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Comment by kens | Hacker News Reader

According to Steve Morse, the 8086 designer:

"Various alternatives for extending the 8080 address space were considered. One such alternative consisted of appending 8 rather than 4 low-order zero bits to the contents of a segment register, thereby providing a 24-bit physical address capable of addressing up to 16 megabytes of memory. This was rejected for the following reasons:

Segments would be forced to start on 256-byte boundaries, resulting in excessive memory fragmentation.

The 4 additional pins that would be required on the chip were not available.

It was felt that a 1-megabyte address space was sufficient. "

Ref: page 16 of https://www.stevemorse.org/8086history/8086history.pdf

Credit to mschaef for finding this.

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And it's been stated in related threads numerous times (and by you as well if I recall correctly) how Intel at that time strangely treated their low pin count limit as almost a religion... apparently that influenced many decisions.

The 8086 was designed to be a 16-bit 8080, and 16 times the address space of that chip was a nice incremental leap forward. If you didn't expect the chip to become the foundation of the PC industry for the next 4 decades, it was probably a reasonable choice. At the same time, Intel made the iAPX432 chip, which was the one that was supposed to be future proof and look how that turned out.

But they revisited and used that alternative later, in designing the 80286. (Launched Feb'82, with ~135K transistors, according to Wikipedia.)

If only there was a famous Bill Gates quote, about how 16MB (the limit of 24-bit physical addresses) ought to be more than enough...

Only in protected mode, and then segments were defined by descriptor table entries having a full 24 bit base address.

> Segments would be forced to start on 256-byte boundaries, resulting in excessive memory fragmentation.

Well, they were dead wrong about /that/.

Depends on how many segments you allocate. With 16 byte alignment, it can be practical to have each object (e.g. a linked list node) in its own segment so that the pointers will be only 16 bits while giving access to 1 MB of address space.

There were even some DOS C compilers that supported it -- similar to near/far/huge pointers. I can't remember what they were called, though.

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