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NASM Boot Disk
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Hi,
I tried having a go at creating my own boot disk that would write text to the screen. The code is here:
It's modified from some code from a tutorial. The modified bit is the part after the begin: label. His tutorial said that the video memory started at 0xB8000 for colour displays and 0xB0000 for mono displays. I have tried both in there but neither of them seem to work. I should be getting two a's in the top left corner (I think) but I get nothing.
I know I'm doing something wrong (probably over simplifying it
) but thats it :lol: Can anyone help me with this?
Thanks, Matt
I tried having a go at creating my own boot disk that would write text to the screen. The code is here:
Assembly Syntax (Toggle Plain Text)
It's modified from some code from a tutorial. The modified bit is the part after the begin: label. His tutorial said that the video memory started at 0xB8000 for colour displays and 0xB0000 for mono displays. I have tried both in there but neither of them seem to work. I should be getting two a's in the top left corner (I think) but I get nothing.
I know I'm doing something wrong (probably over simplifying it
) but thats it :lol: Can anyone help me with this?Thanks, Matt
Hi,
It doesn't work because you've not set up DS segement register, as of what you have in the code, its ambigous and while your at it, its best to leave other regularly used segment registers defined rather keeping them ambigous. In x86 assembly, when you don't specify which segment register like the instruction mov BYTE [0xB8000], 7 it assumes the offset is relative to DS segment, this means if DS=10h for an example the linear address pointed to is B8100h. Also however, for a boot sector you should keep in mind your in 16bit mode and that offset B8000h would need to contain more then 16bits. There are methods to go around this such as an operand size override prefix (opcode 66h). Or to simply write it in 16bit you would use a 0xB800h segment which would look something like:
It doesn't work because you've not set up DS segement register, as of what you have in the code, its ambigous and while your at it, its best to leave other regularly used segment registers defined rather keeping them ambigous. In x86 assembly, when you don't specify which segment register like the instruction mov BYTE [0xB8000], 7 it assumes the offset is relative to DS segment, this means if DS=10h for an example the linear address pointed to is B8100h. Also however, for a boot sector you should keep in mind your in 16bit mode and that offset B8000h would need to contain more then 16bits. There are methods to go around this such as an operand size override prefix (opcode 66h). Or to simply write it in 16bit you would use a 0xB800h segment which would look something like:
Assembly Syntax (Toggle Plain Text)
hi
i'm beginner in assembly language and now , reading "
Jeff Duntemann" book for this purpose . ( off topic : this book is the best ! :mrgreen: )
@ RYU :
can you insert here comlete code for build simple boot disk here ? ( print "hello world" message for example! )
and how i can use & copy it in disk for use ?
at the end, where i can found about boot loaders Programming ?
thank's alot & have a fun
i'm beginner in assembly language and now , reading "
Jeff Duntemann" book for this purpose . ( off topic : this book is the best ! :mrgreen: )
@ RYU :
can you insert here comlete code for build simple boot disk here ? ( print "hello world" message for example! )
and how i can use & copy it in disk for use ?
at the end, where i can found about boot loaders Programming ?
thank's alot & have a fun
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Join Date: Jul 2006
Posts: 155
Reputation: 
Solved Threads: 10
Junior Poster
Hi VIDOCQ,
Take a look at:
http://www.osdever.net/
http://www.mega-tokyo.com/osfaq2/
and finally this:
http://www.mega-tokyo.com/forum/
These websites contain comprehensive resources on making a complete OS and many other issues related to operating systems
Good luck.
Take a look at:
http://www.osdever.net/
http://www.mega-tokyo.com/osfaq2/
and finally this:
http://www.mega-tokyo.com/forum/
These websites contain comprehensive resources on making a complete OS and many other issues related to operating systems
Good luck.
Hi,
Typically for boot sectors its best if we use BIOS interrupts to print text to the screan because this will reduce the code size where we only have 512 bytes to do everything. However to elaberate with the problem in this thread I'll show an example from my own bootsector with a bit of midifications. Note that I use MASM which I know isn't widely used so you will have to make the adjustments with your assembler syntax and directives.
Some things to note: The bootsector is made for FAT12 floppy bootsector, the BPB isn't necessary, but being in windows can make a lot of fusses without it. The far jump to __CSIP isnt necessary either, but if you do things later on such as installing interrupt routines and software interrupt to them, it ensures the routine has the correct return address and not to oblivion because the theres no standard that specifies the BIOS to set CS segment apon entering the bootsector or even all normal calls which the return address is abolute with CS segment. However the standard is the bootsector must be loaded at linear address 7C00h. The source also needs to be linked image relative, meaning it starts at offset 0. You may encounter other examples that use ORG 7C00h which means you would use zero based segments. Because of the ORG 7C00h in MASM creating images 7C00h + the code size I choose to base my code on a 7C00h segment (CS=ES=DS=7C0h) in this case ES was used to point to a video memory segment. Video memory usually will start at B8000h however this in now always the case, here I've assumed so, and detecting which mode the VGA is currently in is another issue. Hope this helps.
