Building Assembler programs
Assembly process
More assembler programs
Types of instructions
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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.
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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:
| Assembly directives |
Meaning |
| .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 |
use any editor program to create the source file. Type the following lines:
; 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.
Save the file with the following name: exam1.asm
Don't forget to save this in ASCII format.
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.
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.
Execute the executable program
C:\>exam1[enter]
Remember, this assembler program changes the size of the computer program.
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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.
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 upon 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.
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 the 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
Save the file with the following name: exam2.asm
Don't forget to save this in ASCII format.
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
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:\>
Execute the executable program
C:\>ejem11[enter] * C:\>
This assembler program shows the asterisk character on the computer screen
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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.
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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 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.
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The unconditional jumps in a written program in assembler language are given by the jmp instruction; a jump is to modify 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. These loops are used
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