7 Jan
2015
7 Jan
'15
12:14 a.m.
On 2015-01-06 23:36, Ronald Natalie wrote:
Another quaint bit of history was when we made the
jump to actually running the kernel in split-I/D mode. My good friend Joe Pistritto
wrote the JHU boot loader for that. The 512-byte boot loader that was the standard UNIX
one was used to load Joe’s split I/D booter. It had a better support of the UNIX file
system, but the question was how do you get from a non-split I/D program into the split
I/D program. Joe’s solution was rather clever. He put an instruction that stored the
processor status word with the new kernel mode at the top of the boot loader’s address
space. As he did the store the PC rolled over and now it was running at the new mode at
location zero.
??? Are you sure you remember that right?
The change from non split I/D to split I/D is not in the processor
status word. Also, the last address of memory, before you enable the
MMU, is actually the PSW. You can't have code there.
Years later I found that others had solved the problem
by just setting up the kernel registers and executing a trap which switched the modes.
I always thought Joe’s solution was more elegant. The kernel started the same way any
other UNIX program would start.
I'm probably missing a whole bunch of detail here, as I'm not fully
following what was done.
Also, I fail to even spot the problem. Enabling split I/D space is just
a bit in the MMU, but even after, you can have the same memory pages in
both page tables, in essence making it a noop. Of course, being able to
have data outside your code means you can have so much more code, in
addition to more data, that you'd just would want to keep them split.
But that means just setting up the two page tables appropriately, load
the memory as needed, and then enable the MMU and the split I/D, and
you're done.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol