[1718627440]

That's how stacks on my desk grow and how everything grows in reality. I wouldn't numerate stacked things on my desk from the top, since this constantly changes. You also wouldn't name the first branch of a tree (the plant) to be the top-most one.

In your example "the stack grows down", seems to be wrong in the image.

[bignerd_95]

Thanks! I tried to rewrite the final sentence

[1718627440]

Yeah, but does that really help? The phrases "growing down/up" still exist and now you defined them to mean the opposite. This issue still didn't go away, since heap and stack still grow in different directions. Can't you just start drawing from the bottom of the blackboard, and it will be obvious? Coordinate systems also typically work that way.

[bignerd_95]

Yes, I draw the heap starting at the top of the board and the stack starting at the bottom of the board and grow them toward each other. That works fine in a one-off explanation.

The problem is that most textbooks draw the opposite, so the student leaves my lecture, opens a book or a slide deck, and now “down” means a different thing.

It gets worse when they get curious and look at a real process with /proc/<pid>/maps. Linux prints mappings from low address to high address as you scroll down (which matches my representation). That is literally reversed from the usual textbook diagram. Students notice and ask why the book is “wrong.”

So I've learned I have to explicitly call this out as notation.

Same story as in electronics class still teaching conventional current flow (positive to negative), even though electrons move the other way (negative to positive). Source: https://www.allaboutcircuits.com/textbook/direct-current/chp.... Historical convention, and then pedagogy has to patch it forever.

[ofalkaed]

Starting at the bottom of the blackboard would be backwards from how it prints in the terminal when you cat /proc/<pid>/maps.

[1718627440]

The way it's printed in the terminal is honestly backwards to me. This seams to only come from the scroll direction of the terminal, and because this is not a drawing, but a simple list. Every other tool like a debugger show it in the opposite direction and in all illustrations I have read it's that way too.

[bignerd_95]

So you use debuggers. Good. Then you can confirm that the program counter is incremented after each instruction, and that you read assembly from top to bottom. That means smaller addresses are at the top and larger addresses are at the bottom. This matches my layout, and it also matches what you see in the terminal in /proc/<pid>/maps.

[1718627440]

Incrementation direction is orthogonal to up/down. When I disassemble a program, the addresses are all over the place and not ordered.

> That means smaller addresses are at the top and larger addresses are at the bottom.

No, my mental model is the exact opposite and this matches the jargon out there.

> also matches what you see in the terminal in /proc/<pid>/maps

I think of this as a sorted list, not as a display or description of a model.

When I drive on a road, I think of think of things on the road near me to have lower addresses in my coordinate system and things further away as having higher addresses. When I write a list of things I see, this will be from left-to-right then from top-to-bottom on a sheet of paper, because it is a list that follows the writing directions of my language/script. When I look at a traffic sign things nearer to me will be at the bottom and things far away at the top, because that's the agreed-upon mental model of a road. When I look at my navi, things near me will be in the center and things far away from me at the edge of the display.

When I write down points in the first sector of the coordinate system, I might order things according to the x-coordinate ascending top-to-bottom. That doesn't mean I would draw the axis inverted.

The correspondence of physical addresses to position is entirely non-linear and also three dimensional so there is no natural top and bottom especially when we are talking about virtual addresses.

When I get taught a new concept I want to get to know the model everyone uses. I will not like a teacher, that tells me a different ordering which is different from how everyone else does it, because this the output of some random command on some random OS, which actually shows a list, not a graph of a memory model. (Sorry that's harsh, of course I still appreciate didactic simplification.)

Maybe the issue is that you consider the stack to be so important to determine the model of the whole process space. When I would draw a stack on it's own, I DO draw it from top-to-bottom. But when I draw a whole process space, I do not, because everything else is mapped/allocated from bottom-to-top. When you invert the direction of the mental model, yes the stack now grows from bottom-to-top. But no, the other things are now allocated from top-to-bottom instead. This are more things: the text, libraries, mmap'd files, and most-used thing: the heap are all allocated inverted now. And the most important thing, the index for all that: the addresses now are allocated from top-to-bottom.