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Overview of Assembly Language
Chapter 9
S. Dandamudi
Outline
• Assembly language
statements
• Data allocation
• Where are the operands?
• Overview of assembly
language instructions





 Addressing modes
»
»
»
»
Register
Immediate
Direct
Indirect
• Defining constants
 EQU and = directives
• Data transfer instructions
 mov, xchg, and xlat
Arithmetic
Conditional
Logical
Shift
Rotate
• Macros
• Illustrative examples
 PTR directive
2003
 S. Dandamudi
Chapter 9: Page 2
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Assembly Language Statements
• Three different classes
 Instructions
» Tell CPU what to do
» Executable instructions with an op-code
 Directives (or pseudo-ops)
» Provide information to assembler on various aspects of the
assembly process
» Non-executable
– Do not generate machine language instructions
 Macros
» A shorthand notation for a group of statements
» A sophisticated text substitution mechanism with parameters
2003
 S. Dandamudi
Chapter 9: Page 3
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Assembly Language Statements (cont’d)
• Assembly language statement format:
[label]
mnemonic
[operands]
[;comment]
 Typically one statement per line
 Fields in [ ] are optional
 label serves two distinct purposes:
» To label an instruction
– Can transfer program execution to the labeled instruction
» To label an identifier or constant
 mnemonic identifies the operation (e.g., add, or)
 operands specify the data required by the operation
» Executable instructions can have zero to three operands
2003
 S. Dandamudi
Chapter 9: Page 4
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Assembly Language Statements (cont’d)
 comments
» Begin with a semicolon (;) and extend to the end of the line
Examples
repeat:
CR
inc
EQU
result ; increment result
0DH
; carriage return character
• White space can be used to improve readability
repeat:
inc
2003
result
 S. Dandamudi
Chapter 9: Page 5
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation
• Variable declaration in a high-level language such
as C
char
int
float
double
response
value
total
average_value
specifies
» Amount storage required (1 byte, 2 bytes, …)
» Label to identify the storage allocated (response, value, …)
» Interpretation of the bits stored (signed, floating point, …)
– Bit pattern 1000 1101 1011 1001 is interpreted as
 -29,255 as a signed number
 36,281 as an unsigned number
2003
 S. Dandamudi
Chapter 9: Page 6
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
• In assembly language, we use the define directive
 Define directive can be used
»
»
»
»
To reserve storage space
To label the storage space
To initialize
But no interpretation is attached to the bits stored
– Interpretation is up to the program code
 Define directive goes into the .DATA part of the
assembly language program
• Define directive format
[var-name]
2003
D?
init-value [,init-value],...
 S. Dandamudi
Chapter 9: Page 7
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
• Five define directives
DB
DW
DD
DQ
DT
Define
Define
Define
Define
Define
Byte
Word
Doubleword
Quadword
Ten bytes
;allocates 1 byte
;allocates 2 bytes
;allocates 4 bytes
;allocates 8 bytes
;allocates 10 bytes
Examples
sorted
response
value
float1
float2
2003
DB
DB
DW
DD
DQ
’y’
?
;no initialization
25159
1.234
123.456
 S. Dandamudi
Chapter 9: Page 8
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
• Multiple definitions can be abbreviated
Example
message
DB
DB
DB
DB
DB
can be written as
message DB
’B’
’y’
’e’
0DH
0AH
’B’,’y’,’e’,0DH,0AH
• More compactly as
message
2003
DB
’Bye’,0DH,0AH
 S. Dandamudi
Chapter 9: Page 9
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
• Multiple definitions can be cumbersome to
initialize data structures such as arrays
Example
To declare and initialize an integer array of 8 elements
marks DW
0,0,0,0,0,0,0,0
• What if we want to declare and initialize to zero
an array of 200 elements?
