3 ms·
You mean 32k words, not 32k bytes, right[1]? And AFAIK by V5 or V6, Unix could use split instruction and data if the MMU supported it giving a bit more headroo
by kjs3 2y ago
You mean 32k words, not 32k bytes, right[1]? And AFAIK by V5 or V6, Unix could use split instruction and data if the MMU supported it giving a bit more headroom. But, yeah, memory was very tight, and a lot of very clever tricks were used to get around it.
[1] Even worse, the top 4kW/8kB was reserved for I/O.
- p_l 2y agoI meant 32k bytes - PDP-11 was byte-addressed, not word-addressed. The 64kB address space was split in half between kernel and userland in so called "high and low moby" scheme (as it required minimal logic latching onto single address line). And for I/D split you needed appropriate CPU model. the top 8kB "I/O page" is reserved as part of the kernel space, not userspace, so it does not impact as much the userspace part.
- kjs3 2y agoAh, I misunderstood your point. And while the PDP-11 was byte addressable, the doco often talked about memory size in words. Carry on.