Masm to Nasm

yousafsajjad · 07-02-2010, 06:36 PM

Can anyone help me convert the following code from MASM to NASM

Code:

.486p
_CODE	SEGMENT  use16 para PUBLIC 'CODE'

DESCRIPTOR	MACRO	name, limit, base, flags
name	label	dword
	dw	limit and 0FFFFh
	dw	base and 0FFFFh
	db	(base and 0FF0000h) shr 16
	db	flags and 0FFh
	db	((flags and 0F00h) shr 4) or ((limit and 00F0000h) shr 16)
	db	(base and 0FF000000h) shr 24

	ENDM

;The GDT for protected mode operation.

align	4
GDTstart label	qword
DESCRIPTOR	nulldesc,	   0, 0,    0h	; null descriptor
DESCRIPTOR	datadesc,    0FFFFFh, 0,  092h	; data segment
DESCRIPTOR	stackdesc,    0FFFFh, 0,  092h	; stack segment
DESCRIPTOR	fatdatadesc, 0FFFFFh, 0, 0892h	; BIG data segment
DESCRIPTOR	fatcodedesc, 0FFFFFh, 0, 0C9Eh	; BIG code segment
DESCRIPTOR	codedesc,     0FFFFh, 0,  09Eh	; code segment
DESCRIPTOR	realdatadesc, 0FFFFh, 0,  093h	; real mode data segment
GDTend	label	qword

;In-memory GDT pointer for the lgdt call.

align	4
GDTptr		label	fword
gdtlimit	dw	GDTend - GDTstart	;Size of the GDT.
gdtbase 	dd	0			;Linear address of GDT.

save_ds dw	?
save_es dw	?
save_fs dw	?
save_gs dw	?
save_ss dw	?

A20_State db	?			; 0 = A20 is already un-wrapped
					; 1 = We need to remove A20 wrap

_CODE	ends
_CODE	segment

paulsm4 · 07-02-2010, 08:18 PM

Hi -

It actually looks syntactically correct as-is?

Q: What specific errors did you encounter when you tried to assemble it with nasm?

It also looks like it might be a good way to trash (deliberately or otherwise) your OS's memory mapping.

Q: What exactly are you trying to do with this code, anyway?

yousafsajjad · 07-03-2010, 12:35 AM

Yes it is totally correct for MASM.
I have not tried much to code it up in nasm because I am not much familiar with equivalent syntax for Nasm. I know its kinda basic but I didn't find much straight forward thing.
I am suppose to write up assembly code for Linux that will switch between protection mode and real mode to trigger some SMI interrupts.

yousafsajjad · 07-06-2010, 07:08 PM

i have figured out most of the code but still i cant seem to figure out one thing .. can anyone please help me with the following:

Code:

DESCRIPTOR    MACRO    name, limit, base, flags
name    label    dword
    dw    limit and 0FFFFh
    dw    base and 0FFFFh
    db    (base and 0FF0000h) shr 16
    db    flags and 0FFh
    db    ((flags and 0F00h) shr 4) or ((limit and 00F0000h) shr 16)
    db    (base and 0FF000000h) shr 24
    ENDM
 
;The GDT for protected mode operation.
    align    4
GDTstart label    qword
DESCRIPTOR    nulldesc,       0, 0,    0h    ; null descriptor
DESCRIPTOR    datadesc,    0FFFFFh, 0,  092h    ; data segment
DESCRIPTOR    stackdesc,    0FFFFh, 0,  092h    ; stack segment
DESCRIPTOR    fatdatadesc, 0FFFFFh, 0, 0892h    ; BIG data segment
DESCRIPTOR    fatcodedesc, 0FFFFFh, 0, 0C9Eh    ; BIG code segment
DESCRIPTOR    codedesc,     0FFFFh, 0,  09Eh    ; code segment
DESCRIPTOR    realdatadesc, 0FFFFh, 0,  093h    ; real mode data segment
GDTend    label    qword

I dont understand "DESCRIPTOR nulldesc, 0, 0, 0h" and other lines like that ..

resetreset · 07-07-2010, 02:04 AM

OK, first of all I don't understand MASM directives, but what I think those "DESCRIPTOR" lines mean are - they're setting aside space in memory for the Global Descriptor Table, which is needed for Protected Mode operation. Please do a Google to find out exactly what it is, or search on the alt.lang.asm and alt.os.assembly newsgroups.
You can't ENTER Protected Mode when you're within Linux, as Linux already does that as part of its bootup - it's a Protected Mode OS, and you DEFINITELY can't enter Real Mode when you're within Linux.
What are SMI Interrupts, btw?

yousafsajjad · 07-07-2010, 11:28 AM

Quote:

Originally Posted by resetreset

You can't ENTER Protected Mode when you're within Linux, as Linux already does that as part of its bootup - it's a Protected Mode OS,

I would be calling an assembly function from C and would be passing a physical address to it. I know the memory region where I can do some stuff in so I do not think that it would be a problem to jump back and forth from protected to real mode.

Quote:

Originally Posted by resetreset

What are SMI Interrupts, btw?

SMM is entered via the SMI (system management interrupt), which is caused by:
* Motherboard hardware or chipset signaling via a designated pin of the processor chip. This signal can be an independent event.
* Software SMI triggered by the system software via an I/O access to a location considered special by the motherboard logic (port 0B2h is common).
* An IO write to a location which the firmware has requested that the processor chip act on.

System Management Mode (SMM) is an operating mode in which all normal execution (including the operating system) is suspended, and special separate software (usually firmware or a hardware-assisted debugger) is executed in high-privilege mode.

resetreset · 07-07-2010, 01:19 PM

Quote:

Originally Posted by yousafsajjad

I would be calling an assembly function from C and would be passing a physical address to it. I know the memory region where I can do some stuff in so I do not think that it would be a problem to jump back and forth from protected to real mode.

Regardless of what "memory region to do some stuff in" you have, you CANNOT go to Real Mode from within Linux. What you're talking about is probably V86 Mode.

yousafsajjad · 07-07-2010, 02:24 PM

hmm .. i can look into it .. in the mean while .. i have another question .. I have an .obj file which was compiled on windows using masm. Can I link my c file with that .obj file and use it linux???

Sergei Steshenko · 07-07-2010, 02:38 PM

Quote:

Originally Posted by yousafsajjad

hmm .. i can look into it .. in the mean while .. i have another question .. I have an .obj file which was compiled on windows using masm. Can I link my c file with that .obj file and use it linux???

Look up 'binutils' and specifically 'objcopy'.