 There is a better way of doing this than repeating zero
200 times in the above statement
» Assembler provides a directive to do this (DUP directive)
2003
 S. Dandamudi
Chapter 9: Page 10
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
• Multiple initializations
 The DUP assembler directive allows multiple
initializations to the same value
 Previous marks array can be compactly declared as
marks DW
8 DUP (0)
Examples
table1 DW
message DB
Name1
DB
10 DUP (?)
;10 words, uninitialized
3 DUP (’Bye!’) ;12 bytes, initialized
;
as Bye!Bye!Bye!
30 DUP (’?’)
;30 bytes, each
;
2003
 S. Dandamudi
initialized to ?
Chapter 9: Page 11
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
• The DUP directive may also be nested
Example
stars DB 4 DUP(3 DUP (’*’),2 DUP (’?’),5 DUP (’!’))
Reserves 40-bytes space and initializes it as
***??!!!!!***??!!!!!***??!!!!!***??!!!!!
Example
matrix DW 10 DUP (5 DUP (0))
defines a 10X5 matrix and initializes its elements to 0
This declaration can also be done by
matrix DW 50 DUP (0)
2003
 S. Dandamudi
Chapter 9: Page 12
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
Symbol Table
 Assembler builds a symbol table so we can refer to the
allocated storage space by the associated label
Example
.DATA
value
sum
marks
message
char1
2003
name
DW
DD
DW
DB
DB
0
0
10 DUP (?)
‘The grade is:’,0
?
 S. Dandamudi
offset
value
sum
marks
message
char1
0
2
6
26
40
Chapter 9: Page 13
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
Correspondence to C Data Types
Directive
DB
DW
DD
DQ
DT
2003
C data type
char
int, unsigned
float, long
double
internal intermediate
float value
 S. Dandamudi
Chapter 9: Page 14
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
LABEL Directive
 LABEL directive provides another way to name a
memory location
 Format:
name
LABEL
type
type can be
BYTE
WORD
DWORD
QWORD
TWORD
2003
1 byte
2 bytes
4 bytes
8 bytes
10 bytes
 S. Dandamudi
Chapter 9: Page 15
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Allocation (cont’d)
LABEL Directive
Example
.DATA
count LABEL WORD
Lo-count DB 0
Hi_count DB 0
.CODE
...
mov Lo_count,AL
mov Hi_count,CL
 count refers to the 16-bit value
 Lo_count refers to the low byte
 Hi_count refers to the high byte
2003
 S. Dandamudi
Chapter 9: Page 16
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands?
• Operands required by an operation can be
specified in a variety of ways
• A few basic ways are:
 operand in a register
– register addressing mode
 operand in the instruction itself
– immediate addressing mode
 operand in memory
– variety of addressing modes
direct and indirect addressing modes
 operand at an I/O port
– discussed in Chapter 19
2003
 S. Dandamudi
Chapter 9: Page 17
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Register addressing mode
 Operand is in an internal register
Examples
mov
mov
mov
EAX,EBX ; 32-bit copy
BX,CX
; 16-bit copy
AL,CL
; 8-bit copy
 The mov instruction
mov
destination,source
copies data from source to destination
2003
 S. Dandamudi
Chapter 9: Page 18
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Register addressing mode (cont’d)
 Most efficient way of specifying an operand
» No memory access is required
 Instructions using this mode tend to be shorter
» Fewer bits are needed to specify the register
• Compilers use this mode to optimize code
total := 0
for (i = 1 to 400)
total = total + marks[i]
end for
 Mapping total and i to registers during the for loop optimizes
the code
2003
 S. Dandamudi
Chapter 9: Page 19
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Immediate addressing mode
 Data is part of the instruction
» Ooperand is located in the code segment along with the
instruction
» Efficient as no separate operand fetch is needed
» Typically used to specify a constant
Example
mov
AL,75
 This instruction uses register addressing mode for
destination and immediate addressing mode for the
source
2003
 S. Dandamudi
Chapter 9: Page 20
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Direct addressing mode
 Data is in the data segment
» Need a logical address to access data
– Two components: segment:offset
» Various addressing modes to specify the offset component
– offset part is called effective address
 The offset is specified directly as part of instruction
 We write assembly language programs using memory
labels (e.g., declared using DB, DW, LABEL,...)
