Chapter 2 programming concepts - I

CHAPTER 2
PROGRAMMING CONCEPTS-
1

ASSEMBLY LANGUAGE
PROGRAMMING
To code efficiently in assembly
language for a particular processor
,the prerequisites are
a good knowledge of the internal
architecture of the processor and
addressing modes

THE ASSEMBLY PROCESS
An assembler is a translator that
translates source instructions( in
symbolic language)
into target instructions (in machine
language)
on a one to one basis.

FEATURES OF ASSEMBLERS
Labels are used for memory addresses
Labels are used for constants
Macros are allowed

INSTRUCTIONS AND DIRECTIVES
Instructions are executable statements
Directives are non-executable .
Directives are also called pseudo
instructions.
Directives aid the assembly process.

WHAT THE ASSEMBLER DOES
 It takes the source code (in assembly
language) and converts it to the
object code in machine language

THE FORWARD REFERENCE PROBLEM
In assemblers ,when a label which has not
yet been defined ,is encountered,
it is called the forward reference problem
or
 future symbol problem or
 the problem of unresolved references .

PASSES OF AN ASSEMBLER
 In the first reading or ‘pass’ of the assembler , it
looks for label definitions and inserts them in the
symbol table after assigning them addresses .
 In pass 2 , the actual translation of assembly code
to machine code is done .

ASSEMBLERS FOR X86
 NASM ,FASM ,MASM ,TASM and HLA
 FASM and NASM –can run under DOS, Linux and
Windows
 TASM and MASM are very popular
 It is found now that Windows 7 ,8 etc do not
directly support DOS based 16 bit programs –
so 16 bit assemblers may not work with
Windows 7 directly.

WHY MASM ?
 Microsoft has written considerable documentation
 Third parties have written assembly language
reference manuals for MASM.
 The versions of MASM 6.0 and above have a lot
more features (aimed at simplification in writing
code )than previous versions .
 Working Principle ?

MEMORY MODELS
 To write programs, we have to define segments
where segment registers must be initialized
 Will take to use the full segment model
 MASM 6.0 and above have incorporated certain
shortcuts to make programme simple
 They are called Dot Models.
 To specify a segment ,write
. MODEL MODEL NAME
 Different models tell the assemble how to use
segements and to provide sufficient space for the
object code.
 simplest way is tiny model and small model

 Tiny model is used when code, data and stack will all fit
into one segment with maximum of 64 K
 Small model can have one data segment and one code
segment each of which has a maximum size of 64 K
TINY Model
 Program
.MODEL TINY ;choose single segment model
.CODE ; start of code segment
.STARTUP ; start of program
MOV AL,67H ; move 67H to AL
MOV BL,45H ; move 45H to BL
ADD AL,BL ; add BL to DL
MOV DL,AL ; copy AL to DL
.EXIT ; exit to DOS
END ; program end

 Save it a as tiny.asm
 Open MASM and go to the BIN directory.Use the
following commands
ml tinym.asm ;for assembling and linking
ml/Fl tinym.asm ;for the list file

THE RESULT OF ASSEMBLING AND LINKING (
DOS COMMAND WINDOW)
C:masm6.14BIN>ml tinym.asm
Microsoft (R) Macro Assembler Version 6.14.8444
Copyright (C) Microsoft Corp 1981-1997. All rights
reserved.
Assembling: tinym.asm
Microsoft (R) Segmented Executable Linker Version
5.60.339 Dec 5 1994
Copyright (C) Microsoft Corp 1984-1993. All rights
reserved.
Object Modules [.obj]: tinym.obj /t
Run File [tinym.com]: "tinym.com"
List File [nul.map]: NUL
Libraries [.lib]:
Definitions File [nul.def]:

