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Comment by CyberDildonics | Hacker News Reader

It was really the 386 that was the beginning of modern computing, since it had a mmu.

Several operating systems on 286 (eg Xenix, Coherent, OS/2) used its MMU for multitasking and memory protection. See https://en.wikipedia.org/wiki/Intel_80286#Protected_mode

Protected mode on the 286 allowed 24-bit addressing, enabling access to 16 MB of memory, but lacked virtual memory and required a reboot to return to real mode. The 386 introduced virtual memory through paging, 32-bit addressing for 4 GB of memory, and virtual 8086 mode for running multiple 8086 programs simultaneously without compromising security.

https://flint.cs.yale.edu/feng/cos/resources/BIOS/procModes....

https://en.wikipedia.org/wiki/Protected_mode

The 286 protected mode did not allow for a 32-bit flat address space and was heavily half-baked in other ways, e.g. no inbuilt way to return the CPU to real mode without a slow and fiddly CPU-reset.

It was architecturally a 16-bit CPU so a flat 32-bit address space would be a non sequitur. If you wanted flat 32-bit addressing, there was a contemporary chip that could do it with virtual memory: Motorola 68010 + the optional external MMU. (Or if you were willing to do some hoops, even a 68000.. see the Sun-1)

Coherent was the first Unix-like OS I ran, on a 386SX box. I think it was Coherent 4.x.

Except the 486 had hardware floating point, essential for technical work.

That's an advancement but that's a matter of speed an simplicity. An MMU is a huge before and after, it's still the biggest separator of CPUs today. The most important detail to understand a CPU is whether it has an MMU.

An MMU is pretty much necessary for robust multitasking. Without it, you are at the whim of how well software behaves. Without it, it is more difficult for developers to create well behaved software. That also assumes good intentions from programmers, since an MMU is necessary for memory protection (thus security).

While emulating an FPU results in a huge performance penalty, it is only required in certain domains. In the world of IBM PCs, it was also possible to upgrade your system with an FPU after the fact. I don't recall seeing this option for IBM compatibles. While I have seen socketed MMUs on other systems, I don't know whether they were intended as upgrade options.

By the way, "the i486SX was a microprocessor originally released by Intel in 1991. It was a modified Intel i486DX microprocessor with its floating-point unit (FPU) disabled." (https://en.wikipedia.org/wiki/I486SX)

You could buy a 8087 for your 8086 or 8088, the 486DX just moved it on chip.

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