» Assembler computes the offset value for the label
– Uses symbol table to compute the offset of a label
2003
 S. Dandamudi
Chapter 9: Page 21
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Direct addressing mode (cont’d)
Examples
mov
AL,response
» Assembler replaces response by its effective address (i.e., its
offset value from the symbol table)
mov
table1,56
» table1 is declared as
table1
DW
20 DUP (0)
» Since the assembler replaces table1 by its effective address,
this instruction refers to the first element of table1
– In C, it is equivalent to
table1[0] = 56
2003
 S. Dandamudi
Chapter 9: Page 22
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Direct addressing mode (cont’d)
• Problem with direct addressing
 Useful only to specify simple variables
 Causes serious problems in addressing data types such
as arrays
» As an example, consider adding elements of an array
– Direct addressing does not facilitate using a loop structure
to iterate through the array
– We have to write an instruction to add each element of the
array
• Indirect addressing mode remedies this problem
2003
 S. Dandamudi
Chapter 9: Page 23
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Indirect addressing mode
• The offset is specified indirectly via a register
 Sometimes called register indirect addressing mode
 For 16-bit addressing, the offset value can be in one of
the three registers: BX, SI, or DI
 For 32-bit addressing, all 32-bit registers can be used
Example
mov
AX,[BX]
 Square brackets [ ] are used to indicate that BX is
holding an offset value
» BX contains a pointer to the operand, not the operand itself
2003
 S. Dandamudi
Chapter 9: Page 24
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
• Using indirect addressing mode, we can process
arrays using loops
Example: Summing array elements
 Load the starting address (i.e., offset) of the array into
BX
 Loop for each element in the array
» Get the value using the offset in BX
– Use indirect addressing
» Add the value to the running total
» Update the offset in BX to point to the next element of the
array
2003
 S. Dandamudi
Chapter 9: Page 25
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Loading offset value into a register
• Suppose we want to load BX with the offset value
of table1
• We cannot write
mov
BX,table1
• Two ways of loading offset value
» Using OFFSET assembler directive
– Executed only at the assembly time
» Using lea instruction
– This is a processor instruction
– Executed at run time
2003
 S. Dandamudi
Chapter 9: Page 26
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Loading offset value into a register (cont’d)
• Using OFFSET assembler directive
 The previous example can be written as
mov
BX,OFFSET table1
• Using lea (load effective address) instruction
 The format of lea instruction is
lea
register,source
 The previous example can be written as
lea
BX,table1
2003
 S. Dandamudi
Chapter 9: Page 27
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Where Are the Operands? (cont’d)
Loading offset value into a register (cont’d)
Which one to use -- OFFSET or lea?
