Nopl

During research for my Geode Repair projects, I found something incredibly weird so this page is my attempt to write it down.

Background

The AMD Geode LX800 CPU is an incredibly strange CPU. Despite being branded AMD, it's a descendant of the Cyrix MediaGX with a ton of features stapled on.

It implements the following instruction sets:

• Pentium (i586)
• Pentium Pro (i686)
• MMX
• Extended MMX
• 3DNow!
• Enhanced 3DNow!
• Extra AMD Geode instructions

Notably the machine does not support PAE (Physical Address Extension). This may be why the CPUID identifies as family 5 (Pentium), not family 6 (Pentium Pro). This is all according to my reading of the AMD Geode LX Processors Data Book.

In contrast, some CPUs like the VIA C3 marked their CPU as family 6 without implementing the instruction set specified in the Pentium Pro Family Developer's Manual Volume 2. Most notably the 'CMOV' instruction.

But in theory, you could run an i686 system on the Geode without PAE. In practice it's a bit more complicated.

Undocumented i686 instructions

In 1995 Intel released the Pentium Pro and its developer manual documenting the entire instruction set.

By 1997 Christian Ludloff had created a map of 2 byte x86 opcodes. This confirmed Intel's documentation, but included a few unknown opcodes: 0F 34, 0F 35, as well as 0F 18 through 0F 1F.

In 1997 those first two opcodes 0F 34 and 0F 35 were determined to be Pentium II SYSENTER and SYSEXIT instructions despite Intel only documenting this later in 1999. It turns out the instructions were available on the Pentium Pro but broken. See SYSENTER, Where Are you? for a good summary of this situation.

Hilariously enough, the Geode LX800 identifies as a Pentium but supports the SYSENTER and SYSEXIT instructions introduced in the Pentium Pro and finally made useful in the Pentium II. Weird.

In 1998 Christian Ludloff documented in his updated map of 2 byte x86 opcodes that the 0F 18 through 0F 1F range of opcodes were hinting NOPs. The first being the 0F 18 opcode which maps to PREFETCHh instructions. I believe this information was documented first in the Intel Architecture Optimization Reference Manual.

Later in 2003 Christian Ludloff clarified in an email thread Undocumented opcodes (HINT_NOP) that these hinting NOPs were declared by Intel in their 1995 patent US5701442. The idea behind this patent from my reading is that you can encode a program written in another ISA as a series of opcodes that are run as NOPs on older machines and the new ISA on a newer machine.

I'm not sure why, but third party x86 CPUs aside from AMD didn't implement these NOPs. Perhaps Intel kept this patent close to their heart? Or maybe it's just not worth spending silicon and research on NOPs that nobody used?