What Is MO:DCA? (Mixed Object Document Content Architecture)
MO:DCA — Mixed Object Document Content Architecture — is the data-stream architecture that defines how an AFP file is built. When people say "AFP," the bytes on disk are MO:DCA.
Structured fields
A MO:DCA document is a flat sequence of structured fields.
Each begins with a 0x5A carriage-control byte, a length, and a
three-byte identifier (a type plus a category), followed by flags, a sequence
number, and the field's data. The identifier says what the field is — Begin
Page, Presentation Text, Include Object, and so on.
The document hierarchy
Structured fields nest into a predictable tree of begin/end brackets:
- BDT … EDT — the document
- BPG … EPG — a page
- BAG … EAG — the active environment group (page geometry, fonts, color)
- object brackets for text, images, bar codes, and graphics
Triplets
Many fields carry triplets — self-identifying sub-parameters (a length, an id, and data) that add details like a fully qualified resource name, a measurement unit, or a color. They tile a field's data area exactly.
See it for yourself
The content inside MO:DCA comes from the object architectures (PTOCA, IOCA, BCOCA, GOCA, FOCA). readAFP parses the MO:DCA stream and shows the whole structured-field tree — every field, its decoded values, and its triplets — beside a render of the page. Open an AFP file and expand the tree to watch the hierarchy above in real data.