[TUHS] history of virtual address space

[brantley coile via TUHS]

I don't know if system designers thought that every cycle executed outside of system state was wasted or not, but the idea of four billion bytes of RAM begin installed in the system seemed like a far away fantasy in 1978. The most one could ever cram into a 780 was 128 MB. I was working on a machine with 256 KB and it seemed quite large. The big machines (IBM 370/158 and CDC Cyber 70/74) only had 4 MB and 128 KB. (The 128KB were 60 bits, I grant you.) 

Again, no opinion on designers attitudes towards pesky users.

Brantley

> On Jan 20, 2026, at 2:58 PM, Phil Budne via TUHS <tuhs at tuhs.org> wrote:
> 
> The "feature" of having user and system use non-overlapping virtual
> address ranges makes reading user data (eg system call arguments) easy (*)
> 
> The VAX reserving a full half of the address space for the system
> software has always struck me of typical of the mindset of
> kernel/monitor developers thru the ages: Cycles expended outside of
> the system state (ie; by users) are wasted(**)!!
> 
> (*) PDP-10's with paging systems all implemented special a "previous
> context" flavor of the XCT (execute) instruction for moving data
> between system and user spaces (systems with more physical memory than
> the full/original 256K word 18-bit virtual address space were common
> from the start.
> 
> (**) I first noticed it at DEC in the 80's; TOPS-20 monitor developers
> hating on EMACS users wasting time by context switching on each
> keystroke.