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Originally Posted by VIDOCQ 
@ RYU :
can you insert here comlete code for build simple boot disk here ? ( print "hello world" message for example! )
and how i can use & copy it in disk for use ?
at the end, where i can found about boot loaders Programming ?
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.386P
.MODEL TINY
OPTION M510
OPTION SCOPED
OPTION PROLOGUE :none
OPTION EPILOGUE :none
OPTION CASEMAP :none
OPTION OFFSETEGMENT
OPTION SEGMENT :USE16
assume cs:none, ss:none, ds:none, es:none, gs:none, fs:none
ORG 0
$BOOT SEGMENT USE16
;========================================
jmp short __CSIP
nop
; ================================================================= ;
;;;;;;;; Bios Parameters Block & Extended Block ;;;;;;;;;
; ================================================================= ;
OemString db "MYOS...." ; 0
BytesPerSector dw 512 ; 8
SectorsPerCluster db 1 ; 10
ReservedSectors dw 1 ; 11
TotalFATs db 2 ; 13
MaxRootEntries dw 224 ; 14
TotalSectorsSmall dw 2880 ; 16
MediaDescriptor db 240 ; 18
SectorsPerFAT dw 9 ; 19
SectorsPerTrack dw 18 ; 21
NumHeads dw 2 ; 23
HiddenSectors dd 0 ; 25
TotalSectorsLarge dd 0 ; 29
;;;;;;;; Extended Parameters ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
DriveNumber db 0 ; 33
Flags db 0 ; 34
Signature db 41 ; 35
VolumeID dd 23232 ; 36
VolumeLabel db "BOOT SYS" ; 40
SystemID db "FAT12 " ; 51
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
szHelloWorld db "HelloWorld", 0
__CSIP:
cli ; disable interrupts
db 0EAh
dw offset start ; jump far 07C0:start
dw 000h ; this is to ensure CS=7C0h for calls that returns from
;absolute address which is relative to CS segment
start:
mov ax, 07C0h
mov ds, ax ; DS=07C0h
mov ax, B800h
mov es, ax ; ES=B800h prepare ES to write to video memory
mov ax, 80h
mov ss, ax ; SS=80h (bottom of the stack is 800h linear)
mov ax, 800h
mov sp, ax ; SP=800h ( top of the stack is SS:800h == 1000h linear)
mov si, offset szHelloWorld
xor di, di ; start at ES:[0] == B8000 linear
mov ax, 0700h ; 15:8 attribute 7:0 text character
PrintLoop:
mov al, [si] ; load a character
test al, al
jz short HALT ; if(ZF==1) indicates test is zero, we are finished.
mov es:[di], ax ; copy attribute and character to vmem
add di, 2 ; adjust offset in video memory by 2 (attribute+character)
inc si ; adjust our offset by 1
jmp short PrintLoop ;continue
HALT:
hlt
jmp short HALT
ORG 1FEh
$BOOTSIG dw 0AA55h ; last two bytes must contain this signature
END
Some things to note: The bootsector is made for FAT12 floppy bootsector, the BPB isn't necessary, but being in windows can make a lot of fusses without it. The far jump to __CSIP isnt necessary either, but if you do things later on such as installing interrupt routines and software interrupt to them, it ensures the routine has the correct return address and not to oblivion because the theres no standard that specifies the BIOS to set CS segment apon entering the bootsector or even all normal calls which the return address is abolute with CS segment. However the standard is the bootsector must be loaded at linear address 7C00h. The source also needs to be linked image relative, meaning it starts at offset 0. You may encounter other examples that use ORG 7C00h which means you would use zero based segments. Because of the ORG 7C00h in MASM creating images 7C00h + the code size I choose to base my code on a 7C00h segment (CS=ES=DS=7C0h) in this case ES was used to point to a video memory segment. Video memory usually will start at B8000h however this in now always the case, here I've assumed so, and detecting which mode the VGA is currently in is another issue. Hope this helps.
Hello, I have to write a bootloader using AMD64 assembler. I am a newbie in it. Do you think it will work if I just change the 16/32 bit registers to the coresponding ones of the AMD64 architecture? Are there any other changes to make? Thank you for the bootloader codes above.
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Originally Posted by maxesanu
Hello, I have to write a bootloader using AMD64 assembler. I am a newbie in it. Do you think it will work if I just change the 16/32 bit registers to the coresponding ones of the AMD64 architecture? Are there any other changes to make? Thank you for the bootloader codes above.
Perhaps you should let us know when you find out.
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