Opcodes
An opcode is a 1-byte FixedUInt that tells the reader what the next expression represents
and how the bytes that follow should be interpreted.
The meanings of each opcode are organized loosely by their high and low nibbles.
| High nibble | Low nibble | Meaning |
|---|---|---|
0x0_ to 0x3_ | 0-F | E-expression, opcode is the address |
0x4_ | 0-7 | E-expression, opcode is the address |
8-F | E-expression with extended address | |
0x5_ | 0-7 | Symbols with symbol address |
8-9 | Annotations | |
A | Reserved | |
B | Tagless-element List | |
C | Tagless-element S-expression | |
D-F | Reserved | |
0x6_ | 0-8 | Integers from 0 to 8 bytes wide |
9 | Reserved | |
A-D | Floats | |
E-F | Booleans | |
0x7_ | 0-F | Decimals |
0x8_ | 0-C | Short-form timestamps |
D | Reserved | |
E | null.null | |
F | Typed nulls | |
0x9_ | 0-F | Strings |
0xA_ | 0-F | Symbols with inline text |
0xB_ | 0-F | Lists |
0xC_ | 0-F | S-expressions |
0xD_ | 0 | Empty struct |
1 | Reserved | |
2-F | Structs | |
0xE_ | 0 | Ion version marker |
1-7 | Directives | |
8-A | Template placeholders | |
B | Absent Argument | |
C-D | NOP | |
E | Struct switch modes (SID ↔ FlexSym) | |
F | Delimited container end | |
0xF_ | 0 | Delimited list start |
1 | Delimited S-expression start | |
2 | Delimited struct start (SID Mode) | |
3 | Delimited struct start (FlexSym Mode) | |
4 | E-expression with FlexUInt macro address followed by FlexUInt length prefix | |
5 | Integer with FlexUInt length prefix | |
6 | Decimal with FlexUInt length prefix | |
7 | Timestamp with FlexUInt length prefix | |
8 | String with FlexUInt length prefix | |
9 | Symbol with FlexUInt length prefix and inline text | |
A | List with FlexUInt length prefix | |
B | S-expression with FlexUInt length prefix | |
C | Struct with FlexUInt length prefix (SID Mode) | |
D | Struct with FlexUInt length prefix (FlexSym Mode) | |
E | Blob with FlexUInt length prefix | |
F | Clob with FlexUInt length prefix |
In addition, some opcodes have different meanings when used as tagless types.
| High nibble | Low nibble | Meaning |
|---|---|---|
0x6_ | 0 | FlexInt integer value |
0xE_ | 0 | FlexUInt integer value |
1-8 | FixedUInt integer values from 1 to 8 bytes wide | |
A | Symbol value with FlexUInt symbol address | |
E | Symbol FlexSym |