EXAMPLE SHOWING LIST FILE
.MODEL TINY ; choose single segment model
0000 .CODE ; start of code segment
.STARTUP ; start of program
0100 B0 67 MOV AL,67H ; move 67H to AL
0102 B3 45 MOV BL,45H ; move 45H to BL
0104 02 C3 ADD AL,BL ; Add BL to AL
0106 8A D0 MOV DL,AL ; copy AL to DL
.EXIT ; Exit to DOS
END ; assembler to stop reading
•On the left hand side, we see the offsets within the code segments in
which the code is saved.
• offsets are generated by assemblers
•We also see opcodes corresponding to each instruction
• example (1st Instruction), 0100H is the offset in the code segment
where the first instruction is stored
• BO 67 is the opcode of MOV AL, BH

 if we add directive.listall, the list file is changed as
shown below,

 .Listall has listed the instructions corresponding to
.EXIT
 .EXIT is a shortcut, which is actually transformed to
two instructions:
MOV AH,4CH
INT 21H

USING THE DEBUGGER
 To enter the debugger ,type debug tinym.com .
 We get an underscore as the prompt.
 On typing ‘r’, we can see the contents of the
registers ,before execution of the program .
 Now type ‘u’, which is the command for
unassembling. (i.e Logical address of the first
instruction)

C:masm6.14BIN>debug tinym.com
-r
AX=0000 BX=0000 CX=010C DX=0000 SP=0000 BP=0000
SI=0000 DI=0000
DS=13AD ES=13AD SS=13BD CS=13BD IP=0100 NV UP EI PL
NZ NA PO NC
13BD:0100 B067 MOV AL,67
-u
13BD:0100 B067 MOV AL,67
13BD:0102 B345 MOV BL,45
13BD:0104 02C3 ADD AL,BL
13BD:0106 8AD0 MOV DL,AL
13BD:0108 B44C MOV AH,4C
13BD:010A CD21 INT 21

COM AND EXE FILES
 com files have only one segment (CS)
 run file generated by the tiny model is a command
(.com) file, rather than an executable (.exe) file
 shown in example 2.2 ; Run File [tiny.com]:
“tinym.com”
 later seen that run files will be obtained as
‘executable’ with .exe
 A tiny model generates only a com file ,
while any other memory model generates an exe file

FEATURES OF A COM FILE
 Size is limited to 64K
 Only one segment ,which is the code segment
 Data is defined in this code segment
 Code starts at offset 0100 H ,just after the PSP
(program prefix segment ) of DOS
 Smaller file compared to exe files ,because it
does not have the 512 byte header block

 previous examples have seen the tactics of
running and analysing a single segment assembly
language program running in MASM
 We see now, the listing corresponding to another
program which uses tiny model

 Above Eg, shows a sequence of instructions that copy
various data between 16 and 8-bit registers
 The act of moving data from one register to another
changes only the destination register, never the
source.

 Listing and it shows various assembly language
instructions that use immediate addressing
 only 8-bit can be placed into 8-bit register
 16 bit copied to 16 bit registers
 look at 2nd instruction

 MOV AL, C
 here assembler translates ASCII ‘C’ to its hex equivalent
43H
 all data written in memory will be in hex format
 Thus, the decimal 45 is found to 2DH in the listing
 A data (byte, word) starting with the hex character
A,B,C,D,E,F must be preceded by a 0.
 else it gives error
 i.e MOV AL, EFH gives assembly error
 shud be rewritten as MOV AL, 0EFH
 Similarly MOV BX, C456H as MOV BX, 0C456H

DEFINITION OF DATA TYPES
 Before we go to two-segment model
 need to understand a few directives of the
assembler, that describes different kinds of data
 May be Bytes, Words etc..
 We have to define data and assign labels to their
corresponding addresses
 Defining data implies allocating space for data
 Data is accordingly using directives
 some of the Data Definitions used in MASM are:

 Eg shows data being placed in code segment itself
 In tiny model, we can have data and code in the same
segment, with the instruction that the size of the segemnt
should not exceed 64Kbytes
 NUM1, NUM2 and NUM3 are locations which store data
 NUM1 is a byte location, while NUM2 and NUM3 are word
locations

THE SMALL MODEL
 Here two segments are used –
A code segment and a data segment

Chapter 2 programming concepts - I

Data segment with labels and off sets corresponding

DUP DIRECTIVE
 is used to replicate a given number of characters.
 Eg, we need to fill up a number of locations in the data
segment with the same word or byte
 NUMS DB 10 DUP(0) fills up with 0s the 10 byte locations
starting with the label NUM
 STARS DB 5 DUP(‘#’) fills up 5-byte locations starting at
location STARS, with the ASCII value of the character #
 BLANK DB 10 DUP(?) reserves 10-byte spaces starting
from location with the label BLANK, but these are not
initialized, means whatever data is there will remain same
 WRDS DW 4 DUP(FF0FH) fills up 4 word locations with
the word FF0FH

