Transcript eel3801-1-assembly

EEL 3801
Part III
Assembly Language Programming
EEL 3801C
Assembly Language Programming
• The basic element of an assembly program
is the statement.
• Two types of statements:
• Instructions: executable statements that actually
do something.
• Directive: provide information to assist the
assembler in producing executable code. For
example, create storage for a variable and initialize it.
EEL 3801C
Assembly Programming (cont’d)
– Assembly language instructions equate one-toone to machine-level instructions, except they
use a mnemonic to assist the memory.
• Program control: Control the flow of the program and
what instructions to execute next (jump, goto).
• Data transfer: Transfer data to a register or to main
memory.
• Arithmetic: add, subtract, multiply, divide, etc.
EEL 3801C
Assembly Programming (cont’d)
• Logical: > < = etc.
• Input-output: read, print etc.
EEL 3801C
Statements
• A statement is composed of a name, a
mnemonic, operands and an optional
comment. Their general format is as
follows:
[name] [mnemonic] [operand(s)] [;comments]
EEL 3801C
Names, labels
– Name: Identifies a label for a statement or for
a variable or constant.
• Can contain one or more of several characters (see
page 56 of new textbook).
• Only first 31 characters are recognized
• Case insensitive.
• First character may not be a digit.
EEL 3801C
Names, labels
• The period “.” may only be used as the first character.
• Cannot use reserved names.
• Can be used to name variables. Such when put in
front of a memory allocation directive. Can also be
used to define a constant. For example:
count1
db
50
; a variable (memory
allocation directive db)
count2
equ
100
; a constant
EEL 3801C
Names, labels
• Can be used as labels for statements to act as place
markers to indicate where to jump to. Can identify a
blank line. For example:
label1:
mov
ax,10
mov
bx,0
jmp
label1
.
.
.
label2:
EEL 3801C
; jump to label1
Mnemonics
– Mnemonic: identifies an instruction or
directive. These were described above.
• The mnemonics are standard keywords of the
assembly language for a particular processor.
• You will become familiar with them as time goes on
this semester.
EEL 3801C
Operands
– Operands are various pieces of information that
tells the CPU what to take the action on.
Operands may be a register, a variable, a
memory location or an immediate value.
10  immediate value
count  variable
AX  register
[0200]  memory location
– Comments: Any text can be written after the
statement as long as it is preceded by the “;”.
EEL 3801C
Elements of Assembly Language
for the 8086 Processor
• Assembler Character Set These are used
to form the names, mnemonics, operands,
variables, constants, numbers etc. which are
legal in 8086 assembly.
• Constant: A value that is either known or
calculated at assembly time. May be a
number or a string of characters. Cannot be
changed at run time.
EEL 3801C
Elements of Assembly Language…
(cont.)
• Variable: A storage location that is
referenced by name. A directive must
be executed identifying the variable
name with the location in memory.
• Integers: Numeric digits with no
decimal point, followed by the radix
mentioned before (e.g., d, h, o, or b).
Can be signed or unsigned.
EEL 3801C
Elements of Assembly Language…
(cont.)
• Real numbers: floating point number made
up of digits, a decimal point, an optional
exponent, and an optional leading sign.
(+ or -) digits.digits [exponential (+ or -)] digits
EEL 3801C
Elements of Assembly Language…
(cont.)
• Characters and strings:
• A character is one byte long.
• Can be mapped into the binary code equivalent
through the ASCII table, and vice-versa.
• May be enclosed within single or double quotation
marks.
• Length of string determined by number of characters
in string, each of which is 1 byte. For example:
EEL 3801C
Elements of Assembly Language…
(cont.)
“a” – 1 byte long
‘b’ – 1 byte long
“stack overflow” – 14 bytes long
‘abc#?%%A’ – 8 bytes long
EEL 3801C
Example of Simple Assembly
Program
• The following simple program will be
demonstrated and explained:
mov ax,5
; move 5h into the AX register
add ax,10
; add 10h to the AX register
add ax,20
; add 20h to the AX register
mov sum,ax
; store value of AX in variable
; ending the program
mov ax, 4C00H
int 21
EEL 3801C
Example (cont.)
• The result is that at the end of the program,
the variable sum, which exists somewhere in
memory (declaration not shown), now
accepts the value accumulated in AX,
namely, 35h.
