What is the longest possible instruction encoding?

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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19874
Location: In your JS exploiting you and your system

revolution 18 Mar 2023, 04:33

It is documented that the maximum allowable instruction length is 15 bytes for all 386+ CPUs. This limit is imposed by the decoder. For earlier CPUs, the 286 was 10 bytes. And even earlier 186 and 8086 had no limit.

I tried to find instructions that reach the limits.

For 8086/186 it is too easy. We can get an infinite length instruction by simply adding more redundant prefixes. And after wrapping around at 64kB it will keep executing forever. But if the idea of redundant prefixes is not permitted then what are the longest instructions?

80286:

Code:

use16
lock add word[es:bx+di+0x1234],0x1234 ; longest I can find is 8 bytes.

80386:

Code:

use16
lock add dword[fs:ebx+edi+0x12345678],0x12345678 ; 15-bytes

For 32 bit:

Code:

use32
lock add dword[fs:ebx+edi+0x12345678],0x12345678 ; only 13-bytes
vinserti64x4 zmm0,zmm1,[fs:ebx+edi+0x12345678],0x45 ; only 13-bytes

How about 64-bit mode?

Code:

use64
lock add qword[fs:ebx+edi+0x12345678],0x12345678 ; 15-bytes

Where are the missing bytes for use32? How to get 10-bytes in the 80286? All without cheating by using redundant or invalid prefixes?

18 Mar 2023, 04:33

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8268
Location: Kraków, Poland

Tomasz Grysztar 18 Mar 2023, 08:17

revolution wrote:

How to get 10-bytes in the 80286? All without cheating by using redundant or invalid prefixes?

It is not possible, as far as I know. My copy of 286 manual states this:

INTEL 80286 PROGRAMMER'S REFERENCE MANUAL wrote:

Do not Duplicate Prefixes. The 80286 sets an instruction length limit of 10 bytes. The only way to violate this limit is by duplicating a prefix two or more times before an instruction. Exception 6 occurs if the instruction length limit is violated. The 8086/8088 has no instruction length limit.

The 386 manual has its own version of this paragraph:

INTEL 80386 PROGRAMMER'S REFERENCE MANUAL wrote:

The 80386 sets a limit of 15 bytes on instruction length. The only way to violate this limit is by putting redundant prefixes before an instruction. Exception 13 occurs if the limit on instruction length is violated. The 8086/8088 has no instruction length limit.

18 Mar 2023, 08:17

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19874
Location: In your JS exploiting you and your system

revolution 18 Mar 2023, 08:50

I suspect there were some widely used applications for the 8086/186 that had redundant prefixes for 10-byte instructions. And Intel didn't want to break them all by imposing an 8-byte limit on the 286. The 286 would have been a commercial failure if it couldn't run the existing 8086 apps.

18 Mar 2023, 08:50

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