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Diffstat (limited to 'doc/0/0-value.md')
| -rw-r--r-- | doc/0/0-value.md | 27 |
1 files changed, 16 insertions, 11 deletions
diff --git a/doc/0/0-value.md b/doc/0/0-value.md index b84a528..61d01d9 100644 --- a/doc/0/0-value.md +++ b/doc/0/0-value.md @@ -135,6 +135,13 @@ different unit sizes. For instance, `istr` (interned string) pointers may use byte-addressing, meaning the 32-bit index limits the `istr` heap to 4 GiB only, while list pointers use 64-bit (8-byte) addressing, allowing for a 32 GiB heap. +Note: The separation of heaps is a possibility, not a guarantee. In practice, +the various 32-bit indexes may share a single value domain, in which case list +pointers, heap pointers, etc. would never have equal 32-bit index values; they +may or may not, depending on current implementation details. The unit size of +each type of 32-bit index is also not guaranteed, but is at least 8 bytes for +list pointers, since a list must contain at least one 64-bit Zisp Value. + ### List pointers In Zisp, a list is a contiguous array of a fixed number of Values. To improve @@ -156,12 +163,12 @@ lists can't needlessly trigger the code branch that handles list pointers. Note that "list pointer" and "list heap" are slightly misleading terms, since arbitrary-length lists can be allocated on the main heap as Array objects with element type Value. In this case, they are represented by a main heap pointer, -and the generic list API hides the difference. +and a generic list API may hide the difference. ### Heap pointers Regular heap objects are represented by this pointer type, which uses a 32-bit -index into the main heap, in the lower portion of the 48-bit payload. +heap index in the lower portion of the 48-bit payload. Of the 16 high bits of the payload, the upper 8 are used to immediately encode the type of the heap object, and the remaining 8 are used for internal metadata @@ -171,8 +178,10 @@ This means our 64-bit Values can be checked against heap types by comparing the 24 high bits to a combined constant: the 16 high bits that indicate it's a main heap index, plus 8 more bits encoding a specific heap type. -Some heap types may actually reside in their own separate heap, meaning the -32-bit index may refer to yet another memory region rather than the main heap. +Different types may reside in different heaps, meaning the 32-bit index values +of different heap types may or may not share value domains. The index value of +two heap pointers, of different types, being equal, is neither a guarantee that +they share a memory location, nor is it ruled out. ### String pointers @@ -180,13 +189,9 @@ An `istr` is a string of up to 255 arbitrary bytes, that is typically interned, fulfilling a similar purpose to symbols in Lisp and Scheme. If uninterned, we could consider the 'i' to mean *intermediate* length string instead. -Of the 48-bit payload value, the lower 32 bits are an offset into a dedicated -virtual memory region for this type only, bounding total memory use to 4 GiB, -which should be more than enough. - -The higher 16 bits of the payload are divided in two halves. The upper 8 bits -directly encode the length, which cannot be zero; the lower 8 bits are used for -garbage collection metadata. +Of the 48-bit payload value, the low 32 bits are a heap index, while the higher +16 bits are divided into 8 high bits for a non-zero length, and 8 low bits for +internal metadata such as for garbage collection. The empty string is represented as a *short string* instead; see below. |