Re: locore.s' history?

[Max Okumoto <okumoto@xxxxxxxx>]

Thomas E. Spanjaard wrote:
> Zera William Holladay wrote:
>
> > Hi, I've been reading "Operating System Source Code Secrets Volume."  The
> > authors imply (I can't remember the exact quote) that locore has a
> > history.  Does anyone know where this name came from, or where it first
> > appeared?  My intuition tells me "locore" means low memory (as in old
> > magnetic core memory)?  Just curious.
>
>
> "locore" means "low core", and core refers to the RAM which used to be 
> magnetic drums. It was called "core memory" because it was closer to the 
> processing hardware than the tapes, disks et al. It got this name due to 
> the place in the memory address space where the code in this file got 
> loaded: the lowermost part of RAM, address 0 usually. This was some 
> requirement in the early days, although I'm not sure why.
>
> Cheers,
>         -- Thomas E. Spanjaard
>            tgen@xxxxxxxxxxxxx

Some of the older processors initialized the PC (program counter) to
zero when reset.  Thus people put the 'boot' code a location zero.
Thus the firmware rom was initially mapped starting at location 0.

There was a hardware trick where writes to low memory was read mapped
to the boot firmware, and write mapped to ram.  The first thing the
boot code did was to copy itself to the same location.

	char	*mem = 0;
	for (int i = 0; i < size_of_rom; ++) {
		mem[i] = mem[i];
	}

	disable_rom();

After the copy was done it would unmap the rom, and then would execute
out of 'fast ram'. (Roms in the past where really slow)

I think the 68k from motorola did that.

				Max