EQU DIRECTIVE
 Used to equate names to constants
 The assembler just replaces the names by the values
mentioned
 Eg: TEMP EQU 34
PRICE EQU 199

ORG DIRECTIVE
 ORG is a directive which means ‘origin’.
 In the context of assembly language programming,
it can change the location of storage of data or
code in memory
 i.e programmer gets the freedom to decide the
offset of data or code when it is stored.

FULL SEGMENT DEFINITION
 Is a traditional model of MASM
 In the simplified memory model, it is left to the loader
software to initialize the segment registers
 In traditional model we use directives to define segments
and instructions to initialize the segment registers
 This is called full segment definition
 CS & SS registers are automatically initialized by the
loader
 DS & ES will have to be initialized by the programmer

LETS REWRITE THE BELOW PROGRAM USING FULL
SEGMENT DEFINITION

SALIENT FEATURES OF THIS MODEL
 the data segment has been given the name DAT. The data
within the segment is enclosed between the SEGMENT
and ENDS directives which are similar to the parentheses
for a segment
 similarly CS is named as COD and the contents of this
segment also have been enclosed between the same
directives
 DS register has been initialized by the first two instructions.
DS cannot use immediate addressing. So DAT corresponds
to a number, has to be loaded to AX, then transfer to DS
 The value of DAT is loaded into DS.
 CS register needs not to be initialized in the program. This
is done by the loader by default.

INSTRUCTION DESIGN
 We have discussed about assemblers and how to run those
programs
 Now lets study the core of the processor and investigate the
process of how machine codes have been designed.
 Manual Coding is a idea of thinking the possibility of doing
manual or hand coding, as it is called taking an assembly
instructions, looking up or finding out its machine code, and feeding
directly to the processor.
 It was happening during 8085 processor as it has limited
instruction sets and look up table was sufficient for hand coding.
 Instruction Set Architecture (ISA) gives almost appropriate
definition, defined as the part of computer architecture related to
programming, including the native data types, instructions,
registers, addressing modes, memory Arch., interrupt and
exception handling, and external I/O. It includes a specification of
the set of opcodes (machine language), i.e., the native commands
implemented by a particular CPU design.

INSTRUCTION SET DESIGN OF 8086
 8086 has instruction size varies from one byte to five bytes
 This makes the processes of assembly, disassembly and
instruction decoding complicated because the instruction
length needs to be calculated for each instruction.
 An instruction should have the complete information for
fetching and executing an instruction.
 It should have the following information
1. Opcode corresponding to the operation carried out
2. Size of the Operands
3. Various addressing modes

FORMAT OF THREE BYTES OF AN
INSTRUCTION

PREFIX:
 This is an optional byte and need to be
used only to change the operation, e.g. segment
override prefix

FIRST BYTE
 Considered as the first byte of an instruction
 Here the operation code (opcode) which is 6 bytes long.
 This is the code which defines the operation to be carried
out. The two other bits are D & W
 W (1-bit) – operand size.
W = 1, word operand; W = 0, means byte operand.
 D (1-bit) – Direction bit.
D = 1, register is destination;
D = 0, register is source

SECOND BYTE
 MOD (2-bit) – Register bits.
REG (3-bit) – the identifying code of the register used.
R/M (3 bits) – Specifying a register or memory operand.
The MOD and R/M bits together specify the addressing
mode of the instruction.
 All the instructions need not have the D & W bits. In such cases,
the size of the operand is implicit, and the direction is irrelevant.

DESIGNING A CODE
 Requires a lot of information such as codes or registers a
table showing the MOD and R/M bits corresponding to
various combinations of addressing modes
 Intel manual for the opcodes and formats of instructions.

CODES OF GENERAL-PURPOSE REGISTERS

CODES PERTAINING TO ADDRESSING MODES –
MOD AND R/M BIT PATTERNS

Chapter 2 programming concepts - I

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Chapter 2 programming concepts - I