use32 directive not will add prefix 0x66?

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Byte

Joined: 21 Mar 2026
Posts: 6

Byte 27 Mar 2026, 13:50

fasm for DOS

Code:

format binary

_inc_addr   EQU  01000h
use32

ORG 0A000h

Main:
  mov   EAX, 00000010h
  mov   EBX, _inc_addr
  mov   [EBX], EAX
.....

Disassembly: (lDebug)

Code:

 B81000     mov  ax, 010
 0000         add  [bx+si], al
 BB0010     mov  bx, 1000
 0000         add  [bx+si], al
 8903         mov  [bp+di], ax

Why B81000000, not 66B810000000 ?
Why

Code:

8903         mov  [bp+di], ax

27 Mar 2026, 13:50

Core i7

Joined: 14 Nov 2024
Posts: 150
Location: Socket on motherboard

Core i7 27 Mar 2026, 15:31

Apparently your disassembler is in 16-bit mode, because this design works correctly.

Code:

format binary
inc_addr  equ 1000h
use32
org 0A000h

start: mov  eax,10h
       mov  ebx,inc_addr
       mov  [ebx],eax
       ret

Description:
Filesize:	10.19 KB
Viewed:	3 Time(s)

27 Mar 2026, 15:31

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