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docs: corrects docs on noun representation

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Jonathan Paprocki 2023-06-22 16:22:34 -04:00
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docs/stack.md
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@ -11,25 +11,27 @@ By treating a 0 MSB as the tag for a direct atom, we can compute directly with d
| MSBs | Noun |
|------|-----------------------|
| 0 | Direct Atom |
| 10 | Cell Pointer |
| 110 | Indirect Atom Pointer |
| 10 | Indirect Atom Pointer |
| 110 | Cell Pointer |
A direct atom is an atom which fits in a machine word, less one bit for the tag. It is stored directly.
An indirect atom is an atom which is too big to be a direct atom.
It is thus represented as a tagged pointer.
The memory referenced is at least 64-bit aligned.
The first 64 bits are the size of the atom in bytes.
This is followed by that number of bytes containing the atom in little-endian order.
An indirect atom is an atom which is too big to be a direct atom. It is thus represented as a tagged
pointer. The memory referenced is 64-bit aligned. The first 64 bits are metadata, followed by the
size in 64-bit words, then the actual value of the atom, whose number of words
is equal to the size in little-endian order. The metadata field is primarily used for
the mug. The first three bits of the size are reserved for a possible forwarding pointer tag.
A cell is represented as a tagged pointer.
The memory referenced is adjacent machine words.
The machine word at the pointer is the noun representation of the head (left side/first) of the cell.
The machine word directly following (higher in memory) is the noun representation of the tail (right side/second) of the cell.
A cell is represented as a tagged pointer. The memory referenced is adjacent machine words.
The machine word at the pointer is metadata. The machine word directly following (higher in memory)
is the noun representation of the head, followed by the noun representation of the tail. The metadata
field is primarily used for the mug.
During collection the memory for indirect atoms and cells may be rewritten to indicate a _forwarding pointer_.
This is discussed further in the "Memory Layout" section.
When an indirect atom is copied from one stack frame to another, its size is replaced with a
forwarding pointer. When a cell is copied from one stack frame to another, its
head is replaced with a forwarding pointer.
## Memory Layout