Populate pr_Arguments on the fake struct Process

crates/volamos-core/src/dispatch.rs

@@ -2361,6 +2361,12 @@ impl<C: Cpu + 'static> Runtime<C> {
         cpu.set_data_register(DataRegister(0), line_len);
         cpu.set_pc(config.entry);
 
+        // pr_Arguments: the same command-line buffer A0/D0 just got,
+        // also reachable later via the process struct -- see
+        // `exectask::PR_ARGUMENTS_OFFSET`'s doc for why a real guest
+        // startup may read this instead of (or in addition to) A0/D0.
+        mem.write_u32(task + exectask::PR_ARGUMENTS_OFFSET, args_addr);
+
         // ReadArgs's default source (rdargs == NULL): the same
         // command-line buffer just built above, per crate::dosargs's
         // module docs. Registers hold this too, but a guest program is
@@ -3737,6 +3743,29 @@ mod tests {
         assert_eq!(bytes, b"COMMENT \"a whole sentence\" \n");
     }
 
+    #[test]
+    fn pr_arguments_points_at_the_same_command_line_buffer_as_a0() {
+        // Found running the real `sidick/micropython` Amiga port:
+        // its own startup reads argv from `pr_Arguments` (the
+        // `struct Process` field), not from A0/D0 -- left at NULL, it
+        // always decided "no arguments" and fell back to its REPL
+        // even with a real script path passed on the command line.
+        // See `exectask::PR_ARGUMENTS_OFFSET`'s doc.
+        let rt = runtime_with_program_and_args(&[RTS], vec!["script.py".to_string()]);
+        let a0 = rt.cpu.address_register(AddressRegister(0));
+        let pr_arguments = rt
+            .mem
+            .read_u32(rt.task + crate::exectask::PR_ARGUMENTS_OFFSET);
+        assert_eq!(
+            pr_arguments, a0,
+            "pr_Arguments should point at the same buffer A0 does"
+        );
+        assert_eq!(
+            crate::guestmem::read_c_string(&rt.mem, pr_arguments),
+            b"script.py \n"
+        );
+    }
+
     #[test]
     fn empty_args_still_produce_a_bare_newline_command_line() {
         let rt = runtime_with_program_and_args(&[RTS], Vec::new());

crates/volamos-core/src/exectask.rs

@@ -321,6 +321,23 @@ pub const PR_CLI_OFFSET: u32 = 172;
 /// called once from `crates/volamos/src/main.rs` after `set_vfs`).
 /// Found needed running the real SAS/C `sc` compiler (issue #17).
 pub const PR_HOMEDIR_OFFSET: u32 = 188;
+/// `pr_Arguments`: `STRPTR`, offset 204 (`pr_HomeDir` 188 + 4, `pr_Flags`
+/// 192, `pr_ExitCode` 196, `pr_ExitData` 200, `pr_Arguments` 204). Per
+/// `<dos/dosextens.h>`: "Arguments passed to the process at start".
+/// This runtime already builds the exact same command-line buffer for
+/// `A0`/`D0` at process entry (see `crate::dispatch::Runtime::new`) --
+/// `pr_Arguments` just needs to point at it too, since a real
+/// AmigaOS process (via `RunCommand`/`CreateNewProc`) gets both: A0/D0
+/// are the entry-time convention a `crt0` reads once at startup, but
+/// `pr_Arguments` is the same string surviving in the process struct
+/// for any code to re-read later (e.g. after `A0`/`D0` have long since
+/// been clobbered). Found running the real `sidick/micropython`
+/// Amiga port (its own startup reads argv from `pr_Arguments`, not
+/// A0/D0): left at 0/NULL, it always decided "no arguments" and fell
+/// back to its REPL even when a script path was passed, regardless of
+/// A0/D0 being correct -- confirmed by cross-checking against vamos,
+/// which does set this field (`amitools`' `Process.init_args()`).
+pub const PR_ARGUMENTS_OFFSET: u32 = 204;
 /// `sizeof(struct Process)` per `<dos/dosextens.h>` -- `pr_CLI`'s own
 /// offset (172) plus every field after it (`pr_ReturnAddr`/
 /// `pr_PktWait`/`pr_WindowPtr`/`pr_HomeDir` 4 each = 16, `pr_Flags` 4,