• Explain program.
EEL 3801C
More Complex Example (cont.)
1:title Hello World Program (hello.asm)
2:
3:; this program displays “Hello, World”
4:
5:dosseg
6:.model small
7:.stack 100h
8:
9:.data
10: hello_message db ‘Hello, World!’,0dh,0ah,’$’
11:
EEL 3801C
More Complex Example (cont.)
12:
.code
13: main
proc
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
main
24:
end
mov ax,@data
mov ds,ax
mov ah,9
mov dx,offset hello_message
int 21h
mov ax,4C00h
int 21h
endp
main
EEL 3801C
More Complex Example (cont.)
The program is explained as follows:
• Line 1: Contains the title directive. All characters
located after the title directive are considered as
comments, even though the ; symbol is not used.
• Line 3: Comment line with the ; symbol
• Line 5: The dosseg directive specifies a standard
segment order for the code, data and stack segments.
The code segment is where the program instructions
are stored. The data segment is where the data
(variables) are stored. The stack segment is where
the stack is maintained.
EEL 3801C
More Complex Example (cont.)
• Line 6: The .model directive indicates the memory
architecture to be used. In this case, it uses the
Microsoft small memory architecture. It indicates this
by the word small after .model.
• Line 7: This directive sets aside 100h of memory for
the stack. This is equivalent to 256 bytes of memory
(162 = 256).
• Line 9: The .data directive marks the beginning of
the data segment, where the variables are defined
and memory allocated to them.
EEL 3801C
More Complex Example (cont.)
• Line 10: The db directive stands for “define byte”,
which tells the assembler to allocate a sequence of
memory bytes to the data that follow. 0dh is a
carriage return and 0ah is the linefeed symbol. The $
is the required terminator character. The number of
memory bytes is determined by the data themselves.
Hello_message is the name of the variable to be
stored in memory, and the db allocates memory to it
in the size defined by the data following it.
• Line 12: The directive .code is the indication of the
beginning of the code segment. The next few lines
represent instructions.
EEL 3801C
More Complex Example (cont.)
• Line 13: The proc directive is used to declare the
main procedure called “main”. Any name could have
been used, but this is in keeping with the C/C++
programming requirement that the main procedure be
called the main function. The first executable
instruction following this directive is called the
program entry point - the point at which the program
begins to execute.
EEL 3801C
More Complex Example (cont.)
• Line 14: The instruction mov is used to copy the
contents of one address into the other one. The first
operand is called the destination address, while the
second one is called the source address. In this
particular use, we tell the assembler to copy the
address of the data segment (@data) into the AX
register.
• Line 15: Copies the content of AX into DS, which is a
register used to put the data segment, the default
base location for variables.
EEL 3801C
More Complex Example (cont.)
• Line 17: This instruction places the value 9 in the AH
register. Remember that from the page.com program,
this is the register used to store the name of the DOS
subroutine to be called with the int 21 instruction.
• Line 18: This instruction places the address of the
string to be identified in the DX register. Remember
that this is the offset, where the default base segment
is already identified in the DS register as the base
segment for the data segment. Since the address of
the variable hello_message begins at the beginning of
the data segment, identified by DS, we only need to
supply the offset for the variable.
EEL 3801C
More Complex Example (cont.)
• Line 19: The instruction int 21, as we saw before,
takes the name of the function from the DX register,
which in this case is 9. DOS funtion 9, incidentally,
sends the contents of DX register to the VRT output
device. The DX register contains the address of the
string to be sent.
• Line 21 and 22: These instructions represent the
equivalent of an end or stop statement. This is
different from that done for page.com because this
will be an executable program (.exe), rather than a
.com program. More on this later.
• Line 23: Indicates the end of the main procedure.
EEL 3801C
More Complex Example (cont.)
• Line 24: The END directive – the last line to be
assembled. The main next to it indicates the program
entry point.
EEL 3801C
More Complex Example (cont.)
– The program may seem overly complicated for
such a simple program.
– But remember that assembly language
corresponds one-to-one with machine language
instructions.
– Note that it takes only 562 bytes of memory
when assembled and compiled.
EEL 3801C
More Complex Example (cont.)
– The same program written in a high level
language will require several more machine level
instructions to carry out the same thing.
