Before you can say "this variable lives in BSS," you must already be fluent in a stack of tiny ideas: what a byte is, what an address is, what "read-only" means, what "grows up" means. The parent note quietly assumes all of them. This page builds each one from absolute zero, in the order they depend on each other.
Why start here? Because everything in the topic — a variable, an instruction, a pointer — is ultimately just a run of bytes. If you don't picture the box, "4 MB in BSS" is meaningless noise.
The picture shows memory as a vertical ruler of boxes. Each box is one byte. To the left of each box is its position number — that number is the whole game, and it has a name: the address.
This distinction is the heart of why the parent has two separate segments for globals. Build it carefully.
The picture makes the split obvious: a box holding 5 must physically carry that 5 in the file on disk (→ Data segment). A box that is just "zero" can be described by a sticky note — "reserve N boxes, all zero" — costing almost nothing on disk (→ BSS segment). This is the entire reason Static and global variables — storage duration separates Data from BSS.
Look at the arrows in the figure. The heap (burnt-orange) starts in the middle and its top edge climbs upward every time you malloc. The stack (teal) starts near the ceiling and its bottom edge sinks downward every time a function is called. They rush toward the empty gap between them — this is exactly the parent's "they share the gap in the middle." If they ever touch, you get a stack-overflow / heap-overflow, which connects to Buffer overflow and stack smashing.
Read it top-down: the byte feeds address, address feeds every notion of position and direction, and permission + duration decide which segment a value lands in. All four streams pour into the Memory Layout topic.
Recall Self-test: can you answer each before peeking?
What is a byte, in one sentence? ::: The smallest named chunk of memory, holding one number from 0 to 255.
What does "address" mean? ::: The position number of a byte on the memory ruler.
What does &x give you? ::: The address (box number) where x starts, not its value.
Why is 0x2000 "higher" than 0x1000? ::: It is a bigger number, so it is a box further down the ruler.
How many bytes is a typical int? ::: 4.
Why does an initialized global cost disk space but a zero one doesn't? ::: The non-zero value is real information stored in the file; all-zeros is described by a size note only.
What does "read-only" let you do and forbid? ::: Read the bytes, but never write them (writing crashes).
"Heap grows up" means what about addresses? ::: Each new heap block lands at a higher address.
"Stack grows down" means what about addresses? ::: Each new call frame lands at a lower address.
Which marker tracks the heap's top edge? ::: The program break.
Which marker tracks the stack's top edge? ::: The stack pointer (e.g. rsp).
Do all globals have static storage duration? ::: Yes, even without the static keyword.
