Instruction addressing and execution

Chapter 2
Instruction Addressing and Execution

Recalling main concepts
• Segment: special areas defined to contain CODE,
DATA and STACK
• Paragraph boundary: location evenly divisible by 16
or 10H

Recalling main concepts
Stack Segment
Data Segment
Code Segment
SS
DS
CS
Segment
Registers

Types of programs
• *.COM and *.EXE files
• *.COM: consists of one segment containing code, data
and stack
• *.exe: separate code, data and stack segments

Loading *.exe program
• Access *.exe from disk
• 256-byte Program Segment Prefix (PSP) on a
paragraph boundary
• Store the program immediately following the PSP
• Load address of PSP in the DS & ES
• Load code segment in CS, set IP
• Load address of the stack to SS, set SP
• Transfer control to the program for execution

.model small,
.stack
.data
message db "Hello everybody! I am learning assembly
language!","$"
.code
main proc
mov ax, seg message
mov ds,ax
mov ah,09
lea dx,message
int 21h
mov ax,4c00h
int 21h
main endp
end main
Assembly
directive to
define
memory
model to use
in the
program

Real and Protected mode
Real Mode
16-bit
Protected
Mode
32-bit Protected
Mode
Segment base
address
20-bit
24-bit,
from
descriptor
32-bit,
from descriptor
Segment size
(limit)
16-bit, 64K
bytes (fixed)
16-bit, 1-64K
bytes
20-bit, 1-1M bytes or
4K-4G bytes
Segment
protection
no yes yes
Segment
register
segment base
address / 16
selector selector

Protected mode
-Is a type of memory utilization, available on
Intel 80286 and later
-Support:
protection: each program is protected
from interference from other programs.
extended memory : Enables a single
program to access more than 640K of memory.
virtual memory : Expands the address
space to over 1GB.
Multitasking:

Booting process
What is booting?
• The process of starting or restarting a computer
cold boot
Process of turning on
a computer after it
has been powered off
completely
warm boot
Process of
restarting a
computer that is
already powered on
Also called a warm
start

Booting process
How does a personal computer boot up?
processor
BIOS
hard disk
CD-ROM drive
(RAM) memory
modules
CMOS
floppy disk drive
Step 6
expansion cards

BIOS Boot process
BIOS routineFFFF0H
Interrupt
Vector
Table
BIOS Data
Areas
Access
the bootstrap
loader
Check ports
Initialize
devices

.model small,
.stack
.data
message db "Hello everybody! I am learning assembly
language!","$"
.code
main proc
mov ax, seg message
mov ds,ax
mov ah,09
lea dx,message
int 21h
mov ax,4c00h
int 21h
main endp
end main
Assembly
directive to
define stack
to use in the
program

STACK
• The word is from data structure
• Last In, First Out (LIFO) mechanism
• STACK in OS has three main functions:
• Contains return address
• Data
• Content of present registers

STACK
• PUSH
• Decrease SP by 2 and store a value there
• POP
• Return a value from stack and increase SP by 2

Lesson plan
• Review loading an *.exe file
• Concept of execution of instructions
• Practice:
• Execution of instructions

Access *.exe from disk
256-byte Program Segment Prefix (PSP)
on a paragraph boundary
Store the program immediately
following the PSP
Load address of PSP in the DS & ES
Load code segment in CS, set IP
Load address of the stack to SS, set SP
Transfer control to the program for
execution
Loading *.exe file

Loading *.exe file
• The sequence of segments (code, data, and stack)
is given
• SS: contains the address of the beginning of the
stack
• CS: contains the address of the beginning of the
code segment
• DS: contains the address of the beginning of the
data segment
• SP: contains the size of stack

Practice
2B360H
PSP
Stack Segment
Data Segment
Code Segment
Memory

Practice
2B360H
PSP
Stack Segment
Data Segment
Code Segment
Memory
PSP 2B360H
PSP size 100H
Offset 0H
SS 2B460H
(stored as
2B46)
2B46HSS

Practice
2B360H
PSP
Stack Segment
Data Segment
Code Segment
Memory
2B46HSS
PSP 2B360H
PSP size 100H
Offset 30H
70H
CS 2B500H
(stored as 2B50)
CS 2B50H

Practice
2B360H
PSP
Stack Segment
Data Segment
Code Segment
Memory
2B46HSS
CS 2B50H
2B36H
2B36H
DS
ES
SP 0030H

Instruction Execution and Addressing
• Executing an instruction include
• Fetch the next instruction, put to a queue (QUEUE: FIFO
vs. STACK LIFO)
• Decode the instruction
• Execute the instruction

Example
4AF0CS 0013IP
4AF13H
CS segment address: 4AF00H
IP offset: 0013H
________________________
+
Instruction address:
04B1DS

Example
4AF0CS 0013IP
4AF13H
A01200
Memory
04B1DS
Decode instruction:
AO: MOV [0012] to AL

Example
4AF0CS 0013IP
4AF13H
A01200
Memory
04B1DS
DS segment address:04B10H
IP offset: 0012H
________________________
+
Data address: 04B22H

Example
4AF0CS 0013IP
4AF13H
A01200
Memory
04B1DS
04B22Data address:
04B22H
| 1B
AXAX
AH AL
1B

Practice
• Viewing memory location using DEBUG
At DOS prompt, type: DEBUG
• Checking Serial and parallel port:
D 40:00
• Checking system equipment
D 40:10

Practice
• Viewing memory location using DEBUG
At DOS prompt, type: DEBUG
• Checking Serial and parallel port:
D 40:00
• Checking system equipment
22 C8 C822
reverse
Number of parallel printer ports=11(binary)=3 (15,14)
Number of serial ports=100(binary)=4 (11-9)
Number of diskette devices=00(binary)=1 (7,6)
Initial video mode =10 (5,4) (80x25 color)
Coprocessor present: 1 (1)
Diskette drive is present: 0 (0)

Practice (cont.)
• Checking the keyboard status
• D 40:17 (With Numlock and Caplock)
• Checking video status
• D 40:49
• D 40:84

Practice (cont.)
• Checking copyright notice & serial number
• D FE00:0
• Checking ROM BIOS date
D FFFF:5

Practice (cont.)
Running the first assembly program step by step
using EMU 8086

Instruction addressing and execution

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