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> So DEC's PDP-8 and OS-8 directly inspired the entire PC-compatible industry, the whole x86 computer industry. I disagree w/ the claim that the PDP-8 directly

by ibiza 5y ago

> So DEC's PDP-8 and OS-8 directly inspired the entire PC-compatible industry, the whole x86 computer industry. I disagree w/ the claim that the PDP-8 directly inspired x86. At the operating system level, maybe: OS/8 -> CP/M -> DOS. At the architecture level, no: PDP-8 -> 8080? The x86 architecture is _nothing_ if not a slavish progression from 8080, 8085, 8086, 80186, 80286, 80386, ... https://en.wikipedia.org/wiki/PDP-8#Description https://en.wikipedia.org/wiki/PDP-8#Description https://en.wikipedia.org/wiki/Intel_8080#Registers https://en.wikipedia.org/wiki/Intel_8080#Registers

retrac 5y ago
I said the 8086 is somewhat like the PDP-11, not the PDP-8! The PDP-8 was an extremely simple machine and quite unlike anything from the 70s really, mini or micro. I would definitely agree the 8080 itself was not inspired or influenced by the PDP-11. The timing is just wrong for that, if nothing else. The earliest ancestor to the 8080 was designed in the late 60s, before the PDP-11 came out. And of course the 8086 was designed in the 8080's lineage, even source level compatible somewhat. I think you just misread me, but since I thought more about it I'll outline my argument more clearly. Nearly all the changes from the 8080 to the 8086 were changes that, in practice, made it more like the PDP-11s: * The 8080 was an accumulator machine. While the 8086 retains some specialized registers and the superficial register file of the 8080, it's largely a general register machine. It can, for example add any two registers together, or add any register and memory together. It can use any of several registers for an index register. * Segmentation is basically a poor man's split I/D + paging on the PDP-11. While there's no protection, it's similar enough that the 8086 was the first microcomputer UNIX was ported to. * The 8080 had a single stack intended pretty much just for subroutine call/returns. Annoyingly it limited what you could push. The 8086 has a general stack, and a second stack pointer too. This is mimicking a common programming model on the PDP-11. As such, C is easy to implement on the 8086 and rather hard on the 8080. (Though Intel explicitly cited Pascal as the intent there at release, so this isn't that strong of a point, I suppose.) * Similar orthogonal byte and word operations for most instructions. * Even the 8086 assembly mnemonics were very similar to the PDP-11 (or VAX) mov/movb/bne/beq/cmp/inc/dec compared to mov/mov.b/jne/jeq/cmp/inc/dec.
- ibiza 5y ago
  Very good points. I should have made a top-level comment as I was refuting the article's rather strong assertion, not yours.

by ibiza 5y ago

retrac 5y ago

I said the 8086 is somewhat like the PDP-11, not the PDP-8! The PDP-8 was an extremely simple machine and quite unlike anything from the 70s really, mini or micro. I would definitely agree the 8080 itself was not inspired or influenced by the PDP-11. The timing is just wrong for that, if nothing else. The earliest ancestor to the 8080 was designed in the late 60s, before the PDP-11 came out. And of course the 8086 was designed in the 8080's lineage, even source level compatible somewhat. I think you just misread me, but since I thought more about it I'll outline my argument more clearly. Nearly all the changes from the 8080 to the 8086 were changes that, in practice, made it more like the PDP-11s: * The 8080 was an accumulator machine. While the 8086 retains some specialized registers and the superficial register file of the 8080, it's largely a general register machine. It can, for example add any two registers together, or add any register and memory together. It can use any of several registers for an index register. * Segmentation is basically a poor man's split I/D + paging on the PDP-11. While there's no protection, it's similar enough that the 8086 was the first microcomputer UNIX was ported to. * The 8080 had a single stack intended pretty much just for subroutine call/returns. Annoyingly it limited what you could push. The 8086 has a general stack, and a second stack pointer too. This is mimicking a common programming model on the PDP-11. As such, C is easy to implement on the 8086 and rather hard on the 8080. (Though Intel explicitly cited Pascal as the intent there at release, so this isn't that strong of a point, I suppose.) * Similar orthogonal byte and word operations for most instructions. * Even the 8086 assembly mnemonics were very similar to the PDP-11 (or VAX) mov/movb/bne/beq/cmp/inc/dec compared to mov/mov.b/jne/jeq/cmp/inc/dec.

ibiza 5y ago
Very good points. I should have made a top-level comment as I was refuting the article's rather strong assertion, not yours.