A bug on lgdt code generation?

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humeafo

Joined: 12 Mar 2010
Posts: 3

humeafo 12 Mar 2010, 01:23

my dos program 16-bit segment code:

lgdt fword [gdtr_]

seems generate the 66 prefix, while this is not necessary according to intel manual?

12 Mar 2010, 01:23

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20623
Location: In your JS exploiting you and your system

revolution 12 Mar 2010, 01:46

Intel Manual wrote:

If operand-size attribute is 32 bits, a 16-bit limit (lower 2 bytes of the 6-byte data operand) and a 32-bit base address (upper 4 bytes of the data operand) are loaded into the register. If the operand-size attribute is 16 bits, a 16-bit limit (lower 2 bytes) and a 24-bit base address (third, fourth, and fifth byte) are loaded. Here, the high-order byte of the operand is not used and the high-order byte of the base address in the GDTR or IDTR is filled with zeros.

12 Mar 2010, 01:46

humeafo

Joined: 12 Mar 2010
Posts: 3

humeafo 12 Mar 2010, 01:51

yes， you're right. I misunderstand the meaning on my fist glance of the manual.
So fasm 1.50 generated the wrong code without 66h while not fasm 1.69.

12 Mar 2010, 01:51

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 12 Mar 2010, 01:53

Another source of info:

Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3A
- 9.10 INITIALIZATION AND MODE SWITCHING EXAMPLE
-- 9.10.2 STARTUP.ASM Listing

Code:

.
.
.
171 DB 66H; execute a 32 bit LGDT
172 LGDT TEMP_GDT_scratch

If you remove "fword" the prefix will not be generated, but keep in mind revolution's post.

12 Mar 2010, 01:53

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20623
Location: In your JS exploiting you and your system

revolution 12 Mar 2010, 01:54

If you don't need the 0x66 prefix and want a 24bt base address then just use this:

Code:

gdtr_: df 0x123456
;...
lgdt [gdtr_]

12 Mar 2010, 01:54

humeafo

Joined: 12 Mar 2010
Posts: 3

humeafo 12 Mar 2010, 01:58

thanks, problem solved.

12 Mar 2010, 01:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20623
Location: In your JS exploiting you and your system

revolution 12 Mar 2010, 03:44

LocoDelAssembly wrote:

If you remove "fword" the prefix will not be generated ...

You also have to make sure the variable definition for gdtr_ is not an fword type, hence the colon (:) after the variable name in my post above.

12 Mar 2010, 03:44

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