Linked by Hadrien Grasland on Fri 30th Dec 2011 08:24 UTC
Hardware, Embedded Systems In the world of alternative OS development, portability across multiple architectures is a challenging goal. Sometimes, it may be intrinsically hard to come up with hardware abstractions that work well everywhere, but many times the core problem is one of missing information. Here, I aim at learning more about the way non-x86 architectures deal with CPU IO ports, and in particular how they prevent user-mode software from accessing them.
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The headaches of Legacy Design.
by Snial on Fri 30th Dec 2011 09:54 UTC
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On a non-x86 architecture it's easy and simple, you just map the memory addresses used for I/O out of user-space.

If we look at it a different way, I/O access is merely equivalent to an extra address bit, that is, you need an extra IO signal on a CPU, which could have been used to provide an extra address bit.

So, specific I/O instructions not only complicate work for the instruction set, compilers and device driver portability, but reduce memory space by a factor of two: and on the original 8-bit 8080 from which the x86 inherits its I/O model; the extra 64Kb would have been really valuable.

An I/O address space also doesn't really help even as an address space because virtually all practical computers from the 8-bit era onwards contained both I/O addressed hardware and memory-mapped hardware.

Video memory (actually an output device) was a common example. It was also true of the early 80s x86 PCs where addresses from 0xA0000 to 0xFFFFF were reserved for I/O, because the 1024 addresses provided by their (incompletely decoded) I/O slots weren't enough even then, 30 years ago.

So as you note, I/O portability is a problem for x86 PCs too since some I/O will be memory-mapped.

So why did Intel do this? Two reasons spring to mind: Firstly, the early Intel processors were really seen as embedded systems for controlling I/O, having a separate I/O signal reduced hardware (though this isn't convincing, you would still need additional decoding to handle more than 1 chip).

Secondly, and more probably, Intel were really memory chip producers and were new to processor design; employing people with no prior experience in computer architecture to design their early processors. Consequently, they simply copied I/O concepts from old minicomputers such as the pdp-8 and HP2100 without quite understanding why these machines had such features.

Hope this helps! (Julian Skidmore, designer of the DIY 8-bit computer FIGnition).

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