– Written in Turbo C++, the executable program
would take 8772 bytes of memory to store.
EEL 3801C
Specifics of ALP – Data Definition
Directives
• A variable is a symbolic name for a location
in memory. This is done because it is easy
to remember variables, but not memory
locations. It is like an aka, or a pseudonym.
EEL 3801C
Data Definition Directives
• Variables are identified by labels. A label
shows the starting location of a variable’s
memory location. A variable’s offset is the
distance from the beginning of the data
segment to the beginning of the variable.
EEL 3801C
Data Definition Directives (cont.)
• A label does not indicate the length of the
memory that the variable takes up.
• If a string is being defined, the label offset is
the address of the first byte of the string
(the first element of the string).
– The second element is the offset + 1 byte.
– The third element is offset +2 bytes.
EEL 3801C
Data Definition Directives (cont.)
• The amount of
memory to be
allocated is
determined by
the directive
itself.
DB
Define byte
DW
Define word
DF
DP
Define
doubleword
Define far
pointer
DQ
Define quadword
DT
Define tenbytes
DD
EEL 3801C
Define Byte
• Allocates storage for one or more 8-bit
values (bytes). Has the following format:
[name] DB initialvalue [,initialvalue]
• The name is the name of the variable.
Notice that it is optional.
EEL 3801C
Define Byte (cont)
• initialvalue can be one or more 8-bit numeric
values, a string constant, a constant
expression or a question mark.
• If signed, it has a range of 127 to –128, If
unsigned, it has a range of 0 – 255.
EEL 3801C
Define Byte - Example
char
signed1
signed2
unsigned1
unsigned2
db
db
db
db
db
‘A’
-128
127
0
255
;
;
;
;
;
ASCII character
min signed value
max signed value
min unsigned value
max signed value
EEL 3801C
Define Byte (cont)
• Multiple values: A sequence of 8-bit
numbers can be used as initialvalue. They
are then grouped together under a common
label, as in a list.
• The values must be separated by commas.
list db 10,20,30,40
EEL 3801C
Define Byte (cont)
•
•
•
•
•
The 10 would be stored at offset 0000;
20 at offset 0001;
30 at offset 0002; and
40 at offset 0003,
where 0001 represents a single byte.
EEL 3801C
Define Byte (cont)
• A variable’s value may be left undefined.
This can be done by placing a ‘?’ for each
byte to be allocated (as in a list).
count db ?
EEL 3801C
Define Byte (cont)
• A string may be assigned to a variable, each
of whose elements will be allocated a byte.
c_string
db
‘This is a long string’
• The length of a string can be automatically
determined by the assembler by the ‘$’
symbol.
• See page 65 of new book for details.
EEL 3801C
Define Word
• Serves to allocate memory to one or more
variables that are 16 bits long. Has the
following format:
[name] DW initialvalue [,initialvalue]
• The name is the name of the variable.
Notice that it is optional.
• initialvalue can be one or more 16-bit
numeric values, a string constant, a constant
expression or a question mark.
EEL 3801C
Define Word (cont)
• If signed, it has a range of 32,767 to –
32,768,
• If unsigned, it has a range of 0 – 65,535.
EEL 3801C
Define Word (Example)
var
signed1
signed2
unsigned1
unsigned2
var-bin
var-hex
var-mix
dw
dw
dw
dw
dw
dw
dw
dw
1,2,3 ; defines 3 words
-32768 ; smallest signed value
32767 ; largest signed value
0
; smallest unsigned value
65535 ; largest signed value
1111000011110000b
4000h
1000h,4096,’AB’,0
EEL 3801C
Reverse Storage Format
• The assembler reverses the bytes in a word
when storing it in memory.
– The lowest byte is placed in the lowest address.
– It is re-reversed when moved to a 16-bit register.
value
dw
B6
2A
EEL 3801C
2AB6h
Define Doubleword – DD
• Same as DB and DW, except the memory
allocated is now 4 bytes (2 words, 32 bits).
[name] DD initialvalue [,initialvalue]
• The name is the name of the variable.
Notice that it is optional.
• initialvalue can be one or more 32-bit
numeric values, either in dec., hex or bin.
form, string const., a const. Expression, or ?
EEL 3801C
Multiple Values
• A sequence of 32-bit numbers can be used
as initialvalue.
