In OS-9 for the 6809, command line parameters are passed to a program through a specific convention using the process's data area and registers at startup.

At Program Entry, the CPU Registers Contain:

• X — points to the start of the parameter string in memory
• Y — points to the top of the data area (the program's allocated memory)
• U — points to the bottom of the data area (start of the stack/data)
• D (A:B) — contains the size of the parameter string

The Parameter String

The parameter string is a raw text buffer containing everything typed after the command name on the shell line. For example, typing:

myprogram foo bar -x

...results in X pointing to foo bar -x, with D holding its byte length.

Key characteristics of the parameter string:

• It is not null-terminated — you must use the length in D to know where it ends
• It ends with a carriage return (0x0D), which counts as part of the string
• It may be empty (D = 1, pointing to just the CR) if no arguments were given
• The string lives in the system's I/O buffer area, not in your program's data area, so you should copy it if you need to keep it

Memory Layout at Entry

Low address
┌─────────────────────┐  ← U (data area base / stack bottom)
│   Program Stack     │
│         ↓           │
│    (grows down)     │
│                     │
│   Static Data       │
└─────────────────────┘  ← Y (top of data area)
High address

Separately, in system buffer:
┌─────────────────────┐  ← X
│  "foo bar -x\r"     │  (D bytes long)
└─────────────────────┘

Parsing Responsibility

OS-9 does no parsing for you. Unlike Unix which gives you argc/argv, OS-9 hands you the raw string and leaves tokenization entirely to the program (or to a library like the one in the C runtime). You walk the string byte by byte, using the length in D as your bounds, splitting on spaces or other delimiters as needed.

Standard I/O

At entry, the program's standard I/O paths are already open:

• Path 0 — standard input
• Path 1 — standard output
• Path 2 — standard error

These are inherited from the shell and ready to use via OS-9 I$Read / I$Write system calls.

This convention is quite minimal and low-level compared to later systems, which is typical of OS-9's design philosophy of keeping the kernel small and fast on 8/16-bit hardware.