 Use OFFSET if possible
» OFFSET incurs only one-time overhead (at assembly time)
» lea incurs run time overhead (every time you run the program)
 May have to use lea in some instances
» When the needed data is available at run time only
– An index passed as a parameter to a procedure
» We can write
lea
BX,table1[SI]
to load BX with the address of an element of table1 whose
index is in SI register
» We cannot use the OFFSET directive in this case
2003
 S. Dandamudi
Chapter 9: Page 28
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Default Segments
• In register indirect addressing mode
 16-bit addresses
» Effective addresses in BX, SI, or DI is taken as the offset into
the data segment (relative to DS)
» For BP and SP registers, the offset is taken to refer to the stack
segment (relative to SS)
 32-bit addresses
» Effective address in EAX, EBX, ECX, EDX, ESI, and EDI is
relative to DS
» Effective address in EBP and ESP is relative to SS
 push and pop are always relative to SS
2003
 S. Dandamudi
Chapter 9: Page 29
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Default Segments (cont’d)
• Default segment override
 Possible to override the defaults by using override
prefixes
» CS, DS, SS, ES, FS, GS
 Example 1
» We can use
add
AX,SS:[BX]
to refer to a data item on the stack
 Example 2
» We can use
add
AX,DS:[BP]
to refer to a data item in the data segment
2003
 S. Dandamudi
Chapter 9: Page 30
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions
• We will look at three instructions
 mov (move)
» Actually copy
 xchg (exchange)
» Exchanges two operands
 xlat (translate)
» Translates byte values using a translation table
• Other data transfer instructions such as
movsx (move sign extended)
movzx (move zero extended)
are discussed in Chapter 12
2003
 S. Dandamudi
Chapter 9: Page 31
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
The mov instruction
 The format is
mov
destination,source
» Copies the value from source to destination
» source is not altered as a result of copying
» Both operands should be of same size
» source and destination cannot both be in memory
– Most Pentium instructions do not allow both operands to
be located in memory
– Pentium provides special instructions to facilitate
memory-to-memory block copying of data
2003
 S. Dandamudi
Chapter 9: Page 32
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
The mov instruction
 Five types of operand combinations are allowed:
Instruction type
Example
mov
register,register
mov
DX,CX
mov
mov
mov
mov
register,immediate
register,memory
memory,register
memory,immediate
mov
mov
mov
mov
BL,100
BX,count
count,SI
count,23
 The operand combinations are valid for all instructions
that require two operands
2003
 S. Dandamudi
Chapter 9: Page 33
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
Ambiguous moves: PTR directive
• For the following data definitions
.DATA
table1
DW
20 DUP (0)
status
DB
7 DUP (1)
the last two mov instructions are ambiguous
mov
BX,OFFSET table1
mov
SI,OFFSET status
mov
[BX],100
mov
[SI],100
 Not clear whether the assembler should use byte or word
equivalent of 100
2003
 S. Dandamudi
Chapter 9: Page 34
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
Ambiguous moves: PTR directive
• The PTR assembler directive can be used to
clarify
• The last two mov instructions can be written as
mov
mov
WORD PTR [BX],100
BYTE PTR [SI],100
 WORD and BYTE are called type specifiers
• We can also use the following type specifiers:
DWORD for doubleword values
QWORD for quadword values
TWORD for ten byte values
2003
 S. Dandamudi
Chapter 9: Page 35
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
The xchg instruction
• The syntax is
xchg
operand1,operand2
Exchanges the values of operand1 and operand2
Examples
xchg
xchg
xchg
EAX,EDX
response,CL
total,DX
• Without the xchg instruction, we need a
temporary register to exchange values using only
the mov instruction
2003
 S. Dandamudi
Chapter 9: Page 36
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
The xchg instruction
• The xchg instruction is useful for conversion of
16-bit data between little endian and big endian
forms
 Example:
mov
AL,AH
converts the data in AX into the other endian form
• Pentium provides bswap instruction to do similar
conversion on 32-bit data
bswap
32-bit register
 bswap works only on data located in a 32-bit register
2003
 S. Dandamudi
Chapter 9: Page 37
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
The xlat instruction
• The xlat instruction translates bytes
• The format is
xlatb
• To use xlat instruction
» BX should be loaded with the starting address of the translation table
» AL must contain an index in to the table
– Index value starts at zero
» The instruction reads the byte at this index in the translation table and
stores this value in AL
– The index value in AL is lost
» Translation table can have at most 256 entries (due to AL)
2003
 S. Dandamudi
Chapter 9: Page 38
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Data Transfer Instructions (cont’d)
The xlat instruction
Example: Encrypting digits
Input digits: 0 1 2 3 4 5 6 7 8 9
Encrypted digits: 4 6 9 5 0 3 1 8 7 2
.DATA
xlat_table DB ’4695031872’
...
.CODE
mov
GetCh
sub
xlatb
PutCh
...