• They are then grouped together under a
common label, as in a list.
• The values must be separated by commas.
EEL 3801C
Reverse Order Format
• As in define word, the bytes in doubleword
are stored in reverse order as in DW.
• For example,
var
78
dd
12345678h
56 34 12
EEL 3801C
Type Checking
• When a variable is created, the assembler
characterizes it according to its size (i.e., byte,
word, doubleword, etc.).
• When a variable is later referenced, the
assembler checks its size and only allows
values to be stored that come from a register
or other memory that matches in size.
• Mismatched movements of data not allowed.
EEL 3801C
Data Transfer Instructions – mov
• The instruction mov is called the data
transfer instruction.
• A very important one in assembly - much
programming involves moving data around.
• Operands are 8- or 16-bit on the 8086,
80186 and 80286.
• Operands on the 80386 and beyond, they
can also be 32-bits long.
EEL 3801C
Data Transfer Instructions – mov
(cont)
• The format is:
mov
destination,source
– The source and destination operands are selfexplanatory.
• The source can be an immediate value (a constant), a
register, or a memory location (variable). It is not
changed by the operation.
• The destination can be a register or a memory
location (variable).
EEL 3801C
Operands
• Register Operands: Transfer involving only
registers. It is the fastest.
• Source = any register
• Destination = any register except CS and IP
• Immediate Operands
• Immediate value can be moved to a register (all but
IP) or to memory.
• Destination must be of same type as source.
EEL 3801C
Operands (cont.)
• Direct operands
• A variable may be one of the two operands, but not
both. It does not matter which is the variable.
EEL 3801C
Limitations on operands
• There are some limitations on mov:
• CS or IP not destination operand
• Moving immediate data to segment registers.
• Moving from segment register to segment register.
• Source and destination operands of different types.
• Immediate value as destination (obviously!!)
• Memory to memory moves
EEL 3801C
Sequential Memory Allocation
• Memory for variables is allocated sequentially
by the assembler.
• If we call DB several times, such as in:
var1 db 10
var2 db 15
var3 db 20
EEL 3801C
Sequential Memory Allocation
(cont)
• var1 will be the first byte in the data
segment of main memory.
• This segment may be identified by the base
segment and the offset.
• var2 will occupy the next available memory
location, or 1 byte away from the beginning
of the data segment in memory.
EEL 3801C
Sequential Memory Allocation
(cont)
• var 3 will be 2 bytes away from this
starting point.
• This will be the case even if the memory
locations are not labeled, such as in:
db
db
db
10
20
30
EEL 3801C
Offsets
• Many times, memory will be allocated, but
not labeled.
• This is typical of an array, when only the
entire array is labeled, not each cell.
• The address of the array is the address of
the first element (position) of the array.
• All subsequent cells are allocated by adding
an offset to the address of the head element.
EEL 3801C
Offsets
• This is also true when a list of elements is
defined through DB, DW, or DD.
– Example: an array or list of 8-bit numbers whose
memory location is called a-list.
• To access the first element of a-list, we reference the
location in memory corresponding to a-list.
• To access any of the other elements of the array, we
provide an offset to the address of array.
– The second element at array+1, the third at array+2, the
fourth at array+3, etc.
EEL 3801C
Offsets
• To move the value of the 5th element of the
array to register AL:
mov
al
array+4
• The size of the two operands must match.
– Otherwise, an error may result.
• Note that AL is used, not AX - 1 byte.
EEL 3801C
PTR Operator
• Used to clarify an operand’s type. NOT a
pointer.
• It can be placed between the mov command
and the operands, as for example: Used for
readability only. It does not change the size
of the operand in any way.
mov word ptr count,10
mov byte
ptr var2,5
EEL 3801C
XCHG Instruction
• Allows the direct exchange of values
between 2 registers or between a register
and a memory location.
– Very fast
– Needs to obey size constraints
– Used in sorting.
EEL 3801C
Stack Operations
• Already discussed what a stack is.
• Each position in the stack is 2 bytes long –
only 16-bit registers can be copied into the
stack.
• The “bottom” of the stack is in high memory,
and it grows downward.
• Typically, 256 bytes are allocated to the
stack, enough for 128 entries.
