lock bt

[LiaoMi]

Hi,

I get an error in both versions 32-64 ...

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mywindow1.asm(22) : Error A2028: Instruction prefix not allowed
mywindow1.asm: 38 lines, 1 passes, 2 ms, 0 warnings, 1 errors
Microsoft (R) Incremental Linker Version 14.16.27024.1
Copyright (C) Microsoft Corporation.  All rights reserved.

with this source code

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option casemap:none

.const

Green equ 7
Red   equ 5

.DATA
hInstance7   qword 0
CommandLine7 qword 0
hInstance    qword 0
CommandLine  qword 0
hInstance6   qword 0
CommandLine6 qword 0

G_TlsMap DWORD 8 DUP (?)

.code

mainCRTStartup proc

    lock bt G_TlsMap,esi

mov ecx,Green
mov hInstance, rax
mov CommandLine7, rax
mov hInstance7, Red
mov CommandLine6, rax
mov hInstance6, Red
mov CommandLine, rax
mov ecx, eax

ret
align 4
mainCRTStartup endp

end

At the same time macro assembler successfully accepts this command

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Assembling: mywindow1.asm
Microsoft (R) Incremental Linker Version 14.16.27024.1
Copyright (C) Microsoft Corporation.  All rights reserved.

Press any key to continue . . .

Older versions of macro assembler also accept this instruction  :icon_redface:

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BT — Bit Test
Opcode Instruction Op/En 64-bit Mode Compat/Leg Mode Description
0F A3 /r BT r/m16, r16 MR Valid Valid Store selected bit in CF flag.
0F A3 /r BT r/m32, r32 MR Valid Valid Store selected bit in CF flag.
REX.W + 0F A3 /r BT r/m64, r64 MR Valid N.E. Store selected bit in CF flag.
0F BA /4 ib BT r/m16, imm8 MI Valid Valid Store selected bit in CF flag.
0F BA /4 ib BT r/m32, imm8 MI Valid Valid Store selected bit in CF flag.
REX.W + 0F BA /4 ib BT r/m64, imm8 MI Valid N.E. Store selected bit in CF flag.


Protected Mode Exceptions ¶
#GP(0)   If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
If the DS, ES, FS, or GS register contains a NULL segment selector.
#SS(0)   If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)   If a page fault occurs.
#AC(0)   If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD   If the LOCK prefix is used.

Real-Address Mode Exceptions ¶
#GP   If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS   If a memory operand effective address is outside the SS segment limit.
#UD   If the LOCK prefix is used.

Virtual-8086 Mode Exceptions ¶
#GP(0)   If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0)   If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)   If a page fault occurs.
#AC(0)   If alignment checking is enabled and an unaligned memory reference is made.
#UD   If the LOCK prefix is used.

Compatibility Mode Exceptions ¶
Same exceptions as in protected mode.

64-Bit Mode Exceptions ¶
#SS(0)   If a memory address referencing the SS segment is in a non-canonical form.
#GP(0)   If the memory address is in a non-canonical form.
#PF(fault-code)   If a page fault occurs.
#AC(0)   If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3.
#UD   If the LOCK prefix is used.

[jj2007]

The analysis is correct, but what do you need it for? The code assembles with MASM, with UAsm, you can use this:

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db 0F0h
bt G_TlsMap, esi

And then you run it, and it crashes 8)

[LiaoMi]

The analysis is correct, but what do you need it for? The code assembles with MASM, with UAsm, you can use this:

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db 0F0h
bt G_TlsMap, esi

And then you run it, and it crashes 8)

to throw an exception #UD, it would be possible to use hardcode, I wonder why, some instructions are blocked, but others are not. For example according to intel ..

CLAC—Clear AC Flag in EFLAGS Register
Protected Mode Exceptions
#UD If the LOCK prefix is used.
If the CPL > 0.
If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.
Real-Address Mode Exceptions
#UD If the LOCK prefix is used.
If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.
Virtual-8086 Mode Exceptions
#UD The CLAC instruction is not recognized in virtual-8086 mode.
Compatibility Mode Exceptions
#UD If the LOCK prefix is used.
If the CPL > 0.
If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0.
64-Bit Mode Exceptions
#UD If the LOCK prefix is used.
If the CPL > 0.
If CPUID.(EAX=07H, ECX=0H):EBX.SMAP[bit 20] = 0

The same can be said about the call command
lock clac
lock call

but in this case the macro assembler says exactly the same -

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Assembling: mywindow1.asm
mywindow1.asm(26) : error A2068:instruction prefix not allowed
Microsoft (R) Incremental Linker Version 14.16.27024.1
Copyright (C) Microsoft Corporation.  All rights reserved.

LINK : fatal error LNK1181: cannot open input file 'mywindow1.obj'
Press any key to continue . . .

I cant see the logic in this example  :icon_confused:, intel says you can use it, macro assembler says differently, UASM says, you can not use. The question is, who is right ?! :icon_rolleyes:

[jj2007]

Assembles with MASM version 14.00.23026.0 (which you should never ever use for work because it has very serious bugs):

  int 3
  nops 2
  clac
  nops 2
  lock bt G_TlsMap, esi
  nops 4

This is what Olly sees (both instructions throw exceptions):

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0040637B           ³.  CC                int3
0040637C           ³?  90                nop
0040637D           ³?  90                nop
0040637E           ³?  0F01CA            clac
00406381           ³?  90                nop
00406382           ³?  90                nop
00406383           ³?  F0:0FA335 F881400 lock bt [4081F8], esi            ; LOCK prefix is not allowed
0040638B           ³?  90                nop
0040638C           ³.  90                nop
0040638D           ³?  90                nop

[aw27]

We are talking about a MASM bug which must be reported to Microsoft Corporation (sometimes they fix them, most times not  ::)).

According to Intel:
The LOCK prefix can be prepended only to the following instructions and only to those forms of the instructions
where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCH8B,
CMPXCHG16B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG. If the LOCK prefix is used with one of
these instructions and the source operand is a memory operand, an undefined opcode exception (#UD) may be
generated.

There is no LOCK BT.

[LiaoMi]

Hi AW,

There is no LOCK BT.

:icon_exclaim:

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00406383           ³?  F0:0FA335 F881400 lock bt [4081F8], esi            ; LOCK prefix is not allowed


According to Intel:
The LOCK prefix can be prepended only to the following instructions and only to those forms of the instructions
where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCH8B,
CMPXCHG16B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG. If the LOCK prefix is used with one of
these instructions and the source operand is a memory operand, an undefined opcode exception (#UD) may be
generated.

That's right, I thought about it, the conclusion that the architecture does not forbid me to use ud exceptions and each assembler developer independently decides how to handle such instructions.