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Indexed registers
Author:  Date: 2016-09-26 22:00
Ok, here's an idea I haven't heard of: Set up some registers: ProgramID, ThreadID, and ContextID. These are controlled by the CPU and set up by the OS when spawnning a program or thread. The app programmer only has access to ContextID. The bits of each of these combine to address a memory buffer for the CPU registers. A thread or program task switch simply indexes a different set of registers - no need to preserve registers between task switches. You could even include some instructions for register exchange (EXX) that simply change ContextID. Maybe this mechanism could also supply register renaming functionality...?

The actual size of the registers is implementation specific, it just depends on how the address bits are devied up. This supports the variable vector registers. Of course this would have to be a very fast memory. The idea came from the Page 0 stuff you could do on the good ol' 6502, where the first page of memory could be addressed with 1 byte.

If the actual registers were separate, and the register page could be copied from/to the real registers, you could use ContextID to fill all registers at once,, for very fast function initialization. The compiler could place instructions at program start that would fill up register contexts for critical functions, and call up that register set when entering each function. There's a lot of possibilities. It all depends on the ability to read from/write to this register page very quickly, and the ability to put enough of this memory on the chip. The amount of memory would limit the number of concurrent programs, threads and contexts, as well as the number and size of registers. Maybe you'd have to consider a program and a thread the same thing, which could reduce the size of this memory somewhat.

If done right, I think this could really speed up task switches and function initialization and save stack space. Another idea could be a partial register exchange. ProgramID and ThreadID (or just ThreadID) point you to the proper register bank, but a special command could cause, say R0 thru R15 to always read from ContextID 0, but R16 thru R31 would read from the Context set by this special bank switch command. For a 2-bit ContextID, that provides you another 64 registers to use, 1 bank of 16 at a time. So R0 thru R15 stay the same, but R16 thru R31 are pulled from 1 of 4 banks. Or you do the full context switch, for 32 out 128.

I'm not sure how this would affect other processes, but on the surface, it sounds powerful.

Please note that I am describing 2 opposing possible setups:
Setup #1: The registers live in this memory block, and are one and the same.
Setup #2: The registers are read from this memory block with a GetContext command, and written to the memory block with a PutContext command.

I see benefit in both setups. Unfortunately, I think either setup might have performance problems, unless some very specialized burst read/write hardware is built. But it could be a joy to program. Imagine placing all your immediates and constants into a register context once at the start of your program. Then, when you call your function, a single command sets up all your constants, your loop counts, and your buffer pointers, without using the stack, or even setting registers. Compilers would have to be a bit different, but we're trying for a better design, right?
Could be interesting. You might even be able to justify reducing your number of registers, and depend on bank swapping, which would reduce your instruction set considerably.

Maybe a routine could "push it's context" to a function to achieve by-reference registers. Or copy it's context into another for by-value, without using the stack, or explicitly setting registers.

Is it a crazy idea? It is complex, but could be very powerful. I wonder if it is practical.

Ok, here's my last idea: If the above cannot be done, how about an instruction that pushes registers onto the stack, based on a mask. The push-by-mask opcode is followed by an immediate 32-bit value, with 1 bit dedicated to each register. Registers are always pushed in the same order. To push r0 thru r7, you'd use an immediate value of 011111111b. Or, to push r2, you'd use 0100b. I guess this would be slow, but it would be compact, and useful. Pop would, of course reverse, so the same mask could be used.

 
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