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x86 and Segments

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Started by serratemplar November 29, 2004 12:48 PM

6 comments, last by Dim_Yimma_H 19 years, 4 months ago

serratemplar

1,656

Author

November 29, 2004 12:48 PM

I'm building an x86 app and I'd like to create and use my own memory segments. I've done a good bit of this with the SEGMENT keyword, but even though I (think) I've defined the segment, I'm not able to get the data out of it. =\ I'm hoping someone can tell me how to do this. So, if I do this (note that the data is arbitrary just for example)


        .FARDATA

SEGMENT MyData
    someData    DB 0
                DB 1
                DB ... LOTS MORE DATA
                DB 9999 ; LAST ONE

    otherData   DB 9999
                DB 9998
                DB 9997 ETC LOTS MORE...
                ...
                DB 1
ENDS

How do I access someData or otherData? I figured this would do it...


    PUSH    SEGMENT MyData     ; move the segment address into ES
    POP     ES

    MOV     SI, OFFSET someData; now where in that segment address?     

    MOV     AX, [ES:SI]        ; move first byte into AX...

but this doesn't work. And I tried MOVing 0 *and* OFFSET MyData into SI, and I got the *exact* same result every time...the result being that I seem to start looking at my data midway in. Does *anybody* remember this stuff? I've been scouring the web with google and I've yet to find any documentation that really clarifies how this works.

Kwizatz

1,394

November 29, 2004 12:59 PM

Not sure, but shouldn't you use LEA?

Adam Gleason

100

November 29, 2004 01:44 PM

I wish i could help man but if you guys want to join my Newbie C++ study group that would be awsome go to http://newbiegroup500.tripod.com/

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Oluseyi

2,123

November 29, 2004 01:49 PM

Quote:Original post by Adam Gleason
I wish i could help man but if you guys want to join my Newbie C++ study group that would be awsome go to http://newbiegroup500.tripod.com/

You really shouldn't derail or attempt to hijack other people's threads. Consider this a friendly warning, as I recognize you're new here. It's simply not polite. If you have nothing to add to the topical conversation, move on to another thread.

Edit: Alternately, you could put a small blurb in your signature (go into your control panel), so that you advertise without the above infraction. Be subtle!

Dim_Yimma_H

557

November 29, 2004 01:55 PM

I'm not used to using the SEGMENT keyword but here's how I use to access data segments:

jmp CodeStart	;some data segments with digits	someData DB 0, 1, 9	otherData DB 9, 8, 7CodeStart:	LEA BX, someData	;retrieve address of someData segment	ADD BX, 2		;2 bytes offset to address to get "9"	MOV AL, [BX]		;move the byte at address in BX to AL	;make the digit into ASCII and send	;an interrupt to display it	ADD AL, 48	MOV DL, AL	MOV AH, 2	INT 21H	;exit program	MOV AH, 4Ch	INT 21h

Sorry if I misunderstood the segment you were trying to use.

\Jimmy H

serratemplar

1,656

Author

November 29, 2004 09:37 PM

I've never even seen or heard of the LEA instruction before. I'm writing code for a (brace yourselves) 486 machine...is the LEA instruction for more modern architectures or is it a standard...and what's it do?

I'll google it, like everything else...but like everything else, people are still smarter than google =)

darookie

1,441

November 29, 2004 09:57 PM

LEA (Load Effective Address) is present since the first x86. You can play nice tricks with it on a 486 since it can even be used to perform integer arithmetics (virtually for free since it is implemented on a different processor unit - AGU [Address Generation Unit), not the ALU [Arithmetic Logic(?)]).

The whole (IMHO totally bloated) CISC architecture might have many more or less useful instructions you might not know about, but LEA is a pretty basic and essential one.

Regards,
Pat.

Dim_Yimma_H

557

November 30, 2004 02:19 AM

Instead of this:
LEA BX, someData
you could write like this:
MOV BX, OFFSET someData
I've actually heard that the later one is supposed to be faster, I'm not sure though.

\Jimmy H

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