Assembler programming |
Building Assembler programs
A D V E R T I S E M E N T
In order to be able to create a program, several tools are needed:
First an editor to create the source program. Second a compiler, which is
nothing more than a program that "translates" the source program into an object
program. And third, a linker that generates the executable program from the
object program.
The editor can be any text editor at hand, and as a compiler we will use the
TASM macro assembler from Borland, and as a linker we will use the Tlink
program.
The extension used so that TASM recognizes the source programs in assembler
is .ASM; once translated the source program, the TASM creates a file with the .OBJ
extension, this file contains an "intermediate format" of the program, called
like this because it is not executable yet but it is not a program in source
language either anymore. The linker generates, from a
.OBJ or a combination of several of these files, an executable program, whose
extension usually is .EXE though it can also be .COM, depending of the form it
was assembled.
Assembler Programming
To build assembler programs using TASM programs is a different program
structure than from using debug program.
It's important to include the following assembler directives:
.MODEL SMALL
Assembler directive that defines the memory model to use in the program
.CODE
Assembler directive that defines the program instructions
.STACK
Assembler directive that reserves a memory space for program instructions
in the stack
END
Assembler directive that finishes the assembler program
Let's program
First step
use any editor program to create the source file. Type the following lines:
first example
; use ; to put comments in the assembler program
.MODEL SMALL; memory model
.STACK; memory space for program instructions in the stack
.CODE; the following lines are program instructions
mov ah,1h; moves the value 1h to register ah
mov cx,07h;moves the value 07h to register cx
int 10h;10h interruption
mov ah,4ch;moves the value 4 ch to register ah
int 21h;21h interruption
END; finishes the program code
This assembler program changes the size of the computer cursor.
Second step
Save the file with the following name: examp1.asm Don't forget to save this
in ASCII format.
Third step
Use the TASM program to build the object program.
Example:
C:\>tasm exam1.asm
Turbo Assembler Version 2.0 Copyright (c) 1988, 1990 Borland International
Assembling file: exam1.asm
Error messages: None
Warning messages: None
Passes: 1
Remaining memory: 471k
The TASM can only create programs in .OBJ format, which are not executable by
themselves, but rather it is necessary to have a linker which generates the
executable code.
Fourth step
Use the TLINK program to build the executable program example:
C:\>tlink exam1.obj
Turbo Link Version 3.0 Copyright (c) 1987, 1990 Borland International
C:\>
Where exam1.obj is the name of the intermediate program, .OBJ. This generates
a file directly with the name of the intermediate program and the .EXE
extension.
Fifth step
Execute the executable program
C:\>exam1[enter]
Remember, this assembler program changes the size of the cursor.
Assembly process.
SEGMENTS
The architecture of the x86 processors forces to the use of memory segments
to manage the information, the size of these segments is of 64kb.
The reason of being of these segments is that, considering that the maximum
size of a number that the processor can manage is given by a word of 16 bits or
register, it would not be possible to access more than 65536 localities of
memory using only one of these registers, but now, if the PC's memory is divided
into groups or segments, each one of 65536 localities, and we use an address on
an exclusive register to find each segment, and then we make each address of a
specific slot with two registers, it is possible for us to access a quantity of
4294967296 bytes of memory, which is, in the present day, more memory than what
we will see installed in a PC.
In order for the assembler to be able to manage the data, it is necessary
that each piece of information or instruction be found in the area that
corresponds to its respective segments. The assembler accesses this information
taking into account the localization of the segment, given by the DS, ES, SS and
CS registers and inside the register the address of the specified piece of
information. It is because of this that when we create a program using the Debug
on each line that we assemble, something like this appears:
1CB0:0102 MOV AX,BX
Where the first number, 1CB0, corresponds to the memory segment being used,
the second one refers to the address inside this segment, and the instructions
which will be stored from that address follow. The way to indicate to the
assembler with which of the segments we will work with is with the .CODE, .DATA
and .STACK directives.
The assembler adjusts the size of the segments taking as a base the number of
bytes each assembled instruction needs, since it would be a waste of memory to
use the whole segments. For example, if a program only needs 10kb to store data,
the data segment will only be of 10kb and not the 64kb it can handle.
SYMBOLS CHART
Each one of the parts on code line in assembler is known as token, for
example on the code line:
MOV AX,Var
we have three tokens, the MOV instruction, the AX operator, and the VAR
operator. What the assembler does to generate the OBJ code is to read each one
of the tokens and look for it on an internal "equivalence" chart known as the
reserved words chart, which is where all the mnemonic meanings we use as
instructions are found.
