16 bit os...

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rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 08 Dec 2006, 03:05

I have been working on this for over five hours... I'm trying to get my code to load to the exectuable at 0600h indirectly, like an actual OS would... but it doesn't want to work... Can someone please look at this... I'm not sure whether or not [bx] means the value of bx or the mem addr of it... Thanks...

Code:

use16
org 0x7C00
jmp Start

;DATA
msg db "Hello World!", 13, 10, 0 ;13 = Carriage Return, 10 = New Line, 0 = End of String
err db "Error!", 13, 10, 0
hur db "Hurrah!", 13, 10, 0
buffer db 0

Sectors db 1


Start:
        mov si, msg
        call PrintFunction
        ;call ConsoleFunction ;Reserved for later use
        mov ax, 0000h
        mov es, ax
        mov bx, 0060h
        mov ah, 02h
        mov al, [Sectors] ;Sectors to be read
        mov ch, 0
        mov cl, 2
        mov dh, 0
        int 13h
        jmp ReadFileStart

Hang:
        jmp Hang

ReadFileStart:
        cmp bx, 0xFD00
        je GotoFile
        inc bx
        jmp ReadFileStart

GotoFile:
        inc bx
        shl bx, 1
        add bx, 0000h
        jmp bx

;CONSOLE RESERVED FOR LATER USE
;ConsoleFunction:
;        mov ah, 00h
;        int 0x16
;        mov bp, buffer
;        ;cmp ah, 0x1C
;        ;je Check
;        cmp ah, 0x01
;        je Done
;
;        mov ah, 0x0E
;        int 0x10
;        mov [bp], al
;        inc bp
;        jmp ConsoleFunction

PrintFunction:
        lodsb
        cmp al, 0
        je Done
        mov ah, 0x0E
        int 0x10
        jmp PrintFunction

Done:
        ret

;Fill in emptyness with X's
times 510-($-$$) db 'X'
dw 0xAA55

org 0600h

FHeader db "SOS1","Hello World", 0, 0xFC, "prg", 0, 0xFD
FData:
        jmp ProgStart
        HelloMsg db "Hello world from Small 16-bit OS!", 13, 10, 0
        ProgStart:
                mov si, HelloMsg
                call PrintFunction
;FEnd db "FEND"
times 512-($-$$) db 'X'

08 Dec 2006, 03:05

bubach

Joined: 17 Sep 2004
Posts: 341
Location: Trollhättan, Sweden

bubach 08 Dec 2006, 10:16

the [] thing can be tricky. for a normal variable the name give you the var address, and [] will give you it's contents. for a register bx will give you the content while [bx] give you the content from the address in bx. since registers don't have addresses it's a bit diffrent.

08 Dec 2006, 10:16

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 08 Dec 2006, 12:47

Thanks

08 Dec 2006, 12:47

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 09 Dec 2006, 19:14

I tried something in a hex editor I use (Icy Hexplorer - pretty nice one for free, might I add), I looked up the jmp command in GotoFile, and it jumps to some odd area in memory where there's an X at. So then I tried to comment out GotoFile all together, as well as the contents of ReadFileStart, and put inside of it "jmp FData", and instead of printing the string like it should it reprints the "Hello World!" message (msg) three times.

09 Dec 2006, 19:14

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 09 Dec 2006, 22:50

Does anyone know what it could be doing wrong?

09 Dec 2006, 22:50

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 10 Dec 2006, 20:55

If I'm reading your code right then

1) You load the next sectors to es:bx = 0x0000:0x0060 not 0x0000:0x0600

mov bx, 0060h => mov bx,0x0600

or

mov ax,0x0060
mov es,ax
mov bx,0x0000 ; xor bx,bx

2) It loops until the value of bx is 0xFD00, not the value pointed to by bx, it should also be byte not word:

cmp bx,0xFD00 => cmp byte [bx],0xFD

HTH

10 Dec 2006, 20:55

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 11 Dec 2006, 03:43

You... Are... AWESOME! Very Happy

THANK YOU!!!! This thing's had me stumped for the past weekend! I was doing the segment:offset thing wrong (backwards) when I was calculating the get the absolute address.

11 Dec 2006, 03:43

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 11 Dec 2006, 13:27

hehe I've done it so many times myself it's almost second nature to correct now Laughing

11 Dec 2006, 13:27

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 11 Dec 2006, 16:13

For me it's just something else I'm getting used to. Smile

Is the BX register set apart from the other general registers in the sense that it can act as a pointer?

11 Dec 2006, 16:13

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 11 Dec 2006, 17:45

no. you can use any general register (ax bx cx dx si di bp sp (in 16bit)) as a pointer to a value at the address of the register

e.g.

mov byte [ax],0xFF

or more complex code up to 2 general purpose registers, 1 allowed to have a multiplier, and a constant value:

e.g.

mov byte [di+4*cx+2],dl

This moves the byte value in dl to the address pointed to by di + (4 × cx) + 2. This is useful if di points to the destination of a block of data and your using cx as the loop counter for some algorithm, the constant allows you to adjust as necessary.

11 Dec 2006, 17:45

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 11 Dec 2006, 18:04

No you can't use any register in 16-bit mode http://www.sandpile.org/ia32/opc_rm16.htm

32-bit mode is more flexible http://www.sandpile.org/ia32/opc_rm32.htm , http://www.sandpile.org/ia32/opc_sib.htm .

11 Dec 2006, 18:04

rhyno_dagreat

Joined: 31 Jul 2006
Posts: 487
Location: Maryland, Unol Daleithiau

rhyno_dagreat 15 Dec 2006, 03:44

Thank you all for clarifying that!

15 Dec 2006, 03:44

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