diff --git a/Makefile b/Makefile
index 88bb949..bf603ea 100644
--- a/Makefile
+++ b/Makefile
@@ -53,9 +53,12 @@ dsk: asm
 	for f in res/TITLE.HGR/* res/TITLE.DHGR/*; do rsync --ignore-existing res/GAMEHELP/STANDARD build/GAMEHELP/$$(basename $$f); done
 	bin/buildfileinfo.sh build/GAMEHELP "04" "8000" >>build/log
 	$(CADIUS) ADDFOLDER build/"$(DISK)" "/$(VOLUME)/GAMEHELP" "build/GAMEHELP" >>build/log
-	bin/changebootloader.sh build/"$(DISK)" res/proboothd
+	bin/changebootloader.sh build/"$(DISK)" build/proboothd
 
-asm: asmlauncher asmfx asmprelaunch
+asm: asmproboot asmlauncher asmfx asmprelaunch
+
+asmproboot: md
+	$(ACME) -r build/proboothd.lst src/proboothd/proboothd.a
 
 asmlauncher: md
 	$(ACME) -DBUILDNUMBER=`git rev-list --count HEAD` src/4sports.a 2>build/relbase.log
diff --git a/src/proboothd/proboothd.s b/src/proboothd/proboothd.a
similarity index 97%
rename from src/proboothd/proboothd.s
rename to src/proboothd/proboothd.a
index 554729d..7ccd4b3 100644
--- a/src/proboothd/proboothd.s
+++ b/src/proboothd/proboothd.a
@@ -2,7 +2,7 @@
 ;minimal open/read binary file in ProDOS filesystem
 ;copyright (c) Peter Ferrie 2016-2019
 !cpu 6502
-!to "proboothd",plain
+!to "build/proboothd",plain
 *=$800
 
                 ;zpage usage, arbitrary selection except for the "ProDOS constant" ones
@@ -22,7 +22,8 @@
                 dirbuf    = $1e00       ;for size-optimisation
 
                 !byte 1
-                tay
+                stx unit
+                ldy #0                  ;Yellowstone support
 -               txa
                 jsr scrn2p2
                 and #7
diff --git a/winmake.bat b/winmake.bat
index 5a7ac9f..90f428d 100644
--- a/winmake.bat
+++ b/winmake.bat
@@ -25,6 +25,7 @@ set GIT=git
 if "%1" equ "asm" (
 :asm
 call :md
+call :asmproboot
 call :asmlauncher
 call :asmfx
 call :asmprelaunch
@@ -85,7 +86,7 @@ for %%q in (res\title.dhgr\*) do if not exist build\GAMEHELP\%%~nxq 1>nul copy b
 cscript /nologo bin\dumpcr.js "build\GAMEHELP\*"
 cscript /nologo bin\buildfileinfo.js build\GAMEHELP "06" "6000" >>build\log
 %CADIUS% ADDFOLDER "build\%DISK%" "/%VOLUME%/GAMEHELP" "build\GAMEHELP" >>build\log
-cscript /nologo bin\changebootloader.js "build\%DISK%" res\proboothd
+cscript /nologo bin\changebootloader.js "build\%DISK%" build\proboothd
 goto :EOF
 )
 
@@ -135,6 +136,10 @@ for %%q in (res\title.dhgr\*) do if not exist build\PRELAUNCH\%%~nxq 1>nul copy
 cscript /nologo bin\buildfileinfo.js build\PRELAUNCH "06" "0106" >>build\log
 goto :EOF
 
+:asmproboot
+%ACME% -r build\proboothd.lst src\proboothd\proboothd.a >> build\log
+goto :EOF
+
 :compress
 for %%q in (res\action.dhgr.uncompressed\*) do if not exist res\action.dhgr\%%~nxq %EXOMIZER% res\action.dhgr.uncompressed\%%~nxq@0x4000 -o res\action.hgr\%%~nxq
 for %%q in (res\action.hgr.uncompressed\*) do if not exist res\action.hgr\%%~nxq %EXOMIZER% res\action.hgr.uncompressed\%%~nxq@0x4000 -o res\action.hgr\%%~nxq