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rhyno_dagreat

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

rhyno_dagreat 02 Dec 2006, 02:01

Okay... I'm sure we've all used the general purpose registers, index registers, and segment registers at one point in our programming history. I'm sure a few of us have also used the Stack Pointer (sp) register at one time or another. I feel stupid saying this now, but I just learned there was a Base Pointer (bp) register!

From what I've used with the SP register, it's used to set up the starting point of where the stack will be in memory. I've looked at the 512 Byte OS, and I'm wondering what is the BP register exactly used for? Is it general purpose or something? :-/ Embarassed

Anyways, thanks y'all!

P.S. - Sorry I've been gone for a while, been busy with college! It's great to be back into this though!

02 Dec 2006, 02:01

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 02 Dec 2006, 03:19

The reason for using BP a lot for those 512-byte chalenges is this http://board.flatassembler.net/topic.php?t=6078

Code:

  ;------------------------------------------------------------------
  ; BP-relative addressing
  ; this trick allows us to use addressing with BP + byte sized displacement,
  ; instead of word sized immediate offset, for example "inc [some_label]"
  ; becomes "inc [bp + some_label - BPVAL]". Only usable for offsets from
  ; BPVAL-128 to BPVAL+127

02 Dec 2006, 03:19

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 02 Dec 2006, 03:29

LocoDelAssembly: 512 byte demos are not exactly typical code though...

Anyway, BP/EBP is usually used as a frame pointer (pointer to the base of the stack frame). In 16-bit code, you could not address relative to SP, so if you wanted to access the stack in places other than the current stack pointer, you had to use BP. BP was the most suitable register for this purpose, since unlike the other three addressable registers (BX, SI, DI) a memory operand using BP would default to the SS segment rather than DS.

Here is a (16-bit) example of using BP to access function parameters on the stack.

Code:

        push 100
        push 200
        call add
        ; now AX = 300

add:    push bp
        mov bp,sp
        mov ax,[bp+4]   ; [bp+4] = 200
        add ax,[bp+6]   ; [bp+6] = 100
        pop bp
        ret 4

02 Dec 2006, 03:29

rhyno_dagreat

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

rhyno_dagreat 02 Dec 2006, 03:29

Thanks Loco, Goplat! Makes sense! It seems like I could use that in my own 512 byte 32 bit mode OS! Wink

02 Dec 2006, 03:29

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 02 Dec 2006, 04:12

Goplat, yes, I forgot to talk about the GENERAL use Laughing

(I thought he was wondering about its "over use" on 512 byte OSes only)

02 Dec 2006, 04:12

Plue

Joined: 15 Dec 2005
Posts: 151

Plue 02 Dec 2006, 09:49

bp is usually used for array access in compiler generated code. And also it can be used as a general-purpose register.

02 Dec 2006, 09:49

rhyno_dagreat

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

rhyno_dagreat 04 Dec 2006, 03:43

Thank you all for the help!

04 Dec 2006, 03:43

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