
instructions
Assemblerdirectives
START
LOOP
Operation
EQU
ORIGIN
DATAWORD
RESERVE
ORIGIN
MOVE
MOVE
CLR
ADD
ADD
DEC
BGTZ
MOVE
RETURN
END
Addressing
or data
information
200
204
100
400
100
N,R1
#NUM1,R2
R0
(R2),R0
#4,R2
R1
LOOP
R0,SUM
START
Figure 2.18. Assembly language representation for the program in Figure 2.17.
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CONTD….
• Most assembly languages require statements in a source
program to be written in the form
• Label
•
•
•
•
Operation
Operand(s)
Comment
Label – is optional and is associated with memory address where
the machine language instruction or the data items are stored
Operation field contains the OP-code mnemonic of the desired
instruction or assembler directive
Operand field contains addressing information for accessing one or
more operands, depending on the type of instruction
Comment field is ignored by the assembler program
• Used for documentation purposes
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ASSEMBLY and EXECUTION OF
PROGRAMS
 ASSEMBLER replaces
 All symbols denoting operations and addressing modes with binary
codes
 All names and labels with their actual values.
 In a branch instruction, branch target is not replaced by actual
address.
 Usually it uses relative addressing mode. Assembler computes branch
offset, and puts into the machine instruction
 The assembler keeps track of all names and the numerical values
that correspond to them in a symbol table
 When a name appears a second time, it is replaced with its value from the
table.
 A problem arises if some name appears before it is given a value. Eg.,
forward branching.
 Solution is to scan through the program twice – once to collect the values
of all names and second time to substitute values for all names from
symbol table. Such an assembler is called as TWO-PASS ASSEMBLER
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ASSEMBLY and EXECUTION OF
PROGRAMS
 Assembler stores object program on a magnetic disk
 The object program must be loaded into memory before it
is executed
 A utility program called loader does this. For this loader must
already be in memory
 Loader must know the length of the program and the address in
the memory where it will be stored
 Assembler usually places this info in a header preceding the object
code
 When the object program begins execution, it proceeds to
completion unless there are errors.
 To help debugging, system software usually includes a debugger
program
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NUMBER NOTATIONS
• Can use either decimal, or binary or hexadecimal notation
• Eg., ADD #93, R1
• This is in decimal
• In binary – ADD #%01011101,R1
• Here % tells that we are using a binary number
• In hexadecimal – ADD #$5D, R1
• Here $ denotes the usage of hexadecimal
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Questions
• Mention the steps assembler performs in order to convert
a source program into object program
• What is symbol table? Why do we need it? How does the
assembler populate symbol table?
• What is a loader? What information must assembler pass
on to loader to perform its function? How does the
assembler do it?
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BASIC INPUT/OUTPUT
OPERATIONS
 Till now we have assumed that the data operated in stored in memory
already.
 Now, let’s see how data transfer takes place between computer and
outside world

The I/O operations are very important and the way they are done can significantly
affect the performance of the computer
 The rate of data transfer from the keyboard to a computer is limited by
the typing speed of the user

Few characters per second
 The rate of output transfers from the computer is determined by the rate
at which characters can be transmitted over the link between the
computer and display device.

Typically several thousand characters per second
 Both are much slower than the speed of processor
 Can execute many millions of instructions per second
 Hence need for the mechanisms to synchronize the transfer of data
between them.
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Solution to speed disparity
• On output, the processor sends the first character and
then waits for a signal from the display that character has
been received
• Then it sends the second character and so on
• For input, the processor waits for a signal indicating that a
character key has been struck and that its code is
available in some buffer register associated with keyboard
• Then only will processor proceed to read the code
SIN
Key board
SOUT
Display
Figure 2.19 Bus connection for processor
, keyboard, and display
.
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DATAIN/DATAOUT – buffer
registers
SIN/SOUT – status control
flags
The striking of a key on keyboard does not automatically cause the
corresponding character to be displayed on the screen!
