[TUHS] history of virtual address space

[brantley coile via TUHS]

We got first edition Unix working some years ago. The user program was loaded at

orig = 0 .
core = orig+40000

That's octal so 16k.

I did a port of Seventh Edition where I put the kernel and user programs at zero. That was a mistake. That was in 1986. I then moved the user program to 4K and the kernel to 0x80000000. I got that from BSD. Don't know if it was the first. 

Maybe you could peep at 32V, but I'm sure it loads the kernel at 1<31. It's how the VAX hardware works, if I'm not mistaken.


https://www.tuhs.org/Archive/Distributions/Research/Dennis_v1/PreliminaryUnixImplementationDocument_Jun72.pdf

bwc


> On Jan 19, 2026, at 2:44 PM, ron minnich via TUHS <tuhs at tuhs.org> wrote:
> 
> I was trying to remember how virtual address space evolved in Unix.
> 
> When I joined the game in v6 days, processes had virtual 0->0xffff, with
> the potential for split I/D IIRC, getting you that bit of extra space. The
> kernel also had virtual 0->0xffff, also with split I/D. The kernel
> accessed user data with mov/load from previous address space instructions.
> But also on the 11, trap vectors chewed up a chunk of low physical memory,
> and the diode ROM took out  some chunk of high physical. The bootstrap you
> set from the front panel started with many 1's.
> 
> Later, when we had bigger address spaces on 32- bit systems, page 0 was
> made unavailable in users address space to catch NULL pointers. Did that
> happen on the VAX at some  point, or later?
> 
> Later we got the "kernel is high virtual with bit 31 set" -- when did that
> first happen?
> At some point, the conventions solidify, in the sense that kernels have
> high order bits all 1s, and user has page 0x200000 and up; and user mode
> does not get any virtual address with high order bit set. My rough memory
> is that this convention became most common in the 90s.
> 
> So, for the pre-mmu Unix, what addresses did processes get, as opposed to
> the kernel? Did the kernel get loaded to, and run from, high physical? Did
> processes get memory "above trap vectors" and below the kernel?
> 
> For the era of MMUs, I kmow the data general 32-bit systems put user memory
> in "high virtual", which turned out to be really painful, as so many Unix
> programs used address 0 (unknown to their authors!). What other Unix
> systems used a layout where the kernel was in low virtual?
> 
> I'm not even sure where to find these answers, so apologies in advance for
> the questions.