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ksherlock 2019-11-29 15:56:37 -05:00
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Merlin-16-REL-Format.md Normal file

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* File Type: `$F8` (User #8)
* Aux Type: (offset of relocation table)
Relocatable intermediate files are generated via the `REL` pseudo op. OMF object files may also be converted to REL format with the CON command (via the editor command box).
The file is assembled, and all expressions are evaluated, with an origin of `$8000`. External labels are given an address of `$8000`
Bytes 0 -- (aux type-1) are the assembled data.
This is followed by 0 or more relation records, a 0 byte, 0 or more label entries, and a 0 byte.
## Relocation Records
Relocation records are 4 bytes long. The first bytes is a flag byte. This is followed by a 16-bit offset, and an 8-bit operand byte. For external references, the operand byte is the external symbol number. For local references, the operand byte is the low 8-bits of the assembled operand.
### Shifting
Shifts (16 bit or 8 bit only) use two relocation records. The first record uses a flag byte of `$ff`. This is followed by the offset and operand/symbol number as above.
Following the `$ff` record is the shift record.
* `$d0` is the shift operand.
* bit 2 is the external (`$04`) / internal flag.
* `$d0` is a 16-bit, 1-byte shift (n.b. due to 24-bit address space, high byte of a 16-bit shift is always 0).
* `$d1` is an 8-bit, 2-byte shift
* `$d3` is an 8-bit, 1-byte shift
Other shift sizes are not supported.
This is followed by a 3-byte address of the value before shifting (as always, assuming an origin of `$8000`)
## Labels
The label type and length are encoded in the first byte.
* bit 5 (`$20`) indicates an absolute label (ie, exporting an `EQU`)
* bit 6 (`$40`) indicates a local (ENTry) label.
* bit 7 (`$80`) indicates an EXTernal label.
This is followed by the label and a 3-byte address.
For local and EQU labels, this is the label address (based on the `$8000` origin).
For external labels, this is the external label number + `$8000`.
## References
* Merlin 816 Macro Assembler Manual
* Merlin 16+ source code
* Quick Asm source code
* Empirical testing