Assembly Language Basic Part-2
What is Assembly Language?
l Every PC has a microprocessor that
manages computer's ALU and control activities.
l Each processor has its own set of
instructions for handling various operations like Input and output.
l To operate this activity assembly language
is used.
Why Study Assembly Language?
l It is used to Interface of programs
with OS, processor and BIOS.
l Representation of data in memory and
other external devices.
l It requires less memory and
execution time.
l It allows hardware-specific complex
jobs in an easier way.
Assembler
l Assembler is a type of converter
accept assembly language as input and convert it into equivalent machine
language.
l There are following types of popular
assembler:
1. Netwide Assembler (NASM)
2. Microsoft Assembler (MASM)
3. Borland Turbo Assembler (TASM)
Assembly Basic Syntax
An assembly
program can be divided into three sections:
1. The data section
2. The bss (Block Started Symbol) section
3. The text section
1. The Data Section
l The data section is used for
declaring initialized data or constants.
l This data does not change at runtime.
l The syntax for declaring data
section is:
Syntax: section .data
2. The bss (Block Started Symbol) section:
l The bss section is used for
declaring variables.
l These variables are accessed from
keyboard and used to display result.
l The syntax for declaring bss section
is:
Syntax: section .bss
3. The text section
l The text section is used for keeping
the actual code.
l This section must begin with the
declaration global _start:
l Which tells the kernel where the
program execution begins.
l The syntax for declaring text section
is:
section .text
global _start:
_start:
Syntax of Assembly Language Statements:
l Assembly language statements are
entered one statement per line. Each statement follows the following format:
l [label] mnemonic [operands] [;comment]
l e.g.
abc1: mov ax,
00h ;initialize acc.
The Hello World Program!!!
section
.data
msg db
'Hello, world!', 10 ;our string, 10=\n like C.
len equ $ -
msg ;length
of our dear string
section
.text
global _start ;must be declared for
linker (ld)
_start: ;tells
linker entry point
mov edx,len ;message
length
mov ecx,msg ;message to write
mov ebx,1 ;file
descriptor (stdout)
mov eax,4 ;system
call number (sys_write)
int 80h ;call
kernel
mov eax,1 ;system call number (sys_exit)
int 80h ;call kernel
Compile and execute Program
>nasm -f
elf64 progname.asm ; Compile code
>ld -o
progname progname.o ;Load & create
obj
>./progname ;Execute program
Assembly Languages
Introduction-2
Different Registers
l Data Register
64-bit data register
l RAX
l RBX
l RCX
l RDX
Pointer Registers
Index Register
System Call in 32-bit
l Exit system call:
mov eax,1 ; system call number
(sys_exit)
Int 80h ; call kernel
l Write System call:
mov edx,4 ;message length
mov ecx,msg ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number
(sys_write)
int 80h ;call kernel
System Call 32-bit
|
Name
|
%eax
|
|
sys_read
|
0
|
|
sys_write
|
1
|
|
sys_open
|
2
|
|
sys_close
|
3
|
|
sys_exit
|
60
|
Data Section Data Type
|
Type Specified
|
Bytes addressed
|
|
DB-Define Byte
|
1
|
|
DW- Define Word
|
2
|
|
DD- Double Word
|
4
|
|
QD- Quad Word
|
8
|
|
TB- Tera Byte
|
10
|
BSS Section Data Type
|
Type Specified
|
Bytes addressed
|
|
RESB- Reserved Byte
|
1
|
|
RESW- Reserved Word
|
2
|
|
RESD- Reserve Double
Word
|
4
|
|
RESQ- Reserve Quad
Word
|
8
|
|
REST- Reserve Tera
Byte
|
10
|
Macros
l To avoid repetative code of block in
program macros are used.
l The Syntax for macro definition:
%macro macro_name
number_of_params
<macro body>
%endmacro
Read Macro 64-bit
%macro read
2
mov
rax,0 ;read
mov
rdi,0 ;stdin/keyboard
mov
rsi,%1 ;buf
mov
rdx,%2 ;buf_len
syscall
%endmacro
Write Macro 64-bit
%macro
display 2
mov
rax,1 ;print
mov
rdi,1 ;stdout/screen
mov
rsi,%1 ;msg
mov
rdx,%2 ;msg_len
syscall
%endmacro
Array in Assembly
l The data definition directives can
also be used for defining a one dimensional array.
l Let us define a one dimensional
array of numbers.
Ex.1 NUMBERS DW 34, 45, 56, 67,75, 89
Ex.2 Arr_Name TIMES 8 DW 0
Loop in Assembly
The JMP
instruction can be used for implementing loops. For example, the following code
snippet can be used for executing the loop-body 10 times.
MOV CL, 10
L1:
<LOOP-BODY>
DEC CL
JNZ L1
The
processor instruction set however includes a group of loop instructions for
implementing iteration.
Procedure
l Procedures or subroutines are very
important in assembly language, as the assembly language programs tend to be
large in size.
l Procedures are identified by a name.
l Syntax:
proc_name:
procedure
body
...
Ret
l The procedure is called from another
function by using the CALL instruction. The CALL instruction should have
l The name of the called procedure as
argument as shown below:
Syntax: CALL proc_name
Display Procedure in NASM
;display
procedure for 64bit
display: ;name of procedure,
similar to label
mov rsi,char_answer+15 ;Point
the RSI to the last position of char_answer variable,like array
mov rcx,16 ;RCX=16 will act as
counter for 16 digits result
cnt:
mov rdx,0 ;cnt=label, For division of RDX and RAX make
first RDX=0
mov rbx,16h ;RBX=16, RBX is the divisor. 16 in hex =
10 decimal & getlast digit reminder
div rbx ;Divide RbX,RAX by RBX
cmp dl,09h ;Remainder comes in DL register,
compare it with 9
jbe add30 ;If DL<=9, jump to label
add30
add dl,07h ;add 7 to DL.difference from 9 to
F.
add30: add dl,30h ;label add30, add 30 to dl, means
dl=dl+30 ASCII
mov [rsi],dl ;move the value in DL to the
address at which RSI
dec rsi ;decrement the address in
RSI,pointing to prev.loc.
dec rcx ;decrement
count in RCX
jnz cnt ;jump to cnt lab, till zero flag is not set
scall 1,1,char_answer,16 ;call scall macro to display answer
ret ;return statement