One block of instructions in I/O program causes the character to be
transferred to processor and another block of instructions will
cause the character to be displayed on the screen
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How data is read?
• The technique used is called program – controlled I/O
• Striking a key stores the corresponding character code in
•
•
•
•
a 8-bit buffer register called DATAIN.
To inform the processor that a valid character is in
DATAIN, a status control flag SIN is set to 1.
A program monitors SIN, and when SIN is set to 1, the
processor reads the contents of DATAIN.
When the character is transferred to the processor, SIN is
automatically cleared to 0.
When a second character is read, SIN is again set to 1
and the process repeats
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How data is displayed?
• A buffer register DATAOUT and status control flag SOUT
is used.
• When SOUT is 1, display is ready to receive a character
• Under the control of a program, ( hence program –
controlled I/O), the processor monitors SOUT and when
SOUT is set to 1, the processor transfers a character
code to DATAOUT
• The transfer of character to DATAOUT clears SOUT to 0.
• When the display is ready to receive a second character, again
SOUT is set to 1.
• SIN , SOUT , DATAIN, DATAOUT are part of circuitry called as
device interface, connected to processor through bus
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Programs for I/O
• Assume that SIN is set to 0 initially and SOUT is set to 1.
• This initialization is performed by device control circuits
• How to refer to the buffers DATAIN and DATAOUT?
• Many computers use arrangement called as memory-mapped I/O
• Some memory address values are used to refer to peripheral device
buffer registers such as DATAIN and DATAOUT
• Thus no special instructions required to access the contents of these
registers
• Eg., Movebyte DATAIN, R1 moves the contents of keyboard character
buffer DATAIN to R1
• Similarly, Movebyte R1, DATAOUT
• The status flags SIN and SOUT are automatically cleared when the
buffer registers are referenced
• Movebyte signifies that the operand is a byte, not a word
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Programs for I/O
• It is common to include SIN and SOUT in device status
registers, which have distinct memory addresses
• Assume that the status registers are INSTATUS and OUTSTATUS
respectively for input and output
• Also assume that SIN and SOUT are referred by bit3 in the registers.
• Read operation
• READWAIT Testbit
•
•
• Write operation
• WRITEWAITTestbit
•
•
#3, INSTATUS
Branch=0
READWAIT
Movebyte
DATAIN,R1
#3,OUTSTATUS
Branch=0
WRITEWAIT
Movebyte
R1,DATAOUT
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Move
#LOC,R0
READ
TestBit
Branch=0
MoveByte
#3,INSTATUS
READ
DATAIN,(R0)
ECHO
TestBit
Branch=0
MoveByte
#3,OUTST ATUS
ECHO
(R0),DATAOUT
Compare
#CR,(R0)+
Branch 0
READ
Initialize pointer register R0 to pointto the
address of the firstlocation in memory
wherethe characters are tobe stored.
Wait for a character tobe entered
in the keyboardbuffer DATAIN.
Transferthe character fromDATAIN into
the memory(this clears SIN to 0).
Wait for the display to become ready.
Move thecharacterjust read to the display
buffer register(this clearsSOUT to 0).
Checkif the character just read is CR
(carriage return). If it is not CR, then
branch bac k and read another character.
Also, increment the pointer to store the
next character.
Figure 2.20. A program that reads a line of characters and displays it.
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STACKS and QUEUES
• Stacks mainly used to organize control and information
linkage between main program and the subroutine
• A stack is a list of elements, usually words or bytes, with
the accessing restriction that elements can be added or
removed at one end of the list only.
• This end is called as the top of stack
• The other end is called as bottom
• Hence also called as pushdown stack
• Eg., a pile of trays in a cafetaria
• Storage mechanism is also referred by LIFO ( Last-in-First-Out)
• Push – put an element on top of stack
• Pop – remove an element from top of stack
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Problem
• In a memory, we can store a total of 2k words. The
memory is byte-addressable. How many bits are required
in the address space for this memory, to address all the
bytes? Assume the word length to be 16 bits.
