6 ms·

I think that the face of computing would have changed quite dramatically if Intel had decided to make the x86 segment paragraph size 256 bytes (8 bits) instead

by kabdib 6y ago

I think that the face of computing would have changed quite dramatically if Intel had decided to make the x86 segment paragraph size 256 bytes (8 bits) instead of the useless 4 bits. Imagine not having a 640K (or 1MB) ceiling (without hacks like "expanded" or "extended" memory). It would not have been a big deal for Intel to do this; I'm pretty sure they could have multiplexed four more pins worth of address lines. It would have given the x86 a native 24MB address space, equal to the 68000's physically provisioned 24 bits (which is all most 68K systems supported anyway, certainly the Atari ST and the Mac). For instance, in the mid 80s, VisiCorp was trying really, really hard to fit their products into 640K. They had some whizzy stuff, but the address space problem pretty much killed them. Later, Microsoft and IBM came along, did the disgusting memory-space hacks that plagued users for a decade or more, and VisiCorp foundered.

cesarb 6y ago
> if Intel had decided to make the x86 segment paragraph size 256 bytes (8 bits) instead of the useless 4 bits It would also have increased memory fragmentation. Many memory allocations were aligned to a segment boundary, so you'd be wasting up to 255 bytes instead of up to 15 bytes on each allocation.
- mrlonglong 6y ago
  Actually there aren't any alignment issues with the 8086, as it's still a 8 bit processor at heart.
  - mschaef 6y ago
    Segment alignment. Not read alignment. Large model memory allocators would allocate memory on segment boundaries. This is more or less essential if you need to use more than 64K of memory in your program. An 8-bit paragraph size (rather than the 4 that were used) would've forced all segment allocations to 256-byte boundaries, so that's where the fragmentation issue comes from. Steve Morse (one of the designers of the chip) talks about the issue in page 16 of this: https://www.stevemorse.org/8086history/8086history.pdf https://www.stevemorse.org/8086history/8086history.pdf
Someone 6y ago
“It would have given the x86 a native 24MB address space, equal to the 68000's physically provisioned 24 bit” That would be 16MB (plus possibly 65,536-256 bytes, depending on whether they added a bug on address wraparound (https://en.wikipedia.org/wiki/A20_line) https://en.wikipedia.org/wiki/A20_line)). Also, the 68000 would still have the clearer upgrade path, as the 24 address lines restriction was in the design of the chip, not in its architecture. As long as code didn’t assume addresses were modulo 2²⁴, code would run unmodified on a CPU with over 16MB memory, and use all of it. Also, the idea with the 8086 was that programs would actually use 16-bit pointers for objects, not 20-bit ones. That would be wasteful of memory if the granularity of such addresses were 256 bytes.
- kabdib 6y ago
  I meant 16MB, of course. Duh. Argument still stands.
Narishma 6y ago
Maybe we would still have had a ceiling, it just would have been later at 15MB or so instead of 640KB.
mschaef 6y ago
> I think that the face of computing would have changed quite dramatically if Intel had decided to make the x86 segment paragraph size 256 bytes (8 bits) instead of the useless 4 bits. I've made that argument in the past too, but I tend to think the 8-bit paragraph shift would've been a nice third mode on the 286 (or maybe a second mode on the 186). So a 286 would've had: * Real mode with 4 bit shift * Real mode with 8 bit shift * Protected mode with indirection through descriptor tables It seems plausible to me that it would've been possible to make software that would have binary compatible with either of the two real modes, which was not possible between real and protected. That would've opened up 286 users to easier access to memory beyond 1MB. But I don't know that this would've been a huge change for the industry, aside from killing off the need for LIM/EMS bank switching between 1985 and its death at the hands of Windows 3.0 in 1990. (And towards the later end of that period, the 386 could be made to provide EMS memory without additional hardware.) I think the best argument for it being a major change is that it might have undermined the need for OS/2.... but even then, OS/2 was the way it was, not due to memory constraints, but due to IBM's need to attempt to regain control of a market it'd lost control over. So 8-bit paragraphs would've been nice for big spreadsheet/dBase users in the 1986-88 period, but not much nore than that. (And all of this is just five or six years, really.... no time at all.)

by kabdib 6y ago

cesarb 6y ago

> if Intel had decided to make the x86 segment paragraph size 256 bytes (8 bits) instead of the useless 4 bits It would also have increased memory fragmentation. Many memory allocations were aligned to a segment boundary, so you'd be wasting up to 255 bytes instead of up to 15 bytes on each allocation.

mrlonglong 6y ago
Actually there aren't any alignment issues with the 8086, as it's still a 8 bit processor at heart.
- mschaef 6y ago
  Segment alignment. Not read alignment. Large model memory allocators would allocate memory on segment boundaries. This is more or less essential if you need to use more than 64K of memory in your program. An 8-bit paragraph size (rather than the 4 that were used) would've forced all segment allocations to 256-byte boundaries, so that's where the fragmentation issue comes from. Steve Morse (one of the designers of the chip) talks about the issue in page 16 of this: https://www.stevemorse.org/8086history/8086history.pdf https://www.stevemorse.org/8086history/8086history.pdf

Someone 6y ago

“It would have given the x86 a native 24MB address space, equal to the 68000's physically provisioned 24 bit” That would be 16MB (plus possibly 65,536-256 bytes, depending on whether they added a bug on address wraparound (https://en.wikipedia.org/wiki/A20_line) https://en.wikipedia.org/wiki/A20_line)). Also, the 68000 would still have the clearer upgrade path, as the 24 address lines restriction was in the design of the chip, not in its architecture. As long as code didn’t assume addresses were modulo 2²⁴, code would run unmodified on a CPU with over 16MB memory, and use all of it. Also, the idea with the 8086 was that programs would actually use 16-bit pointers for objects, not 20-bit ones. That would be wasteful of memory if the granularity of such addresses were 256 bytes.

kabdib 6y ago
I meant 16MB, of course. Duh. Argument still stands.

Narishma 6y ago

Maybe we would still have had a ceiling, it just would have been later at 15MB or so instead of 640KB.

mschaef 6y ago

> I think that the face of computing would have changed quite dramatically if Intel had decided to make the x86 segment paragraph size 256 bytes (8 bits) instead of the useless 4 bits. I've made that argument in the past too, but I tend to think the 8-bit paragraph shift would've been a nice third mode on the 286 (or maybe a second mode on the 186). So a 286 would've had: * Real mode with 4 bit shift * Real mode with 8 bit shift * Protected mode with indirection through descriptor tables It seems plausible to me that it would've been possible to make software that would have binary compatible with either of the two real modes, which was not possible between real and protected. That would've opened up 286 users to easier access to memory beyond 1MB. But I don't know that this would've been a huge change for the industry, aside from killing off the need for LIM/EMS bank switching between 1985 and its death at the hands of Windows 3.0 in 1990. (And towards the later end of that period, the 386 could be made to provide EMS memory without additional hardware.) I think the best argument for it being a major change is that it might have undermined the need for OS/2.... but even then, OS/2 was the way it was, not due to memory constraints, but due to IBM's need to attempt to regain control of a market it'd lost control over. So 8-bit paragraphs would've been nice for big spreadsheet/dBase users in the 1986-88 period, but not much nore than that. (And all of this is just five or six years, really.... no time at all.)