EEL 3801C
Stack Operations (cont)
• Identified by the SS register (stack
segment), which identifies the address of the
base location of the stack segment.
• The stack pointer (SP register) indicates the
address of the first element of the stack (top
of the stack).
EEL 3801C
Push Operation
• Puts something (a 16-bit element) at
the top position of the stack.
• Decrements stack pointer (it grows
downward).
• Can put the contents of a register or of
memory (a variable)
• In 80286 and later processors, it can
also place an immediate value.
EEL 3801C
Push Operation
• Has the following form:
push
push
push
ax
ar1
1000h
EEL 3801C
Pop Operation
• The opposite of the push operation.
• Removes the top element in the stack.
• Copies value of top element in stack to
destination indicated in the statement.
• Increments stack pointer.
• Registers CS (code segment) and IP
(instruction pointer) cannot be used as
operands.
EEL 3801C
Pop Operation
• Has the following form:
pop
pop
ax
ar2
EEL 3801C
PUSHF and POPF
• Special instructions that move and remove
the contents of the Flags register onto and
out of the stack.
• These are used to preserve the contents of
these registers in case it is changed and the
old values are to be reinstated.
• See page 56.
EEL 3801C
PUSHA (80186+) and PUSHD
(80386+)
• Pushes the contents of the registers AX, CX,
DX, BX, original SP, BP, SI, and DI on the
stack in this exact order.
• PUSHD does the same for 32-bit registers.
EEL 3801C
POPA and POPD
• Pops the same registers in the reverse order.
EEL 3801C
Arithmetic Instructions
• Form the heart of any program, at any level
of abstraction.
• Integer arithmetic done in 8- or 16-bit
operands.
EEL 3801C
Arithmetic Instructions
– Floating point arithmetic done in one of three
ways:
• 80x87 math coprocessor
• software routines that emulate the coprocessor
• software that converts floating point to integer,
executes the instruction and then converts back to
floating point. (not particularly useful in many cases).
– Section concentrates on integer arithmetic
EEL 3801C
INC and DEC Instructions
• Respectively increment and decrement
the operand by one.
• Form is as follows:
inc
inc
inc
1
dec
1
dec
dec
al ; incr. 8-bit register by 1
bx ; incr. 16-bit register by 1
mem
; incr. memory location by
mem
; decr. memory location by
bx ; decr. 16-bit register by 1
al ; decr. 8-bit register by 1
EEL 3801C
INC and DEC Instructions
• Faster than ADD and SUB instructions
• All status flags are affected except the Carry
Flag.
EEL 3801C
ADD Instruction
• Used to add two numbers together.
• Format is as follows:
add
destination,source
• The high level language equivalent of:
destination = destination + source
EEL 3801C
ADD Instruction
• Takes 2 operands, a source and a
destination.
• Adds the contents of the operands.
• Places the result in the destination.
• Only one memory operand may be used.
• Source may be immediate value, but not
destination.
EEL 3801C
ADD Instruction (cont.)
• The contents of the source remain
unchanged.
• A segment register may not be a destination.
• All status flags are affected.
• Sizes of operands must match (same size).
EEL 3801C
SUB Instruction
• Subtracts a source operand from a
destination operand and stores the result in
the destination.
• Format is as follows:
sub destination,source
• The high level language equivalent of:
destination = destination + (source)
EEL 3801C
SUB Instruction (cont.)
• Takes the two’s complement of source and
adds it to the destination.
• Only one of the operands may be a memory
operand.
• Size of operands must match (same size).
• Segment register may not be the destination
operand.
EEL 3801C
SUB Instruction (cont.)
• Only one of the operands may be a memory
operand.
• Immediate values cannot be destinations.
EEL 3801C
Effects of SUB and ADD on Flag
Registers
• Why the flags?
– If an operation such as ADD overflows (i.e.,
number to be put in the destination exceeds the
size of the destination), then overflow.
– The flag may indicate that the value of the
destination is meaningless.
– The ADD instruction, therefore, affects both the
Carry and Overflow flags.
– Only 1 of these flags (if signed or if unsigned).
EEL 3801C
Flag Registers (cont.)
– Zero Flag: Set when the result of the
instructions inc, dec, add or sub = 0.
– Sign Flag: Set when the result of the
instructions inc, dec, add or sub < 0
– Carry Flag: Used with unsigned arithmetic only,
even though the processor updates it even if
operation is signed.
• For example:
EEL 3801C
Flag Example
mov
ax,00FFh
add
al,1
; AX = 0000h, CF=1
00FFh + 0001h = 0100h
– Since the operand of the add instruction is 8-bit
(AL register), this is an 8-bit operation.
– Therefore, the operation looks more like
FFh + 01h = 100h
EEL 3801C
Flag Example (cont.)
– This is an overflow, so the Carry Flag is set to
indicate that the result was larger than the 8-bit
destination could store.
– Could be fixed by using a 16-bit operation (e.g.,)
add
ax,1
; now it’s a 16-bit operation
; AX = 0100h CF = 0
EEL 3801C
Flag Example (cont.)
• If result of operation is too large for the destination
operand, the Carry flag is set.
• Can also occur in a subtraction operation when
subtracting a larger operand from a smaller one
(signed results are not permissible).
mov
sub
ax,5501h
al,2
; AX = 55FFh
CF=1
• We subtracted 2 from 1, resulting in a negative
number in an unsigned operation.
EEL 3801C
Flag Registers (cont.)
• Overflow Flag: Set by the processor
regardless of the type of operation, but
important only when operation is signed.
– Signed results are useless when this flag is set
– Can result when using add or sub
– But not inc or dec.
EEL 3801C
Flag Registers - Example
• If we add 126 + 2 = 128, in an 8-bit
operation, this will represent an overflow.
• This operation in binary numbers:
01111111 + 00000010 = 10000000
• Equivalent to the two’s complement of –128,
which is completely incorrect, therefore
meaningless in signed integers.
• Therefore, OF = 1.
EEL 3801C
Flag Registers - Example (cont.)
• In the case of subtraction, it gets a little
more complicated.
– If we subtract 2 from –128:
– -128 – 2 = -130 which is an overflow, as it
does not fit in an 8-bit destination.
– In binary, this looks like this:
10000000 (-128 in two’s complement) +
11111110 (-2 in twos’ comp)
= 1 01111110
EEL 3801C
Flag Registers - Example (cont.)
• The first 1, of course, is a ninth digit, which
does not fit into the 8-bit register.
• Thus, the operation looks like it resulted in
01111110
• which is –126.
• So, the OF = 1.
EEL 3801C
Addressing Modes - the OFFSET
• Five (5) different addressing modes in the
intel 8086 family of processors.
• Introduce an operator called offset, which
places the address (actually, the offset) of a
variable in a register.
• To move the address of a variable into a
register, but do not know it directly, the
OFFSET operator returns variable’s address
EEL 3801C
Addressing Modes - Example
• The assembler always knows the address of
each variable (i.e., label).
• For example,
mov
ax,offset
EEL 3801C
avariable
Direct Addressing Mode
• Returns the contents of memory at the offset
of a variable.
• The assembler keeps track of the offset of
every label (variable).
• Thus, by simply referencing the label, the
contents of the memory location assigned to
that label can be accessed.
EEL 3801C
Direct Addressing Mode (cont.)
• The effective address (EA) is the offset (i.e.,
distance) from the beginning of a segment.
• A displacement is either the absolute
address, or the offset of a variable.
EEL 3801C
Direct Addressing Mode Example
count db 20
mov al,count ; AL = 20 moved the
;contents of the label
;count into AL
• Can also be done by referencing the memory
address within brackets,
– for example, [200]
EEL 3801C
Indirect Addressing Mode
• Placing the address of the label in a base or
index register can create a pointer to a label.
• Typically done to represent a complex data
structure, such as an array or a structure.
• We can increment the pointer to point to
elements of the data structure.
EEL 3801C
Indirect Addressing Mode Example
• An array such as shown below.
A B C D E F G H I J K L M N
• Let us define this array as a string in the
following manner:
astring db “ABCDEFGHIJKLMN”
EEL 3801C
Indirect Addressing Mode Example
• To access the 6th element (F), set the
value of a register to the first element of
the array (string).
mov
bx,offset
astring
; moved the
;offset
• Then increment the memory address or
offset, by 5, such as
add
mov
bx,5; add 5 bytes to address in BX
dl,[bx] ; move content of BX into DL
EEL 3801C
Indirect Addressing Mode Example
• If the BX, SI or DI registers are used, the
effective (absolute) address is
understood to be an offset from the DS
register (by default).