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0
Stack
pointer
register
SP
•
•
•
- 28
Current
top element
17
739
Stack
BOTTOM
Normal
practice is
stack grows in
the direction of
decreasing
memory
addresses
k
2 - 1
•
•
•
43
•
•
•
Figure 2.21. A stack of words in the memory.
Bottom
element
Stack
pointer(SP) –
is a processor
register used
to keep track
of the address
of the top
element of
stack
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Push and Pop operations on Stack
• Stack pointer keeps track of the address of the top of
stack
• To Push an element
• Subtract #4,SP
• Move NEWITEM,(SP)
• To Pop an element
• Move (SP),ITEM
• Add #4,SP
• If processor support autoincrement and autodecrement
• Push  Move NEWITEM,-(SP)
• Pop  Move (SP)+,ITEM
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Stacks contd…
• Care has to be taken that stack is not popped when
empty and pushed when full
• Usage of Compare instruction
• Compare src,dst
• [dst] – [src] and affect flags accordingly
• eg/., BOTTOM – at addres 2000
• Till location 1500
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SAFEPOP
Compare
Branch> 0
#2000,SP
EMPTYERROR
Move
(SP)+,ITEM
Check to seeif the stack pointer contains
an addressvaluegreaterthan 2000. If it
does,the stack is empty . Branch to the
routine EMPTYERROR for appropriate
action.
Otherwise,pop the top of the stack into
memory location ITEM.
(a) Routine for a safe pop operation
SAFEPUSH
Compare
Branch 0
#1500,SP
FULLERROR
Move
NEWITEM,– (SP)
Check to seeif the stack pointer
contains an addressvalueequal
to or less than1500. If it does, the
stack is full. Branch to the routine
FULLERROR for appropriateaction.
Otherwise,push the element in memory
location NEWITEM onto the stack.
(b) Routine for a safe push operation
Figure 2.23. Checking for empty and full errors in pop and push operations.
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Queue – First In First Out List
 Data elements are inserted in memory at the increasing
order of memory addresses i.e., at the back of Queue.
 Data elements are retrieved from memory at the decreasing
order of memory addresses i.e. from the front end of Queue.
 Operations such as QINSERT and QDELETE can be performed
on a queue to insert and delete & element respectively.
 A Queue consisting of 6 elements may appear as follows:
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Difference between implementation
of stacks and queues
• Stacks
• One end of a stack is fixed ( bottom ), while the other end rises and
falls as data are pushed and popped. Hence a single pointer is needed
to point to the top of stack at any given time.
• A stack is assigned a fixed amount of space in memory.
• Queues
• Both ends of queue move to higher addresses as data are added at
the back and removed from the front. So two pointers are needed to
keep track of the two ends of the queue.
• Without further control, a queue would continuously move through the
memory of a computer in the direction of higher addresses.
• One way to limit the queue to a fixed region in memory is to use a circular
buffer.
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SUBROUTINES
• In a given program, it is many times needed to perform a
particular subtask on different data values. Such a sub
task is called as subroutine
• Eg., a subroutine to calculate sine function, or to sort a list of
values in increasing or decreasing order
• One copy of the instructions that constitute subroutine is
placed in the memory, and any program that requires the
use of subroutine branches to its starting location.
• This branching to a subroutine is termed as calling the subroutine
• The instruction which performs this is Call instruction
•
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SUBROUTINES - Linkage
• After a subroutine has been executed, the calling program must resume
execution continuing immediately after the instruction that called the
subroutine.
• This is performed by a Return instruction
• Since a subroutine may be called from different places, we need to make provision
to return to appropriate location
• The location where the calling program resumes execution is the location pointed to
by the updated PC while the Call instruction is being executed.
• Hence, need to save the contents of PC by Call instruction to enable correct return.
