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                        V2_OS and other strange brews

Checked up on VS_OS [1] today. Surprise, surprise, they finally released the
source code. Immediately downloaded it and took a look.

Nothing surprising really, except that the source code to the bootsector is
missing. Or rather, there is code to a boot sector, but …

> BOOTIMAGE    DB 0E9H, 011H, 001H, 0FFH, 0FFH, 0FFH, 0FFH, 0FFH, 02BH, 056H
>              DB 032H, 05FH, 046H, 053H, 02BH, 000H, 030H, 030H, 030H, 02EH
>              DB 030H, 030H, 035H, 000H, 080H, 000H, 000H, 000H, 000H, 000H
>              DB 000H, 000H, 044H, 033H, 022H, 011H, 010H, 000H, 000H, 000H
>              DB 000H, 000H, 000H, 000H, 000H, 000H, 00DH, 00AH, 056H, 032H
>              DB 02DH, 04FH, 053H, 020H, 056H, 030H, 02EH, 031H, 020H, 028H
>              DB 043H, 029H, 031H, 039H, 039H, 039H, 020H, 056H, 032H, 05FH
>              DB 04CH, 061H, 062H, 02CH, 020H, 052H, 06FH, 074H, 074H, 065H
>              DB 072H, 064H, 061H, 06DH, 02EH, 00DH, 00AH, 000H, 04CH, 06FH
>              DB 061H, 064H, 069H, 06EH, 067H, 020H, 053H, 079H, 073H, 074H
>              DB 065H, 06DH, 031H, 036H, 000H, 00DH, 00AH, 046H, 061H, 069H
>

And some interesting code like:

>    MOV DI, OFFSET PARTLIST
>    MOV AL, 'f'
>    MOV DS:[DI+0], AL
>    MOV DS:[DI+16], AL
>    MOV AL, 'd'
>    MOV DS:[DI+1], AL
>    MOV DS:[DI+17], AL
>    MOV AL, '0'
>    MOV DS:[DI+2], AL
>    MOV DS:[DI+18], AL
>    MOV AL, 0
>    MOV DS:[DI+3], AL         ; 'FD0',0
>    MOV DS:[DI+19], AL
>

Two things wrong here (at least for 80x86 Assembly):

 1. Using DI instead of EDI in 32-bit mode. This causes an extra byte of
    opcode to be generated for a 16-bit offset.
 2. Moving individual letters into locations. If you are doing this at this
    level, you can do better by:
    >
    >          mov     eax,$00306466           ; 'fd0',0
    >          mov     [edi],eax               ; DS: override
    >          mov     [edi+16],eax            ; not needed
    >
    >       >


In poking around, I found a link to RDOS, [2] another 80x86 operating system
written in Assembly. This one is impressive, if only because it's a
functional OS in about 130,000 lines of Assembly (including TCP/IP). Haven't
had much time to look around this one though.

[1] http://www.v2.nl/nl_os/
[2] http://www.rdos.net/rdos/index.htm

Email author at [email protected]

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