flat assembler - Some questions: [edi+ecx*3] | PUSH | RAM limit

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Teehee

Joined: 05 Aug 2009
Posts: 570
Location: Brazil

Teehee 12 Mar 2011, 18:21

i got a trouble.

How do i pass a value from RM to PM and vice-versa?

I fill the ModeInfoBlock in RM and have all my VBE functions there, but if i call them in PM i got a reset.

Code:

jmp 0x08:pmode

 include 'vbe.asm' ; all vbe functions and ModeInfoBlock here

pmode:

    ; move kernel to addr 0x100000 and jmp to there.

    ; code, code, code..

   call drawsomething ; NOOOOooooooo!!!

maybe bc i didn't move the vbe code together? but what if i want let it in the low memory?

if i change the include place, call drawsomething works, but then i cant call getInfoblock in RM:

Code:


    call getInfoBlock ; OH NOES!!!

    ; code...
jmp 0x08:pmode

pmode:

    ; move kernel to addr 0x100000 and jmp to there.

    ; code, code, code..

   call drawsomething ; YEAH!!!

 include 'vbe.asm' ; all vbe functions and ModeInfoBlock here

Last edited by Teehee on 12 Mar 2011, 18:27; edited 1 time in total

12 Mar 2011, 18:21

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 12 Mar 2011, 18:22

Of course there are pros and cons for both, but Intel chose little-endian format because then you can read a byte and a qword (or any other size) from the same offset:

Code:

; data @ 1234h: 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 ...
mov rax,[1234h] ; rax=0807060504030201h
mov eax,[1234h] ; eax=04030201h
mov  ax,[1234h] ; ax=0201h
mov  al,[1234h] ; al=01h

with each read from memory al (the least significant byte) will still be 01h - the value at the first byte read from 1234h.

With big-endian format you'd have to read a qword from 1234h, dword from 1238h, ..., byte from 123Bh. I've gotten used to little-endian so much that I wouldn't know how to use the other one.

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12 Mar 2011, 18:22

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 12 Mar 2011, 18:41

@Teehee: You must code different subroutines for RM and PM. You can't expect them to work universally. I don't know what's behind your calls, but you definitely need to think these through. You cannot just move them around from RM to PM and vice-versa.

12 Mar 2011, 18:41

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