• The method in which computer calls and returns from subroutines is
referred to as subroutine linkage method
• Simplest way is to save the return address in a specific location, which may be a
register dedicated to perform this function
• Such a register is called as link register
• When the subroutine completes its task, the Return instruction returns to the calling
program by branching indirectly through the link register
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Memory
location
Memory
location
Calling program
200
204
Call
SUB
next instruction
Subroutine SUB
1000
first instruction
Return
1000
PC
204
Link
204
Call
Call instruction performs the
following operations Figure 2.24.
i. Store the contents of PC in
link register
ii. Branch to the target
address specified by the
instruction
Subroutine linkage using a link register.
Return
Return instruction
performs the following
operation
i. Branch to the
address contained in
link register
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SUBROUTINES - NESTING AND
PROCESSOR STACK
• Subroutine nesting – it’s a practice where in one subroutine calls another
subroutine.
• In this case, the return address of the second call is also stored in link register, destroying its
previous contents.
• Hence, need to save the contents of link register in some other location before calling another
subroutine.
• Else the return address of first subroutine is lost
• Subroutine nesting can be carried out to any depth.
• Last subroutine that is called completes computations and returns to the subroutine which called
it.
• Hence return addresses are generated and used in a last-in-first-out order.
• Hence return addresses can be pushed onto a stack.
• Many processors do this automatically as one of the operations performed by call instruction
• A special pointer called stack pointer, points to a stack called processor stack.
• the call instruction pushes the contents of the PC onto the processor stack and loads the
subroutine address into the PC.
• The return instruction pops the return address from the processor stack into the PC
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Subroutines - PARAMETER
PASSING
• When calling a subroutine, a program must provide to the
subroutine the parameters, (operands or their addresses), to
be used in the computation
• The subroutine returns other parameters ( the results of
computation) back.
• This exchange of information between calling program and
subroutine is called as parameter passing
• Can be done in several ways
• Can place the parameters in registers or memory locations
• Placing in processor registers in highly efficient and straight forward
• No of parameters that can be passed is limited by the number of general
purpose registers available
• The parameters may also be placed on processor stack.
• Highly flexible and can handle a large no of parameters
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Addition of N numbers in a loop using subroutines. Parameters are passed
through registers
Calling program
Move
Move
Call
Move
..
.
N,R1
#NUM1,R2
LISTADD
R0,SUM
R1 servesas a counter.
R2 pointsto thelist.
Call subroutine.
Save result.
Clear
Add
Decrement
Branch>0
Return
R0
(R2)+,R0
R1
LOOP
Initialize sumto 0.
Add entry from list.
Subroutine
LISTADD
LOOP
Return to calling program.
Figure 2.25. Program of Figure 2.16 written as a subroutine; parameters passed through registers.
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Program to add a
list on n numbers
written as a
subroutine.
Parameters are
passed through
processor stack
NOTE : parameter
passing either by pass by
value or pass by reference
n – is passed by value
(actual value is passed)
NUM1 – passed by
reference. address is
passed ( not the actual
entries of the list but the
address of first element is
passed
Figure 2.26. Program of Figure 2.16 written as a subroutine; parameters passed on the stack.
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Subroutines – Stack Frame
• The locations which will be used by the subroutine on
stack for private work, constituting a private work space
for the subroutine, which is created at the time the
subroutine is entered and freed up when the subroutine
returns control to the calling program, is called as stack
frame.
• Can also allocate space for local memory variables
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Stack frames
• In addition to stack pointer, it is useful to have another
additional pointer register, called frame pointer (FP)
• The local variables are local to the subroutine, hence can
be allocated space in stack frame associated with the
subroutine
• FP points to the location just above the return address
• Contents of FP remain fixed throughout the execution of
subroutine
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Move FP,-(SP)
Move SP,FP
SP
(stack pointer)
saved [R1]
saved [R0]
localvar3
localvar2
localvar1
FP
(frame pointer)
saved [FP]
Return address
Subtract #12,SP
Stack
frame
for
called
subroutine
param1
Access parameters
using index addr
mode.