2003
BX,OFFSET xlat_table
AL
AL,’0’ ; converts input character to index
; AL = encrypted digit character
AL
 S. Dandamudi
Chapter 9: Page 39
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Pentium Assembly Instructions
• Pentium provides several types of instructions
• Brief overview of some basic instructions:






Arithmetic instructions
Jump instructions
Loop instruction
Logical instructions
Shift instructions
Rotate instructions
• These instructions allow you to write reasonable
assembly language programs
2003
 S. Dandamudi
Chapter 9: Page 40
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Arithmetic Instructions
INC and DEC instructions
 Format:
inc
destination
dec
destination
 Semantics:
destination = destination +/- 1
» destination can be 8-, 16-, or 32-bit operand, in memory
or register
No immediate operand
• Examples
inc
dec
2003
BX
value
 S. Dandamudi
Chapter 9: Page 41
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Arithmetic Instructions (cont’d)
Add instructions
 Format:
add
destination,source
 Semantics:
destination = destination + source
• Examples
add
add
 inc
EBX,EAX
value,35
EAX
is better than
add EAX,1
– inc takes less space
– Both execute at about the same speed
2003
 S. Dandamudi
Chapter 9: Page 42
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Arithmetic Instructions (cont’d)
Add instructions
 Addition with carry
 Format:
adc
destination,source
 Semantics:
destination = destination + source + CF
• Example: 64-bit addition
add
adc
EAX,ECX
EBX,EDX
; add lower 32 bits
; add upper 32 bits with carry
 64-bit result in EBX:EAX
2003
 S. Dandamudi
Chapter 9: Page 43
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Arithmetic Instructions (cont’d)
Subtract instructions
 Format:
sub
destination,source
 Semantics:
destination = destination - source
• Examples
sub
sub
 dec
EBX,EAX
value,35
EAX
is better than
sub EAX,1
– dec takes less space
– Both execute at about the same speed
2003
 S. Dandamudi
Chapter 9: Page 44
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Arithmetic Instructions (cont’d)
Subtract instructions
 Subtract with borrow
 Format:
sbb
destination,source
 Semantics:
destination = destination - source - CF
 Like the adc, sbb is useful in dealing with more than
32-bit numbers
• Negation
neg
destination
 Semantics:
destination = 0 - destination
2003
 S. Dandamudi
Chapter 9: Page 45
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Arithmetic Instructions (cont’d)
CMP instruction
 Format:
cmp
destination,source
 Semantics:
destination - source
 destination and source are not altered
 Useful to test relationship (>, =) between two operands
 Used in conjunction with conditional jump instructions
for decision making purposes
• Examples
cmp
2003
EBX,EAX
cmp
 S. Dandamudi
count,100
Chapter 9: Page 46
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Unconditional Jump
 Format:
jmp
label
 Semantics:
» Execution is transferred to the instruction identified by label
• Target can be specified in one of two ways
 Directly
» In the instruction itself
 Indirectly
» Through a register or memory
» Discussed in Chapter 12
2003
 S. Dandamudi
Chapter 9: Page 47
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Unconditional Jump (cont’d)
Example
• Two jump instructions
 Forward jump
jmp
CX_init_done
 Backward jump
jmp
repeat1
• Programmer specifies
target by a label
• Assembler computes the
offset using the symbol
table
2003
. . .
mov
CX,10
jmp
CX_init_done
init_CX_20:
mov
CX,20
CX_init_done:
mov
AX,CX
repeat1:
dec
CX
. . .
jmp
repeat1
. . .
