Bahasa Assembler :
Praktek Sistem
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Sebelumnya anda sudah download dan instal
Emulator 8086 untuk praktek sistem ini!.
Usually, when a computer starts it will try to load the first
512-byte sector (that's Cylinder 0, Head 0, Sector 1)
from any diskette in your A: drive to memory location 0000h:7C00h
and give it control. If this fails, the BIOS tries to use the MBR of the
first hard drive instead.
This tutorial covers booting up from a
floppy drive, the same principles are used to boot from a hard drive. But
using a floppy drive has several advantages:
- You can keep your existing operating system intact (Windows,
DOS...).
- It is easy to modify the boot record of a floppy disk.
Example of a simple floppy disk boot program:
; directive to create BOOT file:
#MAKE_BOOT#
; Boot record is loaded at 0000:7C00,
; so inform compiler to make required
; corrections:
ORG 7C00h
; load message address into SI register:
LEA SI, msg
; teletype function id:
MOV AH, 0Eh
print: MOV AL, [SI]
CMP AL, 0
JZ done
INT 10h ; print using teletype.
INC SI
JMP print
; wait for 'any key':
done: MOV AH, 0
INT 16h
; store magic value at 0040h:0072h:
; 0000h - cold boot.
; 1234h - warm boot.
MOV AX, 0040h
MOV DS, AX
MOV w.[0072h], 0000h ; cold boot.
JMP 0FFFFh:0000h ; reboot!
new_line EQU 13, 10
msg DB 'Hello This is My First Boot Program!'
DB new_line, 'Press any key to reboot', 0
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Copy the above example to
Emu8086 source editor and press [Compile and
Emulate] button. The Emulator automatically loads ".boot" file
to 0000h:7C00h.
You can run it just like a regular program, or you
can use the Virtual Drive menu to Write 512 bytes at 7C00h to
the Boot Sector of a virtual floppy drive (FLOPPY_0 file in
Emulator's folder).
After writing your program to the Virtual Floppy
Drive, you can select Boot from Floppy from Virtual Drive
menu.
If you are curious, you may write the virtual floppy (FLOPPY_0)
or ".boot" file to a real floppy disk and boot your computer from
it, I recommend using "RawWrite for Windows" from: http://uranus.it.swin.edu.au/~jn/linux/rawwrite.htm
(recent
builds now work under all versions of Windows!)
Note:
however, that this .boot file is not an MS-DOS
compatible boot sector (it will not allow you to read or write data on
this diskette until you format it again), so don't bother writing only
this sector to a diskette with data on it. As a matter of fact, if you use
any 'raw-write' programs, such at the one listed above, they will erase
all of the data anyway. So make sure the diskette you use doesn't contain
any important data.
".boot" files are limited to 512 bytes (sector size). If
your new Operating System is going to grow over this size, you will need
to use a boot program to load data from other sectors. A good example of a
tiny Operating System can be found in "Samples" folder
as:
micro-os_loader.asm
micro-os_kernel.asm
To
create extensions for your Operating System (over 512 bytes), you can use
".bin" files (select "BIN Template" from "File" ->
"New" menu).
To write ".bin" file to virtual floppy,
select "Write .bin file to floppy..." from "Virtual Drive"
menu of emulator:
You can also
use this to write ".boot" files.
Sector at:
Cylinder: 0
Head:0
Sector:
1
is the boot sector!
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Idealized floppy drive and diskette
structure:
For a 1440 kb
diskette:
- Floppy disk mempunyai 2 sisi, dan ada 2 head; one for each side
(0..1), the drive heads move above the surface of the disk on
each side.
- Each side has 80 cylinders (numbered 0..79).
- Each cylinder has 18 sectors (1..18).
- Each sector has 512 bytes.
- Total size of floppy disk is: 2 x 80 x 18 x 512 = 1,474,560
bytes.
To read sectors from floppy drive use INT
13h / AH = 02h.
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