Invoker: Populate $280 same as Bitsy Bye when launching BASIS.SYSTEM

ProDOS 2.4's Bitsy Bye populates $280 with the containing path for the
target file when invoking BASIS.SYSTEM as a fallback interpreter. The
documented ProDOS convention is to set $280 to the path to the system
file itself, but Bitsy Bye does its own thing. For better
compatibility, do the same specifically when launching BASIS.SYSTEM,
but not other interpreters.

Updated behavior comparison:

                Bitsy                DeskTop
/VOL/FILE invoked:
    Prefix      /VOL/                /VOL/
    $2006       FILE                 FILE
    $280        /VOL                 /VOL
    $380        /VOL/BASIS.SYSTEM    /VOL/BASIS.SYSTEM
/VOL/SUBDIR/FILE invoked:
    Prefix      /VOL/SUBDIR/         /VOL/SUBDIR/
    $2006       FILE                 FILE
    $280        /VOL/SUBDIR          /VOL/SUBDIR
    $380        /VOL/BASIS.SYSTEM    /VOL/BASIS.SYSTEM

Also note that Bitsy Bye has the high bits set for '/' characters in
the $280 path (i.e. $AF not $2F) and *all* characters in the $380
path. DeskTop populates both paths with high bits clear.

common.inc

@@ -331,6 +331,7 @@ INVOKER_PREFIX          := $220 ; Prefix to set
 INVOKER                 := $290 ; Invoke other programs
 INVOKER_FILENAME        := $280 ; File to invoke (PREFIX must be set)
 INVOKER_INTERPRETER     := $380 ; If non-empty, use path as interpreter.
+INVOKER_BITSY_COMPAT    := $3C1 ; bit7 = set $280 like Bitsy does
 
 ;;; ============================================================
 ;;; Default Font

desktop/main.s

@@ -890,7 +890,9 @@ sys_disk:
         jsr     SetCursorWatch ; before invoking
 
         ;; Assume no interpreter to start
-        copy    #0, INVOKER_INTERPRETER
+        lda     #0
+        sta     INVOKER_INTERPRETER
+        sta     INVOKER_BITSY_COMPAT
 
         ;; Easiest to assume absolute path later.
         param_call MakePathAbsolute, src_path_buf
@@ -937,7 +939,10 @@ retry:  jsr     GetSrcFileInfo
         ;; Fallback - try BASIS.SYSTEM
 fallback:
         jsr     CheckBasisSystem ; Is fallback BASIS.SYSTEM present?
-        bcc     launch           ; yes, continue below
+    IF_CC                        ; yes, continue below
+        copy    #$80, INVOKER_BITSY_COMPAT
+        bmi     launch          ; always
+    END_IF
         param_jump ShowAlertParams, AlertButtonOptions::OK, aux::str_alert_cannot_open
 
         ;; --------------------------------------------------

lib/invoker.s

@@ -128,6 +128,19 @@ load_target:
         sta     BITSY_SYS_PATH,x
         dex
         bpl     :-
+
+        ;; When launching BASIS.SYTEM, ProDOS 2.4's Bitsy Bye populates
+        ;; $280 with the path containing the target file.
+        BITSY_DIR_PATH := $280
+        bit     INVOKER_BITSY_COMPAT
+      IF_NS
+        ldy     INVOKER_PREFIX
+:       lda     INVOKER_PREFIX,y
+        sta     BITSY_DIR_PATH,y
+        dey
+        bpl     :-
+      END_IF
+
     END_IF
 
         ;; Set return address to the QUIT call below
@@ -149,6 +162,8 @@ exit:   rts
 
         PAD_TO ::INVOKER_INTERPRETER
         .res    ::kPathBufferSize, 0
+        ASSERT_ADDRESS ::INVOKER_BITSY_COMPAT
+        .byte   0
 
 ;;; ============================================================
 

selector/app.s

@@ -1572,6 +1572,7 @@ fail:   jmp     ClearSelectedIndex
         ;; Ensure it's BIN, SYS, S16 or BAS (if BS is present)
 
 check_type:
+        copy    #0, INVOKER_BITSY_COMPAT
         lda     file_info_params::file_type
         cmp     #FT_BASIC
         bne     not_basic
@@ -1591,7 +1592,10 @@ not_basic:
         beq     check_path
 
         jsr     CheckBasisSystem ; Is fallback BASIS.SYSTEM present?
-        beq     check_path
+    IF_EQ
+        copy    #$80, INVOKER_BITSY_COMPAT
+        bmi     check_path      ; always
+    END_IF
 
         ;; Don't know how to invoke
         lda     #AlertID::selector_unable_to_run