 S. Dandamudi
Chapter 9: Page 48
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Unconditional Jump (cont’d)
• Address specified in the jump instruction is not the
absolute address
 Uses relative address
» Specifies relative byte displacement between the target
instruction and the instruction following the jump instruction
» Displacement is w.r.t the instruction following jmp
– Reason: IP points to this instruction after reading jump
 Execution of jmp involves adding the displacement
value to current IP
 Displacement is a signed 16-bit number
» Negative value for backward jumps
» Positive value for forward jumps
2003
 S. Dandamudi
Chapter 9: Page 49
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Target Location
• Inter-segment jump
 Target is in another segment
CS = target-segment (2 bytes)
IP = target-offset (2 bytes)
» Called far jumps (needs five bytes to encode jmp)
• Intra-segment jumps
 Target is in the same segment
IP = IP + relative-displacement (1 or 2 bytes)
 Uses 1-byte displacement if target is within -128 to +127
» Called short jumps (needs two bytes to encode jmp)
 If target is outside this range, uses 2-byte displacement
» Called near jumps (needs three bytes to encode jmp)
2003
 S. Dandamudi
Chapter 9: Page 50
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Target Location (cont’d)
• In most cases, the assembler can figure out the type
of jump
 For backward jumps, assembler can decide whether to
use the short jump form or not
• For forward jumps, it needs a hint from the
programmer
 Use SHORT prefix to the target label
 If such a hint is not given
» Assembler reserves three bytes for jmp instruction
» If short jump can be used, leaves one byte of nop (no operation)
– See the next example for details
2003
 S. Dandamudi
Chapter 9: Page 51
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Example
.
8 0005
EB 0C
9 0007
10 000A
B9 000A
EB 07 90
11
12 000D
13 0010
B9 0014
E9 00D0
14
15 0013
8B C1
2003
.
.
jmp
SHORT CX_init_done
0013 - 0007 = 0C
mov
CX,10
jmp
CX_init_done
nop
0013 - 000D = 07
init_CX_20:
mov
CX,20
jmp
near_jump
00E3 - 0013 = D0
CX_init_done:
mov
AX,CX
 S. Dandamudi
Chapter 9: Page 52
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Example (cont’d)
16
17 0015
18 0016
49
EB FD
. . .
84 00DB
EB 03
85 00DD
86
87 00E0
88
89 00E3
2003
B9 FF00
BA 0020
E9 FF27
repeat1:
dec
CX
jmp
repeat1
0015 - 0018 = -3 = FDH
jmp
SHORT short_jump
00E0 - 00DD = 3
mov
CX, 0FF00H
short_jump:
mov
DX, 20H
near_jump:
jmp
init_CX_20
000D - 00E6 = -217 = FF27H
 S. Dandamudi
Chapter 9: Page 53
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Conditional Jumps (cont’d)
Format:
j<cond>
lab
– Execution is transferred to the instruction identified by
label only if <cond> is met
• Example: Testing for carriage return
read_char:
. . .
cmp
AL,0DH ; 0DH = ASCII carriage return
je
CR_received
inc
CL
jmp
read_char
. . .
CR_received:
2003
 S. Dandamudi
Chapter 9: Page 54
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Conditional Jumps (cont’d)
 Some conditional jump instructions
– Treats operands of the CMP instruction as signed numbers
je
jg
jl
jge
jle
jne
2003
jump
jump
jump
jump
jump
jump
if
if
if
if
if
if
equal
greater
less
greater or equal
less or equal
not equal
 S. Dandamudi
Chapter 9: Page 55
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Conditional Jumps (cont’d)
 Conditional jump instructions can also test values of the
individual flags
jz
jnz
jc
jnc
jump
jump
jump
jump
if
if
if
if
zero (i.e., if ZF = 1)
not zero (i.e., if ZF = 0)
carry (i.e., if CF = 1)
not carry (i.e., if CF = 0)
 jz is synonymous for je
 jnz is synonymous for jne
2003
 S. Dandamudi
Chapter 9: Page 56
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
A Note on Conditional Jumps
• All conditional jumps are encoded using 2 bytes
 Treated as short jumps
• What if the target is outside this range?
• Use this code to get around
target:
. . .
cmp
AX,BX
je
target
mov
CX,10
. . .
target:
traget is out of range for a
short jump
2003
. . .
cmp
AX,BX
jne
skip1
jmp
target
skip1:
mov
CX,10
. . .