8(FP), 12(FP)
Local variables canFigure 2.27.
be accessed using
addresses
-4(FP), -8(FP)
param2
param3
param4
Old TOS
(top-of-stack)
A subroutine stack frame example.
The calling program is
responsible for
removing parameters
from stack frame, some
of which may be results
passed back by the
subroutine
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Stack frames for nested subroutines
• Stack is a proper data structure for holding return
addresses when subroutines are nested
• Complete stack frames for nested subroutines build up on
the processor stack
• Saved contents of FP in the current stack frame are the
frame pointer contents for the stack frame of the
subroutine that called the current subroutine
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Reading Assignment : ( not to be
submitted )
• Read and understand the program and the stack frame
structure that follows. This is the program with nested
subroutines and associated stack frame ( pg 78-80 in text)
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Memory
lo cation
Main
Figure shows
only flow of
control and
data between
subroutines.
Not the actual
computations
Instructions
program
2000
2004
2008
2012
2016
2020
First
2100
2104
2108
2112
3000
P ARAM2, – (SP)
P ARAM1, – (SP)
SUB1
(SP),RESUL T
#8,SP
Place parameters
FP ,– (SP)
SP ,FP
R0 – R3, – (SP)
8(FP),R0
12(FP),R1
Sav e frame poin ter register.
Load the frame poin ter.
Sav e registers.
Get first parameter.
Get second parameter.
Store result.
Restore stack
on stack.
lev el.
subroutine
SUB1
2160
2164
Second
..
.
Mo v e
Mo v e
Call
Mo v e
Add
next instruction
..
.
Commen ts
Mo v e
Mo v e
Mo v eMultiple
Mo v e
Mo v e
..
.
Mo v e
Call
Mo v e
..
.
Mo v e
Mo v eMultiple
Mo v e
Return
P ARAM3,
SUB2
(SP)+,R2
R3,8(FP)
(SP)+,R0
(SP)+,FP
– (SP)
– R3
Place a parameter
on stack.
Pop SUB2
in to R2.
result
Place answ er on stack.
Restore registers.
Restore frame p oin ter register.
Return to Main program.
subroutine
SUB2
Mo v e
Mo v e
Mo v eMultiple
Mo v e
..
.
Mo v e
Mo v eMultiple
Mo v e
Return
FP ,– (SP)
SP ,FP
R0 – R1, – (SP)
8(FP),R0
Sav e frame p oin ter register.
Load the frame p oin ter.
Sav e registers R0 and R1.
Get the parameter.
R1,8(FP)
(SP)+,R0
(SP)+,FP
Place SUB2 result on stack.
Restore registers R0 and R1.
Restore frame p oin ter register.
Return to Subroutine
1.
– R1
Figure 2.28. Nested subroutines.
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FP
[R1] from SUB1
[R0] from SUB1
[FP] from SUB1
2164
param3
[R3] from Main
FP
[R2] from Main
[R1] from Main
[R0] from Main
[FP] from Main
2012
Stack
frame
for
second
subroutine
Stack
frame
for
first
subroutine
param1
param2
Old TOS
Figure 2.29. Stack frames for Figure 2.28.
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ADDITIONAL INSTRUCTIONS
• Instructions so far
• Move
• Load
• Store
• Clear
• Add
• Subtract
• Increment
• Decrement
• Branch
• Testbit
• Compare
• Call
• Return
Even this small set of instructions has a
number of redundancies
Eg., Load and Store instructions can be
replaced by a single Move instruction
Increment by Add and Decrement by
Subtract instruction
Clear by a Move ( moving zero )
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ADDITIONAL INSTRUCTIONS
• Logic instructions
• Not
• Complement all the bits in destination operand
• Not R0 ( complement all bits of R0)
• Can get 2’s complement representation of a negative number
• Not R0 ( R0 contains, say 3)
• Add #1,R0
• Can get 2’s complement of a number by a single instruction
• Negate R0
• Bitwise And , Or
• Performs and & or operations bitwise
• And #$FF000000,R0
• Problem : write a program which checks whether an entered 4letter
word contains Z in the beginning.
