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Index > OS Construction > is something wrong with my PrintString: (16bit and bios)

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hckr83



Joined: 12 Nov 2006
Posts: 86
Location: usa
hckr83
I'm writing an OS to test my emulator I'm making(currently very limited for opcodes and devices)

My code may look a bit wierd to get around my limitation of opcodes but anyway..
Code:
mov ax,ds
mov es,ax
mov si,my_string ;point it to my string
call PrintString

nop
hlt

my_string db 'h','i',0

PrintString: ;PrintString --es:si points to the string
mov ax,es
push ds ;I don't have segment overrides so..
mov ds,ax
;set it up for bios int 10h, ah=0Ah
mov ah,0x0A
mov bx,0
mov bp,0
print_loop:
mov al,[si+bp]
sub al,0 ;eventually replace with cmp
jz print_done ;if it is the end of a null terminated string then stop
mov cx,1
int 0x10
;temp only!
push ax  ;save ax
mov ax,bp ;we just add 1 to bp
add ax,1
mov bp,ax
pop ax
jmp print_loop ;do loop

print_done:
pop ds
ret
    


also I'm sure that every opcode in that code works right

_________________
x86 CPU Emulation library: http://sourceforge.net/projects/x86lib
Post 09 Dec 2006, 18:36
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rhyno_dagreat



Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau
rhyno_dagreat
hckr83, is that all of your OS code? If it is then you're forgetting three things:

  • Start the program at offset 0x7C00 using "org 0x7C00"
  • At the end of the program put "times 510-($-$$) db 0" to fill up the rest of the sector with zeros.
  • After you do the thing listed right above put "dw 0xAA55" which is a word value which tells the BIOS that the sector you were just working in is the boot sector.


If that isn't your problem, did you put anything into ax before you started putting the the value in it into the data segment and extra segment?
Post 09 Dec 2006, 19:37
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hckr83



Joined: 12 Nov 2006
Posts: 86
Location: usa
hckr83
Quote:
# Start the program at offset 0x7C00 using "org 0x7C00"
# At the end of the program put "times 510-($-$$) db 0" to fill up the rest of the sector with zeros.
# After you do the thing listed right above put "dw 0xAA55" which is a word value which tells the BIOS that the sector you were just working in is the boot sector.

I already did the "equivalent" to that(I don't have fdd support yet so I just load it straight into memory)

Quote:
If that isn't your problem, did you put anything into ax before you started putting the the value in it into the data segment and extra segment?

I didn't completely understand what you just said but I AX doesn't have any special data in it, at the first part I use it to mov ES to DS
Post 09 Dec 2006, 19:43
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rhyno_dagreat



Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau
rhyno_dagreat
You should still make sure it contains zero before you do. Just either "mov ax, 0" or "xor ax, ax".
Post 09 Dec 2006, 19:58
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