File types:
- ProDOS TXT or DOS T, with filename extension ".S"
Primary references:
- Reverse engineering
Merlin is an assembler written by Glen Bredon for the Apple II. Initially written for DOS 3.3, an updated version called "Merlin Pro" was published after ProDOS was released. After the Apple IIgs shipped, Merlin Pro was renamed to "Merlin 8", and a IIgs-specific version called "Merlin 16" was released.
Merlin provided its own source code editor. Source files use a customized text format.
Assembly source files have four columns:
- label
- opcode
- operand
- comment
To save space in memory and on disk, the contents of each column are separated by a single space character. Each column starts at a specific column on-screen. Lines starting with '*' are treated as full-line comments.
End-of-line comments start with a semi-colon. The editor doesn't require multiple separator spaces before a comment, but if you type them, they will be part of the file. A line that begins with ';' is considered an end-of-line comment.
The Apple II screen was 80 columns wide, so the code was generally formatted to fit in that space. The default placement of the four columns positioned them at screen positions 1, 10, 16, and 27, respectively (where 1 is the left edge of the screen).
The file contents are generally high ASCII, but spaces in comments and quoted text in operands are encoded as 0x20. Columns are separated with a single space character (0xa0). Encoding spaces this way makes it easy to find the column breaks, because you don't have to worry about tracking whether you're inside quoted text. Lines end with a carriage return (0x8d).
The DOS Toolkit ED/ASM source format was very similar, but didn't use 0x20 for spaces in comments and quoted operands.