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Segment and offset uses


By : JDavid
Date : July 30 2020, 02:00 AM
it should still fix some issue As mentioned in the comments by Nate Eldredge, the segments overlap. Each subsequent 86 Mode segment starts 16 bytes after the prior one. So (as mentioned) there are 4096 (4 Ki) ways to address any one byte.
The exception is to do with A20 handling on later x86 CPUs. On those, if A20 is enabled, memory in the first 64 KiB has less aliases pointing to it. An address that would "wrap" on the 8086, starting with 0FFFFh:0010h, would have pointed into the first 64 KiB (starting with linear address 00_0000h).
code :


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get offset and segment address of string


By : sammysheep
Date : March 29 2020, 07:55 AM
hop of those help? Segments and offsets are antiquated features from before protected mode came to x86 chips.
Once they gained protected mode, they started using selectors to get away from the fact that memory addresses were now virtual rather than physical. And they generally use a flat model nowadays, which has very few selectors, each with a large (and sometimes the same) backing memory block.
code :
#include <stdio.h>
int main (void) {
    int xyzzy = 42;
    printf ("Address is %p\n", &xyzzy);
    return 0;
}

How does the segment:offset addressing scheme work?


By : hainc
Date : March 29 2020, 07:55 AM
it should still fix some issue The segment register value is shifted left by 4 (multiplied by 16) and added to the operand address.
16 * 65536 = 1 megabyte.

access data segment using offset


By : suman ghimire
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further arr1 is an Array of WORDs i.e. 16-bit-values. mov eax,[esi] receives a 32-bit-value in this case two 16-bit-values. So you'll find in EAX the first and the second element of arr1. If you want to fill the whole EAX only with the first value use movzx eax, word ptr [esi] instead. This instruction fills the lower 16 bits of EAX with the element and nullifies the upper 16 bits.

Set stack segment and offset in assembly


By : ZheXing Yang
Date : March 29 2020, 07:55 AM
This might help you I set up 4k stack space begins at end of the bootloader. After that, I read 16 sectors (=8k) of code to address 0x2000:0x0000. It's the kernel of my operating system. And I branched to it. , Try
code :
mov ax,2000h
mov ss,ax
mov sp,4000h

segment:offset in a constant


By : Borkat Rambe
Date : March 29 2020, 07:55 AM
Does that help x86 doesn't have a way to load from an immediate-constant far-pointer in one instruction. There's a far jmp ptr16:16 that can set cs:ip to a 32-bit immediate, but for loads I don't see any option that doesn't use a segment register.
code :
getndrives:
 ;; return in AL (zero-extended to AX; the upper byte of 0040h happens to be 0)
 ;; clobbers: AH, ES
    mov     ax, 0x0040
    mov     es, ax
    mov     al, [es:0x0475]
    ret
getndrives:
 ;; return in AL (zero-extended to AX; the upper byte of 0040h happens to be 0)
 ;; clobbers: AH,  or nothing if you uncomment the push/pop of AX
    push    es
    ;push    ax                 ; you might as well use a reg other than AX to simplify this, if you do want to preserve AH, too.  And also for performance on CPUs that don't rename low8 partial regs separately, so mov al,[mem] has a dependency on the pop.
      mov     ax, 0x0040
      mov     es, ax            ; or use DS if you don't need it in any interrupt handler
    ;pop     ax
      mov     al, [es:0x0475]
    pop     es
    ret
getndrives:
 ;; return in AL
 ;; requires/assumes: DS=0
    mov     al, [0x0475]
    ret
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