Following this process, the assembler reads MOV, looks for it on its chart
and identifies it as a processor instruction. Likewise it reads AX and
recognizes it as a register of the processor, but when it looks for the Var
token on the reserved words chart, it does not find it, so then it looks for it
on the symbols chart which is a table where the names of the variables,
constants and labels used in the program where their addresses on memory are
included and the sort of data it contains, are found.
Sometimes the assembler comes on a token which is not defined on the program,
therefore what it does in these cased is to pass a second time by the source
program to verify all references to that symbol and place it on the symbols
chart.There are symbols which the assembler will not find since they do not
belong to that segment and the program does not know in what part of the memory
it will find that segment, and at this time the linker comes into action, which
will create the structure necessary for the loader so that the segment and the
token be defined when the program is loaded and before it is executed.
More assembler programs
Another example
first step
use any editor program to create the source file. Type the following lines:
;example11
.model small
.stack
.code
mov ah,2h ;moves the value 2h to register ah
mov dl,2ah ;moves de value 2ah to register dl
;(Its the asterisk value in ASCII format)
int 21h ;21h interruption
mov ah,4ch ;4ch function, goes to operating system
int 21h ;21h interruption
end ;finishes the program code
second step
Save the file with the following name: exam2.asm
Don't forget to save this in ASCII format.
third step
Use the TASM program to build the object program.
C:\>tasm exam2.asm
Turbo Assembler Version 2.0 Copyright (c) 1988, 1990 Borland International
Assembling file: exam2.asm
Error messages: None
Warning messages: None
Passes: 1
Remaining memory: 471k
fourth step
Use the TLINK program to build the executable program
C:\>tlink exam2.obj
Turbo Link Version 3.0 Copyright (c) 1987, 1990 Borland International
C:\>
fifth step
Execute the executable program
C:\>ejem11[enter]
*
C:\>
This assembler program shows the asterisk character on the computer screen
Types of instructions.
Data movement
In any program it is necessary to move the data in the memory and in the CPU
registers; there are several ways to do this: it can copy data in the memory to
some register, from register to register, from a register to a stack, from a
stack to a register, to transmit data to external devices as well as vice versa.
This movement of data is subject to rules and restrictions. The following are
some of them:
*It is not possible to move data from a memory locality to another directly;
it is necessary to first move the data of the origin locality to a register and
then from the register to the destiny locality.
*It is not possible to move a constant directly to a segment register; it
first must be moved to a register in the CPU.
It is possible to move data blocks by means of the movs instructions, which
copies a chain of bytes or words; movsb which copies n bytes from a locality to
another; and movsw copies n words from a locality to another. The last two
instructions take the values from the defined addresses by DS:SI as a group of
data to move and ES:DI as the new localization of the
data.
To move data there are also structures called batteries, where the data is
introduced with the push instruction and are extracted with the pop instruction.
In a stack the first data to be introduced is the last one we can take, this is,
if in our program we use these instructions:
PUSH AX
PUSH BX
PUSH CX
To return the correct values to each register at the moment of taking them
from the stack it is necessary to do it in the following order:
POP CX
POP BX
POP AX
For the communication with external devices the out command is used to send
information to a port and the in command to read the information received from a
port.
The syntax of the out command is:
OUT DX,AX
Where DX contains the value of the port which will be used for the
communication and AX contains the information which will be sent.
The syntax of the in command is:
IN AX,DX
Where AX is the register where the incoming information will be kept and DX
contains the address of the port by which the information will arrive.
Logic and arithmetic operations
The instructions of the logic operations are: and, not, or and xor. These
work on the bits of their operators.
To verify the result of the operations we turn to the cmp and test instructions.
The instructions used for the algebraic operations are: to add, to subtract sub,
to multiply mul and to divide div.Almost all the comparison instructions are
based on the information contained in the flag register. Normally the flags of
this register which can be directly handled by the programmer are the data
direction flag DF, used to define the operations about chains. Another one which
can also be
handled is the IF flag by means of the sti and cli instructions, to activate and
deactivate the interruptions.
Jumps, loops and procedures
The unconditional jumps in a written program in assembler language are given
by the jmp instruction; a jump is to moves the flow of the execution of a
program by sending the control to the indicated address.
A loop, known also as iteration, is the repetition of a process a certain
number of times until a condition is fulfilled.
