Early CPU Memory Management

Side fact: Microprocessors did not have virtual memory or paging back then. They operated 1:1 on the bus lines.

Arrays at the hardware level can be implemented. Though, this wasn't always the case. Direct access to memory didn't really become a thing until the late 60s, with the advent of the Memory management unit (MMU). My first experience was with the 6502, and 6510. For instance the 6510 had 64k of addressable memory, but good luck ever having all of it.Many architectures had each word loaded into a register, and you never had direct access to the memory. Those were absolute nightmares by

I believe the 8086, as originally designed and implemented, had a segmented address space with base and bounds registers in the style of the CDC 6600. This rudimentary but effective memory management approach was scrapped by Intel to make the 8086 part commensurate with the stepper reticle and semiconductor process of the time.

Wait--weren't MMUs available back even on the 65/67?

segment registers were a cheap MMU before its age. It was the only way to run code without relocation tables at various addresses. Mind you that at this era the whole operating system fitted in 40kB of RAM! My old turbo-pascal 3.0 editor+compiler was something around 37 kB! Just try to write a hello world of that size nowadays! It's pointless to criticize the past based on 10000 times more powerful hardware nowadays, which doesn't even deliver the same work faster due to the tremendous

For sure, and up until the mid-to-late 80s RAM speeds were higher than processor speeds. So approaches to architecture were totally different.Most minicomputer class processors were sewn together from multiple chips and transistors; even their register sets for the CPU were often not dissimilar from main memory. Texas Instrument's minicomputer (and later microcomputer) architecture even just put registers in RAM. In the microcomputer world, the 6502 got around having a very small registe

It sounds like the same situation as large model on 8088. I'm wondering if any 8-bit systems did anything like this, but I don't think so.

You are correct. I have long forgotten about the mapped memory and was thinking about Unibus device registers having long 18-bit addresses. The 11/70 also had separate instruction and data spaces which did not exist in earlier models.

Many C programmers assume a single flat memory space. Most machines today have that. There's a long history of machines that didn't: Intel 286 in segmented mode, some Pentium variants in segmented mode beyond 4MB (Linux supports this in the kernel), IBM AS/400, Unisys Series B 36-bit word machines, Burroughs 5500/6700/Unisys Series A segmented machines where an address is a file-like path, Motorola CPUs where I/O space and memory space are distinct, and old DEC PDP

didn't we call it 'segmented memory' back in DOS days...?