 S. Dandamudi
Chapter 9: Page 57
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Loop Instructions
Unconditional loop instruction
 Format:
loop
target
 Semantics:
» Decrements CX and jumps to target if CX  0
– CX should be loaded with a loop count value
• Example: Executes loop body 50 times
mov
CX,50
repeat:
<loop body>
loop
repeat
...
2003
 S. Dandamudi
Chapter 9: Page 58
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Loop Instructions (cont’d)
• The previous example is equivalent to
mov
CX,50
repeat:
<loop body>
dec
CX
jnz
repeat
...
 Surprisingly,
dec
jnz
CX
repeat
executes faster than
loop
2003
repeat
 S. Dandamudi
Chapter 9: Page 59
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Loop Instructions (cont’d)
• Conditional loop instructions
 loope/loopz
» Loop while equal/zero
CX = CX – 1
ff (CX = 0 and ZF = 1)
jump to target
 loopne/loopnz
» Loop while not equal/not zero
CX = CX – 1
ff (CX = 0 and ZF = 0)
jump to target
2003
 S. Dandamudi
Chapter 9: Page 60
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Logical Instructions
 Format:
and destination,source
or
destination,source
xor destination,source
not destination
 Semantics:
» Performs the standard bitwise logical operations
– result goes to destination
 test is a non-destructive and instruction
test
destination,source
 Performs logical AND but the result is not stored in
destination (like the CMP instruction)
2003
 S. Dandamudi
Chapter 9: Page 61
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Logical Instructions (cont’d)
Example:
. . .
and
AL,01H ; test the least significant bit
jz
bit_is_zero
<bit 1 code>
jmp
skip1
bit_is_zero:
<bit 0 code>
skip1:
. . .
• test instruction is better in place of and
2003
 S. Dandamudi
Chapter 9: Page 62
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Shift Instructions
• Two types of shifts
» Logical
» Arithmetic
 Logical shift instructions
Shift left
shl destination,count
shl destination,CL
Shift right
shr destination,count
shr destination,CL
 Semantics:
» Performs left/right shift of destination by the value in
count or CL register
– CL register contents are not altered
2003
 S. Dandamudi
Chapter 9: Page 63
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Shift Instructions (cont’d)
Logical shift
 Bit shifted out goes into the carry flag
» Zero bit is shifted in at the other end
2003
 S. Dandamudi
Chapter 9: Page 64
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Shift Instructions (cont’d)
 count is an immediate value
shl
AX,5
 Specification of count greater than 31 is not allowed
» If a greater value is specified, only the least significant 5 bits
are used
 CL version is useful if shift count is known at run time
» Ex: when the shift count value is passed as a parameter in a
procedure call
» Only the CL register can be used
Shift count value should be loaded into CL
mov
CL,5
shl
AX,CL
2003
 S. Dandamudi
Chapter 9: Page 65
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Shift Instructions (cont’d)
Arithmetic shift
 Two versions as in logical shift
sal/sar
sal/sar
2003
destination,count
destination,CL
 S. Dandamudi
Chapter 9: Page 66
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Double Shift Instructions
• Double shift instructions work on either 32- or 64bit operands
• Format
 Takes three operands
shld
dest,src,count ; left shift
shrd
dest,src,count ; right shift
 dest can be in memory or register
 src must be a register
 count can be an immediate value or in CL as in other
shift instructions
2003
 S. Dandamudi
Chapter 9: Page 67
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Double Shift Instructions (cont’d)
 src is not modified by doubleshift instruction
 Only dest is modified
 Shifted out bit goes into the carry flag
2003
 S. Dandamudi
Chapter 9: Page 68
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Rotate Instructions
 Two types of ROTATE instructions
 Rotate without carry
