use32 directive not will add prefix 0x66?

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Byte

Joined: 21 Mar 2026
Posts: 8

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: 152
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:	42 Time(s)

27 Mar 2026, 15:31

Byte

Joined: 21 Mar 2026
Posts: 8

Byte 27 Mar 2026, 18:07

Thanks for the reply.

Core i7 wrote:

Apparently your disassembler is in 16-bit mode...

Maybe.
But isn't the 0x66 prefix in the binary code a sign of using a 32-bit register? Shouldn't the compiler add this prefix to define a 32-bit instruction?

27 Mar 2026, 18:07

Core i7

Joined: 14 Nov 2024
Posts: 152
Location: Socket on motherboard

Core i7 27 Mar 2026, 18:21

it turns out the other way around - the prefix 0x66 is used if you use 16-bit registers in x32 mode.

Code:

format binary
use32
start: mov  eax,100h
       mov  bx,66h
       mov  word[eax],bx
       ret
;------------------------
00000000: B800010000           mov eax, 00000100
00000005: 66BB6600             mov bx, 0066 ;'f'
00000009: 668918               mov [eax], bx
0000000C: C3                   ret

27 Mar 2026, 18:21

Core i7

Joined: 14 Nov 2024
Posts: 152
Location: Socket on motherboard

Core i7 27 Mar 2026, 18:32

By the way, the prefix also works in the opposite direction - if you use 32-bit registers in x16 mode

Code:

format binary
use16
start: mov  eax,100h
       mov  bx,66h
       mov  word[eax],bx
       ret                
;-----------------------
00000000: 66B800010000         mov eax, 00000100
00000006: BB6600               mov bx, 0066 ;'f'
00000009: 678918               mov [eax], bx
0000000C: C3                   ret

27 Mar 2026, 18:32

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20915
Location: In your JS exploiting you and your system

revolution 27 Mar 2026, 20:31

The CPU has modes.

In 32-bit mode all registers are 32-bit without 0x66, and 16-bit with 0x66.
In 16-bit mode all registers are 16-bit without 0x66, and 32-bit with 0x66.

The disassembler has to be told which mode the CPU is in to correctly disassemble the bytes.

CPU modes are changed by writing to CR0 with MOV or SMSW.

There is also 64-bit mode (long mode) where the 0x66 prefix can make registers either 32-bit or 16-bit, or do nothing, depending on the instruction. This is not confusing in any way. Wink

27 Mar 2026, 20:31

Byte

Joined: 21 Mar 2026
Posts: 8

Byte 27 Mar 2026, 20:38

revolution wrote:

In 32-bit mode all registers are 32-bit without 0x66, and 16-bit with 0x66.
In 16-bit mode all registers are 16-bit without 0x66, and 32-bit with 0x66.

Thank you, now I understand.

27 Mar 2026, 20:38

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