How to write boot codes using 8088?

[leetow2003]

I read boot codes in a book,it is written by nasm,look:

Code: Select all

org 07c00h     
  mov ax,cs
  mov ds,ax
  mov es,ax
  call DispStr    
  jmp $           
DispStr:
  mov ax,BootMessage
  mov bp,ax       
  mov cx,16      
  mov ax,01301h  
  mov bx,000ch    
  mov dl,0
  int 10h         
  ret
BootMessage:  db  "hello,OS world!"
times  510-($-$$) db 0  
dw  0xaa55  

I want to rewrite codes using 8088,look:

Code: Select all

code segment
 assume cs:code
 start:
 org 07c00h     
  mov ax,cs
  mov ds,ax
  mov es,ax
  call DispStr    
  jmp $          
DispStr:
  mov ax,offset BootMessage
  mov bp,ax       
  mov cx,16       
  mov ax,01301h   
  mov bx,000ch   
  mov dl,0
  int 10h         
  ret
BootMessage  db  "hello,OS world!"
   db 510-($-$$) dup(0)  ;error
dw  0aa55h           
 code ends
 end start

I know that symbol $$ is unknown in 8088,but I don't how to change?
I want to know how to change code:times 510-($-$$) db 0?
Thank you very much.

[TylerH]

There's these really cool things called code tags, look:

Code: Select all

[code]

[/code]

[osdevkid]

Hi,

Based on my knowledge, '$' & '$$' symbols are specific to NASM compiler, they are not instructions of any specific processor.

I am sorry, if the above information is wrong.

[Artlav]

I want to rewrite codes using 8088,look:
{code}
I know that symbol $$ is unknown in 8088,but I don't how to change?
I want to know how to change code:times 510-($-$$) db 0?

Confusing terminology.
You rewrote the same code from modern NASM notation into older TASM notation.
8088 is a cpu - both things would work at it, or anything later (x86).
NASM and TASM are assemblers - compilers - that translate this code.

As for the error - it's either db 510-$ dup(0) or org 510, depending on how old a TASM you use.

[miker00lz]

hi all. first post. anyway.. maybe this will help you. it's a simple bootloader that i wrote for my real-mode OS kernel that loads my 28 KB kernel into 1000:0000 starting from the second sector of the bootdisk. then it sets up some registers and transfers control to the kernel.

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org 7c00h

jmp entry

spt db 18 ;sectors per track
heads db 2 ;heads

entry:
xor ax, ax
xor bx, bx
xor cx, cx
xor si, si
xor di, di
xor bp, bp
push ax
push ax
push ax
pop es
pop ds
pop ss
mov sp, 0FFFEh
sti

lea si, loadbanner
call printstr

mov cx, 2
mov dx, 0
mov bx, 0
mov ax, 01000h
push ax
pop es
jmp read

readsect:
inc cl
cmp cl, spt
ja nexthead
add bx, 512

read:
push bx
push cx
mov ax, 0201h                            
int 013h
pop cx
pop bx
jc readerr

inc total
call printdot

push cx
mov cx, total
cmp cx, sectorcount
pop cx
jz success

mov retrycount, 0
jmp readsect

readerr:
push ax
mov ax, retrycount
inc ax
mov retrycount, ax
cmp ax, 7 ;allow up to 6 retries
pop ax
jnz read
lea si, errbanner
call printstr
cli
hlt

nexthead:
mov cl, 0
inc dh
cmp dh, heads
jnz readsect

mov dh, 0
inc ch
jmp readsect

success:
mov ax, 01000h
push ax
push ax
push ax
pop ds
pop ss
pop es
mov ax, 0FFFEh
push ax
pop sp 
mov ax, 01000h
push ax
mov ax, 00h
push ax
cli
retf

printstr:
mov ah, 0Eh
mov al, [si]
cmp al, 0
jz goback
mov bx, 0
int 010h
inc si
jmp printstr
goback:
ret

printdot:
push ax
push bx
mov ah, 0Eh
mov al, '.'
int 010h
pop bx
pop ax
ret

total dw 0
retrycount dw 0
sectorcount db 56
loadbanner db 'Loading NamelessOS...', 0
errbanner db 'Error reading data! System halted.', 13, 10, 0

i'm pretty new to x86 assembly still, so i'm sure it could be made better somehow, but it does it's job perfectly and you can change a couple variables if you need to work with different disk geometries. the above code will work unmodified on a 1.44 MB floppy.

[qw]

Code: Select all

org 07c00h     
  mov ax,cs
  mov ds,ax

Classic mistake. You can't rely on CS being zero so the origin may be compeletely off. You should explicitly load zero into DS. Also, you did not set up a stack.

Roel