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ADDITIONAL
INSTRUCTIONS
• Answer
• And $#FF000000, R0
• Compare #$5A000000,R
• Branch=0 YES
• And instruction is often used in practical programming tasks where
all bits of an operand except for some specified field are to be
cleared to zero
• Problem: Write program to clear the bits b10 to b31 in a 4 byte word
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ADDITIONAL INSTRUCTIONS –
• Shift instructions
Shift
and
Rotate
instructions
• For general operands we use logical shifts
• For signed numbers, we use arithmetic shifts ( to preserve the sign of
the number)
• Logical Shifts
• 2 instructions – for left shift and for right shift
• LShiftL count, destination
• LShiftR count, destination
• Count can be given either as immediate operand or given inside a
processor register
• Vacated positions are filled with zeros
• The bits shifted out are passed through the Carry flag, C, and then
dropped
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C
R0
0
before:
0
0
1
1
1
0
.
.
.
0
1
1
after:
1
1
1
0
.
.
.
0
1
1
0
0
(a) Logical shift left
LShiftL
0
#2,R0
R0
C
before:
0
1
1
1
0
.
.
.
0
1
1
0
after:
0
0
0
1
1
1
0
.
.
.
0
1
(b) Logical shift r
ight
LShiftR #2,R0
R0
C
before:
1
0
0
1
1
.
.
.
0
1
0
0
after:
1
1
1
0
0
1
1
.
.
.
0
1
(c) Ar ithmetic shift r
ight
AShiftR #2,R0
Figure 2.30. Logical and arithmetic shift instructions.
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ADDITIONAL INSTRUCTIONS –
Shift and Rotate instructions
• Digit packing
• 2 decimal digits represented in ASCII are located in
memory at byte locations LOC and LOC + 1
• Represent each of these digits in 4-bit BCD and store in a
single byte location PACKED (the result is said to be in
packed-BCD format)
• Hint: rightmost 4 bits of the ASCII code for a decimal digit
correspond to the BCD code for the digit
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Move
MoveByte
LShiftL
MoveByte
And
Or
MoveByte
#LOC,R0
(R0)+,R1
#4,R1
(R0),R2
#$F,R2
R1,R2
R2,PACKED
R0 points to data.
Load first byte into R1.
Shift left by 4 bit positions.
Load secondbyte into R2.
Eliminate high-order bits.
ConcatenatetheBCD digits.
Store the result.
Figure 2.31. A routine that packs two BCD digits.
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ADDITIONAL INSTRUCTIONS –
Shift and Rotate instructions
• Arithmetic Shifts
• Shifting a number one bit left is same as multiplying the
number by 2
• Overflow may occur
• Shifting right is equivalent to dividing the number by 2
• Reminder is lost
• On a right shift sign-bit must be repeated as the fill in bit for the
vacated position, main difference between Arithmetic shift and
logical shift
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ADDITIONAL INSTRUCTIONS –
Shift and Rotate instructions
• Rotate instructions
• 4 in number
• Rotate left without Carry – RotateL
• Rotate left with Carry – RotateLC
• Rotate right without Carry – RotateR
• Rotate right with Carry - RotateRC
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C
R0
before:
0
0
1
1
1
0
.
.
.
0
1
1
after:
1
1
1
0
.
.
.
0
1
1
0
1
(a) Rotate left without carry
RotateL
C
#2,R0
R0
before:
0
0
1
1
1
0
.
.
.
0
1
1
after:
1
1
1
0
.
.
.
0
1
1
0
0
(b) Rotate left with carry
RotateLC #2,R0
R0
C
before:
0
1
1
1
0
.
.
.
0
1
1
0
after:
1
1
0
1
1
1
0
.
.
.
0
1
(c) Rotate r
ight without carr
y
RotateR #2,R0
R0
C
before:
0
1
1
1
0
.
