[nine_k]

PDP-11 wasn't unique. To the contrary, it had many typical features.

- Uniform memory, cache memory is small, optional and transparent.

- A single linear address space; no pages, stack in the same RAM as data.

- A single CPU, with a single scalar ALU, and completely in-order execution; no need for memory barriers.

A typical modern machine larger than an MCU has several level of memory hierarchy which affect performance enormously, the physical RAM is mapped all over the address space, several execution units process data in parallel and often out of strict order, there are many variants of vector (SIMD) instructions, and usually a whole vector co-processor ("graphics card"). This breaks many of the assumptions that C initially ha made, and hardware tries hard to conceal the memory hierarchy (well, your OS may allow you to schedule your threads to the same NUMA domain), to conceal the parallel execution, to conceal the memory incoherence between processing nodes, etc. Well, you sort of can make the compiler infer that mean a vectorized operation, or use an intrinsic.

In my eyes, the C's assumptions about hardware show their age, and also hold the hardware back.

[crazyd987]

My memory of the PDP-11 is different. However, the pdp11 is/was a weird collection of systems, from tiny little single chip implementations to run a VAX console, upto large racks of boards to make up a cpu and fpu like the 11/70. I mainly worked on 11/44's an 11/73's which both shared a 16bit paged address space, (with no cache that I remember).

They had more physical memory (256k and I think 4M) than could be addressed by the instructions(64k).

The pages where 8k - so eight of them, and waving them around required an OS mapping function call.

The IO controllers where asynchronous, and the OS did preemptive multiprocessing and the problem-space was larger than 64k, and faster than the disk-drive, so multi-processing and locks where required to address it.

We used C and assembler on them. C was nicer than assembler to work with.

I don't see a difference of-kind between the pdp-11 and current computers. I do see a difference of 'know-ability' of the software stack that makes up a system.

There are so many external dependencies in the systems I have worked on since, many of them larger than the systems that loaded into that pdp-11, so being certain that there is no fault was almost always a pipe-dream. Automated tests helped - somewhat.

Often, confidence is based on the 'trajectory' of the rate of bugs discovered.