Re: register sets

On Tue, 15 Jul 2025 4:49:55 +0000, John Savard wrote:

On Wed, 16 Apr 2025 23:42:20 -0400, Robert Finch wrote:
>

Another thought is to not include float registers for anything other
than apps. It would save 32 regs per mode, possibly allowing three
register sets to be provided.

>
I had to re-read this to understand it.
>
Some architectures in the past did speed up context-switching between
user programs and the operating system by giving the operating system
its own set of registers.
>
And the operating system doesn't need to do floating-point math, only
user programs that do calculations. So what could go wrong with this
great idea?

OSs often need to move big chunks of memory (disk cache->application)
and thus need the registers that are most powerful when doing MOVs
that is the SIMD registers (at least a few)

The first thing that comes to mind is that even a computer with a GUI,
not merely a time-sharing mainframe, may have multiple user programs
running at once. So when a real-time clock interrupt hits, and it's
time to rotate from one compute-bound user program to another, those
floating-point registers *also* need to be saved to memory.
>
So you will probably need an outer ring of the OS that doesn't use
the special OS-specific set of registers.

Each GuestOS gets to decide for himself when to rotate the work queues.
The higher-ups just decide on which RTIs go to which GuestOSs.

John Savard