.
.
0
1
1
0
after:
1
0
0
1
1
1
0
.
.
.
0
1
ight with carr
y
(d) Rotate r
RotateRC #2,R0
Figure 2.32. Rotate instructions.
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ADDITIONAL INSTRUCTIONS –
Shift and Rotate instructions
• Multiplication and Division
• MultiplyRi,Rj
• Product can be as large as 2n bits for 2 n-bit numbers
• Answer will not necessarily fit in Rj
• Lower order bits in Rj, higher order bits in Rj+1
• Some systems provide Divide Ri ,Rj
• Rj/Ri and quotient in Rj
• May place remainder in Rj+1
• Computers which do not have divide and multiply instructions
can implement same using basic instructions like add, subtract,
shift and rotate
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ENCODING OF MACHINE
INSTRUCTIONS
• Till now we have not used acronyms for instructions
because they are specific to a particular processor in
market ( eg., we used Move instead of MOV )
• To be executed, an instruction must be encoded in a
compact binary pattern. Such encoded instructions are
properly referred to as machine instructions
• The instructions that use symbolic names and acronyms are called
as assembly language instructions, which are converted to
machine instructions using assembler program
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ENCODING OF MACHINE
INSTRUCTIONS
• The type of operation to be performed and the type of
operands used is specified using an encoded binary pattern
referred to as the OP code for the given instruction
• Suppose 8 bits are assigned for OP code to specify upto 256 different
instructions
• If we assume that word length is 32-bits and if we want to fit an entire
instruction into a single memory word, then we are left with 24 bits to
specify rest of the information
• Eg., Add R1,R2 has to specify the registers R1 and R2, in
addition to the OP code
• If system has 16 registers, then 4 bits are required to identify
each register and some additional bits to specify that we are
using register addressing mode
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ENCODING OF MACHINE
INSTRUCTIONS
• Move 24(R0) ,R5
• 16 bits to denote OP code, 2 registers, some bits to express that
source operand uses index addressing mode and that index value
is 24
• Suppose 3 bits are used to specify addressing mode
• We need 6 bits – to denote the chosen addressing mode for both
source and destination operands
• 32 – ( 16+6) = 10 bits left to give the index value ( signed number )
• Eg., LShiftR #2,R0
• Move #$3A, R1 how many bits left to specify immediate value?
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ENCODING OF MACHINE
INSTRUCTIONS
• Branch>0 LOOP
• Here 8 bits for OP code. How many bits for Branch OFFSET?
• What is the limit for the branch target address with respect to
branch insrtuction?
• Jump instruction - uses Absolute or Register Indirect mode to
specify branch target
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ENCODING OF MACHINE
INSTRUCTIONS
• CISC processors use instructions that are complex and
require more than a single word to specify the instruction
• Consider instruction
• Add LOC,R2
• Here 8 bits for OP code, 10 bits for addr mode and specifying the
register, remaining 14 bits cannot hold a 32-bit address of the
absolute addressing mode
• Hence we need to go either for multiple word instruction
• Or
• Register Indirect
• Load address of LOC to R3
• Add (R3),R2
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8
7
7
10
OP code
Source
Dest
Other info
(a) One-word instruction
OP code
Source
Dest
Other info
Memory address/Immediate operand
(b) Two-word instruction
OP code
Ri
Rj
Rk
Other info
(c) Three-operand instruction
Figure 2.39. Encoding instructions into 32-bit words.
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ENCODING OF MACHINE
INSTRUCTIONS
• But how to load 32-bit address to register?
• One way is to use indexed addressing mode to load the
address into register ( for this need to place the address
in a memory location close to the program)
• Other option is to use Logical and Shift instructions –
passing the address by parts
• The restriction that the instruction must fit in a single word
( 32 bit here ) is the style of RISC machines
• Places a restriction that all manipulation of data contained only in
processor registers
• Move (R3) ,R1
• Add R1,R2