• If BP register is used, it is understood to
be an offset from the SS register.
• One can change the default by placing
before the register the base register from
which the offset is calculated.
EEL 3801C
Indirect Addressing Mode Example
• For example:
• To use the BP register, but calculate its
address from the DS register rather than
from the SS:
mov
dl,ds:[bp]; looks in data segment DS
EEL 3801C
Based and Indexed Addressing
Mode
• A register value and a displacement can be
used as addresses to access the contents of
memory.
• The displacement is either a number, or the
offset of a label whose address (offset) is
known by the assembler at compile time.
EEL 3801C
Based and Indexed Addressing
Mode - Example
• For example, to access the number 8:
5 7 1
6
2
8
7
9
0
anarray db 5,7,1,6,2,8,7,9,0
mov bx,5
mov al,anarray[bx]
or
mov al,[anarray+bx]
or
mov al,[anarray+5]
EEL 3801C
Base-Indexed Addressing Mode
• Combining a base register and an index
register forms the effective address.
• Useful for defining 2 dimensional arrays.
• Similar to the Base and indexed, except now
instead of having a displacement that is
either a constant or the address of a label, it
is now the sum of the base and index
registers.
EEL 3801C
Base-Indexed - Example
a2darray
db
10,20,30,40,50
db
60, 70, 80, 90, A0
db
B0, C0, D0, E0, F0
• If we want to access the third element of the
second row, we set a base segment to the
first element of the second row, and then the
offset is the third column.
EEL 3801C
Base-Indexed - Example (cont.)
• For example, let’s say we want to get the
‘80’:
10 20
F0
mov
add
mov
mov
30 40 50 60 70 80 90 A0 B0 C0 D0 E0
bx,offset a2darray
bx,5; base=1st element of row 2
si,2
;offset = 2
al,[bx+si] ; moves the
;value 80 into AL
EEL 3801C
Base-indexed with Displacement
Addressing Mode
• The combination of the 2 addressing modes
above.
• Combines the displacement of a label with
that of a base and index segments.
• The same ‘80’ value in the above array can
be accessed with the operand:
mov
al, array[bx+si]
EEL 3801C
Program Structure
• Program divided into 3 primary segments:
• code segment (pointed to by register CS)
• data segment (pointed to by register DS)
• stack segment (pointed to by register SS)
• Segments can range in size between 10h (16)
bytes and 64K bytes.
• Instructions or data located on these
segments is referenced through the offsets
from the base segment (displacements).
EEL 3801C
Program Structure (cont.)
• Each segment is identified by the .stack,
.code, and .data indicators.
• Note that the SP register points to the next
memory location after the end of the stack
segment.
– This is because the stack grows from high
memory to low memory.
EEL 3801C
Program Structure (cont.)
• The title directive identifies the program
title (up to 128 characters long).
• The dosseg directive tells the assembler to
place the program segments in the standard
order used by the high level languages.
• The .model directive identifies the type of
memory model to be used.
EEL 3801C
Program Structure (cont.)
• The memory models are based on the idea
that a 16-bit address register can only
represent approximately 64K bytes of
memory range.
• Thus, if we have a segment of memory that
is less than or equal to 64K bytes, then we
can easily access that location through a
simple offset from the base segment.
EEL 3801C
Program Structure (cont.)
• This means quicker addressing, since only 1
instruction is necessary to load a 16-bit
address.
• If greater than 64K, however, then we must
change the base segment register as well, as
the offset cannot reach beyond 64K bytes.
• This requires 2 machine instructions, one for
the base segment, and one for the offset.
EEL 3801C
Program Structure (cont.)
• The various models are the following:
– Tiny:
Code + data <= 64K
– Small: Code <= 64K; data <= 64K
– Medium:
Data <= 64K; Code any size
– Compact: Code <= 64K; Data any size
– Large: No restrictions on either one, but arrays
<= 64K
– Huge: No restrictions on any of the three
EEL 3801C
Program Structure (cont.)
• The Tiny model does not result in an
executable file, but rather, in a command file
(.com).
• All others result in .exe files.
• The linker produces a map file which
indicates the location of these segments.
EEL 3801C