Difference between revisions of "X86 microcode system"

Revision as of 14:55, 7 June 2007

Microop Parameter Specialization

Microops are not, in general, written in an absolute sense. They are templates which provide an implementation for a macroop, but need to be specialized using the arguments of the original instruction. In hardware, this appears to be handled in two different ways depending on the origin of the microop. In the case of combinationally generated microops, the instructions are actually generated with the correct parameters in place. This is how the macroops in SPARC work. If the microops come from the ROM, they are specilized afterwards by a special hardware unit which operates on them before they pass on to the rest of the pipeline. The "emulation environment", aka the particulars of the instruction being emulated by the microop sequence, is stored and is used by this specializer to get the values to substitute in. In hardware, this means recognizing a bit pattern and replacing the value with a different one on a match. In C++, this becomes more complicated. There seem to be three options.

• The instructions could be allocated with the correct parameters, similar to the combinational case. This has the benefit that the instruction is totally specialized for it's environment from the very start.
• The instructions could be specialized after the fact by adding a new function. This lets you reuse instructions without having to fully reallocate them.
• The instructions could be specialized by adding in an external blob of data which holds all the relevant parameters.

The simplest approach would be the first, but allocating a new microop for every instruction from the ROM would likely have a fairly high performance overhead. The later two approaches help with that somewhat by letting you skip actually allocating memory, but they prevent you from using more than one version of a microop at a time.

Immediate values are handled by making ROM based microops generate a few combinational microops before actually going to the ROM. These push the immediate parameters into certain microcode registers which the ROM code is written to use.

Microop Allocation

Microops are allocated in two different ways. When they're generated combinationally, they can be set up by their containing macroop when it gets allocated. Existing mechanisms which cache regular instructions also cache the macroop, and the microops end up being saved as well. With microops that come from the microcode ROM, microops aren't stored with the macroop, and the caching mechanism doesn't automatically work.

Also, the microop that should be generated from a particular microopc will depend, probably significantly, on the macroop it was generated for. This ties in with the above topic about parameter specialization, and means that the ROM can't directly hold instatiations of it's microops which would be a handy caching mechanism.

One potential solution would be for the ROM to have it's own cache of microop instantiations. This wouldn't be as simple as in the regular decode case because there aren't handy machine code representations to work with, but the micropc might be used instead. Unlike regular instruction memory, the same micropc should always refer to the same instruction. Note that while this might seem like a reasonable assumption, there are situations I can imagine someone at some point might want to have dynamic microcode.

Another option would be to actually store the microops in the macroop which requested them. The draw back here is that it's possible that you're macroop might need a non constant and large set of microops to implement itself, and it might be difficult to keep track of what's been instantiated and what hasn't.

Yet another solution would be to try to work the microops into the main cache somehow. This doesn't sound like a great idea since they really aren't the same thing.

Macroop Argument Specialization

This sounds like but is very different from microop parameter specialization. This refers to generating groups of macroops which all perform the same operation, but use different arguments from different places including memory.

The solution which will be implemented for now will be to generate the microcode using python. The python will generate a memory based and non-memory based implementation. A preprocessor would work, but it would likely be overkill for what's needed.

To allow the decoder to specify what variant of arguments to use, the system of passing tags into the instruction format will be revamped. It will work essentially the same, but instead of doing replacement on the register arguments, it will need to take advantage of whatever the microop parameter specialization mechanism is to set things up.