» rol (ROtate Left)
» ror (ROtate Right)
 Rotate with carry
» rcl (Rotate through Carry Left)
» rcr (Rotate through Carry Right)
 Format of ROTATE instructions is similar to the SHIFT
instructions
» Supports two versions
– Immediate count value
– Count value in CL register
2003
 S. Dandamudi
Chapter 9: Page 69
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Rotate Instructions (cont’d)
 Bit shifted out goes into the carry flag as in SHIFT
instructions
2003
 S. Dandamudi
Chapter 9: Page 70
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Rotate Instructions (cont’d)
 Bit shifted out goes into the carry flag as in SHIFT
instructions
2003
 S. Dandamudi
Chapter 9: Page 71
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Rotate Instructions (cont’d)
• Example: Shifting 64-bit numbers
 Multiplies a 64-bit value in EDX:EAX by 16
» Rotate version
mov
CX,4
shift_left:
shl
EAX,1
rcl
EDX,1
loop
shift_left
» Doubleshift version
shld
EDX,EAX,4
shl
EAX,4
• Division can be done in a similar a way
2003
 S. Dandamudi
Chapter 9: Page 72
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Defining Constants
• Assembler provides two directives:
» EQU directive
– No reassignment
– String constants can be defined
» = directive
– Can be reassigned
– No string constants
• Defining constants has two advantages:
 Improves program readability
 Helps in software maintenance
» Multiple occurrences can be changed from a single place
• Convention
» We use all upper-case letters for names of constants
2003
 S. Dandamudi
Chapter 9: Page 73
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Defining Constants (cont’d)
The EQU directive
• Syntax:
name
EQU
expression
 Assigns the result of expression to name
 The expression is evaluated at assembly time
Similar to #define in C
Examples
NUM_OF_ROWS
NUM_OF_COLS
ARRAY_SIZE
EQU
EQU
EQU
50
10
NUM_OF_ROWS * NUM_OF_COLS
 Can also be used to define string constants
JUMP
2003
EQU
jmp
 S. Dandamudi
Chapter 9: Page 74
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Defining Constants (cont’d)
• Syntax:
The = directive
name = expression
 Similar to EQU directive
 Two key differences:
» Redefinition is allowed
count = 0
. . .
count = 99
is valid
» Cannot be used to define string constants or to redefine
keywords or instruction mnemonics
Example: JUMP = jmp is not allowed
2003
 S. Dandamudi
Chapter 9: Page 75
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Macros
• Macros can be defined with MACRO and ENDM
• Format
macro_name
MACRO[parameter1, parameter2,...]
macro body
ENDM
• A macro can be invoked using
macro_name [argument1, argument2, …]
Example:
Definition
multAX_by_16
2003
Invocation
MACRO
sal
ENDM
AX,4
 S. Dandamudi
...
mov
AX,27
multAX_by_16
...
Chapter 9: Page 76
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Macros (cont’d)
• Macros can be defined with parameters
» More flexible
» More useful
• Example
mult_by_16
MACRO
sal
ENDM
operand
operand,4
 To multiply a byte in DL register
mult_by_16
DL
 To multiply a memory variable count
mult_by_16
2003
count
 S. Dandamudi
Chapter 9: Page 77
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Macros (cont’d)
Example: To exchange two memory words
 Memory-to-memory transfer
Wmxchg
MACRO operand1, operand2
xchg
AX,operand1
xchg
AX,operand2
xchg
AX,operand1
ENDM
2003
 S. Dandamudi
Chapter 9: Page 78
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.
Illustrative Examples
• Five examples in this chapter
 Conversion of ASCII to binary representation
(BINCHAR.ASM)
 Conversion of ASCII to hexadecimal by character
manipulation (HEX1CHAR.ASM)
 Conversion of ASCII to hexadecimal using the XLAT
instruction (HEX2CHAR.ASM)
 Conversion of lowercase letters to uppercase by
character manipulation (TOUPPER.ASM)
 Sum of individual digits of a number
(ADDIGITS.ASM)
Last slide
2003
 S. Dandamudi
Chapter 9: Page 79
To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.