Download executing a program

Assembling, Linking and Executing Programs
Objective: To cover the steps in assembling, linking, and executing programs
The symbolic instructions that you code in assembly language are known as the source program. You
use an assembler program to translate the source program into machine code, known as the object
program. Finally, you use a linker program to complete the machine addressing for the object program,
generating an executable module.
The following figure provides a chart of the steps required to assemble link, and execute a program.
1. The assembly step involves translating the source code into object code and generating an
intermediate .OBJ (object) file, or module. One of the assembler’s tasks is to calculate the offsets
for every data item in the data segment and for every instruction in the code segment. The
assembler also creates a header immediately in front of the generated .OBJ module; part of the
header contains information about incomplete addresses.
2. The link step involves converting the .OBJ module to an .EXE machine code module. The linker’s
tasks include completing any addresses left open by the assembler and combining separately
assembled programs into one executable module.
3. The last step is to load the program for execution. Because the loader knows where the program is
going to load in memory, it is now able to resolve any remaining address still left incomplete in the
header. The loader drops the header and creates a program segments prefix (PSP) immediately
before the program loaded in memory.
Assembling a Source program
The assembler converts your source statements into machine code and displays any error messages on
the screen. Optional output files from the assembly step are object (.OBJ), listing (.LST), and cross
reference (.CRF or .SBR). You probably often request an .LST file, especially when it contains error
diagnostics or you want to examine the generated machine code.
The Microsoft MASM 6.1 assembler use ML command. The ML command allows you to assemble
and link any number of programs into one executable module. The format for the ML command to
assemble and link is
ML [options] filename.ASM [[options] filename.ASM] ….. [/LINK options]
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Command-line options are case-sensitive and begin with a slash (/) character.
/c
/Fl
/Fr
/Sn
/Zd
Assemble, do not link
Generate a listing (.LST)
Generate a .SBR (cross-reference) file
Suppress listing of the symbol table
Include line number debugging information
A useful command is simply ML -?, which displays the command-line syntax and options
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Using Conventional Segment Definitions
Stack segment
This definition of 32 words is a realistic size for a stack because a large program may require many
interrupts for input/output and calls to subprograms, all involving use of the stack.
If the stack is too small to contain all the items pushed into it, neither the assembler nor the linker
warns you, and the executing program may crash in an unpredictable way.
Data segment.
 FLDD defines a word (two bytes) initialized with decimal value 215
 FLDE defines a word initialized with decimal value 125
 FLDF is coded as a DW with ? in the operand to define a word with an uninitialized constant.
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Code Segment
The assembler recognizes the reference to a segment and assumes its address. Note the machine code to
the left: B8----R. The four hyphens mean that at this point the assembler cannot determine the address
of DATASEG; the system determines this address only when the object program is linked and load for
execution. Because the loader may locate a program anywhere in memory, the assembler leaves the
address open and indicates the fact with an R (for relocatable).
Although the loader automatically initializes SS and CS when it loads a program for execution, it is
your responsibility to initialize DS, and ES if required.
Segments and Groups table
The table provides the length in bytes of each segment, the alignment(all the paragraphs), the combine
type, and the class.
Symbols table
This table provides the names of data fields in the data segment (FLDD, FLDE, and FLDF) and the
labels applied to instructions in the code segment. TYPE F PROC means far procedure. The Value
column gives the offset form the beginning of the segment for names, labels, and procedures.
Two-Pass Assembler
Assemblers typically make two or more passes through a source program in order to resolve forward
references to addresses not yet encountered in the program. During pass 1, the assembler reads the
entire source program and constructs a symbol table of names and labels used in the program. Pass 1
determines the amount of code to be generated for each instruction.
During pass 2, the assembler uses the symbol table that it constructed in pass 1. it knows the length and
relative position of each data field and instruction, it can complete the object code for each instruction.
MASM constructs object code based on what it supposes is the length of each generated machine
language instruction. If there are any differences between pass 1 and pass 2 concerning instruction
lengths, MASM issues an error message “Phase error between passes”.
Linking an object program
MASM 6.1 performs assemble and link with the ML command. The linker performs the following
functions:
Combines, if requested, more than one separately assembled module into one executable program, such
as two or more assembly programs or an assembly program with a C program.
Generate an .EXE module and initializes it with special instructions to facilitate its subsequent loading
for execution.
The output files from the link step are executable (.EXE), map (.MAP), and library (.LIB).
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EXECUTING A PROGRAM
If the .EXE file is in the default drive, you could ask the loader to read it into memory by typing
“A05ASM1”
If you omit typing the file extension, the loader assumes it is an executable .EXE and .COM program.
Run it under DEBUG and use Trace commands to step through its execution.
DEBUG n:A05ASM1.EXE


To view the stack segment, key in D SS:0
To view the code segment, key in D CS:0

To view the contents of the registers, press R followed by <Enter>
Error Diagnostics
The assembler provides diagnostics for any programming errors that violate its rules. The program in
Figure 5-5 is similar to the one in Figure 5-2, except that it has a number of intentional errors inserted
for illustrative purposes.
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