5 ms·

Sure, but learning an old ISA can leave you with a very very wrong idea about how modern processors work. Even x86 assembly paints a very misleading image of ho

Sure, but learning an old ISA can leave you with a very very wrong idea about how modern processors work. Even x86 assembly paints a very misleading image of how modern processors actually work. For example, someone learning x86-64 assembly will likely believe all of the following: - assembly instructions are executed in the order they appear in in the source code - an x86 processor only has a handful of registers - writing to a register is an instruction like any other and will take roughly the same time - the largest registers on an x86 processor are 64-bit

t-3 1y ago
They will be disabused of any of those notions simply by reading the relevant portions of the architecture handbook. In a pedagogical environment that's very simple to arrange.
- tsimionescu 1y ago
  Someone who is just learning to program will not be well served by reading a modern CPU architecture handbook. It is far too complex for someone who doesn't even know yet what a graph is, for example.
  - t-3 1y ago
    They don't have to read the whole thing. Excerpts or specific pages/sections, presented under the guidance of an experienced teacher or mentor, are perfectly digestible. The instruction description pages are the best documentation for looking up how to use instructions as well.
flohofwoe 1y ago
Peeking under the hood is a later step after getting comfortable with assembly coding. E.g. none of those details are really relevant when starting out, instead it makes a lot of sense to do a speed run through computing history in order to really understand why modern CPUs (and computers as a whole) work like they do.
ThrowawayR2 1y ago
All of which are completely irrelevant implementation details hidden behind the ISA. The x86-64 ISA promises execution of instructions in the specified order, a certain number of registers, etc. and that's all they need to know.
- Someone 1y ago
  > The x86-64 ISA promises execution of instructions in the specified order It doesn’t, and out-of-order CPUs don’t do that. https://en.wikipedia.org/wiki/Out-of-order_execution https://en.wikipedia.org/wiki/Out-of-order_execution: “In this paradigm, a processor executes instructions in an order governed by the availability of input data and execution units, rather than by their original order in a program.”
  - flohofwoe 1y ago
    Out-of-order execution is an internal optimization, from the outside results are still guaranteed to be available in order - e.g. the instruction stream appears to be executed in order when observing the CPU from the outside. For instance you don't need to be afraid that an instruction uses garbage inputs just because a previous instruction hadn't finished computing an input value to the instruction. At worst you'll get a pipeline stall if the CPU can't fill the gap with out-of-order executed instructions. On some CPUs it does get tricky once memory is involved though (on ARM, but not on x86).
    - Someone 1y ago
      > from the outside results are still guaranteed to be available in order - e.g. the instruction stream appears to be executed in order when observing the CPU from the outside. > […] > On some CPUs it does get tricky once memory is involved though (on ARM, but not on x86). https://en.wikipedia.org/wiki/Memory_ordering https://en.wikipedia.org/wiki/Memory_ordering: “Among the commonly used architectures, x86-64 processors have the strongest memory order, but may still defer memory store instructions until after memory load instructions.”
  - ThrowawayR2 1y ago

t-3 1y ago

They will be disabused of any of those notions simply by reading the relevant portions of the architecture handbook. In a pedagogical environment that's very simple to arrange.

tsimionescu 1y ago
Someone who is just learning to program will not be well served by reading a modern CPU architecture handbook. It is far too complex for someone who doesn't even know yet what a graph is, for example.
- t-3 1y ago
  They don't have to read the whole thing. Excerpts or specific pages/sections, presented under the guidance of an experienced teacher or mentor, are perfectly digestible. The instruction description pages are the best documentation for looking up how to use instructions as well.

flohofwoe 1y ago

Peeking under the hood is a later step after getting comfortable with assembly coding. E.g. none of those details are really relevant when starting out, instead it makes a lot of sense to do a speed run through computing history in order to really understand why modern CPUs (and computers as a whole) work like they do.

ThrowawayR2 1y ago

All of which are completely irrelevant implementation details hidden behind the ISA. The x86-64 ISA promises execution of instructions in the specified order, a certain number of registers, etc. and that's all they need to know.

Someone 1y ago
> The x86-64 ISA promises execution of instructions in the specified order It doesn’t, and out-of-order CPUs don’t do that. https://en.wikipedia.org/wiki/Out-of-order_execution https://en.wikipedia.org/wiki/Out-of-order_execution: “In this paradigm, a processor executes instructions in an order governed by the availability of input data and execution units, rather than by their original order in a program.”
- flohofwoe 1y ago
  Out-of-order execution is an internal optimization, from the outside results are still guaranteed to be available in order - e.g. the instruction stream appears to be executed in order when observing the CPU from the outside. For instance you don't need to be afraid that an instruction uses garbage inputs just because a previous instruction hadn't finished computing an input value to the instruction. At worst you'll get a pipeline stall if the CPU can't fill the gap with out-of-order executed instructions. On some CPUs it does get tricky once memory is involved though (on ARM, but not on x86).
  - Someone 1y ago
    > from the outside results are still guaranteed to be available in order - e.g. the instruction stream appears to be executed in order when observing the CPU from the outside. > […] > On some CPUs it does get tricky once memory is involved though (on ARM, but not on x86). https://en.wikipedia.org/wiki/Memory_ordering https://en.wikipedia.org/wiki/Memory_ordering: “Among the commonly used architectures, x86-64 processors have the strongest memory order, but may still defer memory store instructions until after memory load instructions.”
- ThrowawayR2 1y ago