and another one

src/a2fc.system.a

@@ -38,11 +38,13 @@ MACHID = $BF98
 inputfilebuffer
          !fill $40
 Start
-         ldy   #(CodeEnd-CodeStart); copy code to lower memory so we can load graphic at $2000
--        ldx   CodeStart-1,y
+         ldy   #(CodeEnd-CodeStart)  ; copy code to lower memory so we can load graphic at $2000
+-        ldx   CodeStart-1,y         ; the last byte fetched is the '3' for kMLIOpenCount
+                                     ; it will be converted later to a '4' for become kMLIReadOrQuitCount
+                                     ; also used for switching from aux to main memory access
          stx   $4f,y
          dey
-         bne   -
+         bne   -                     ; Y is 0 on exit, required later
 ;CheckFor128K
          lda   MACHID
          and   #$30
@@ -52,15 +54,15 @@ Start
 +        jsr   MLI                   ; open file
          !byte kMLIOpen
          !word mliParamForOpen
-         inx
-         stx   <mliparam             ; set parameter count for MLI call, one more than Open
+         inx                         ; convert to kMLIReadOrQuitCount
+         stx   <mliparam             ; set parameter count for MLI call
                                      ; both Read and Quit accept four parameters
          jmp   ReadFile
 
 CodeStart
 !pseudopc $50 {
 mliParamForOpen
-         !byte kMLIOpenCount
+         !byte 3                     ; kMLIOpenCount when used here
          !word inputfilebuffer
          !byte <aFileBuffer
 mliparam
@@ -87,18 +89,19 @@ ReadFile
          inc   <(CopyToAuxHigh)
          bit   <(CopyToAuxHigh)      ; copy until $4000
          bvc   -
-         sta   $C004                 ; read from mainmem, write to mainmem
+SoftswitchBase=*+1
+         sta   $C000,x               ; read from mainmem, write to mainmem
+                                     ; X is 4 from above
          jsr   Read                  ; read second half of graphic (stays in main memory)
                                      ; does not return if error
 ;DHGRShow
          ldx   #(softswitches_e-softswitches)-1
--        ldy   <softswitches,x
+-        ldy   <softswitches,x       ; the last byte fetched is the '0', required later
          sta   (SoftswitchBase),y    ; enable DHGR graphics display
          dex
          bpl   -
 ;WaitForKey
-SoftswitchBase=*+1
--        lda   $C000
+-        lda   (SoftswitchBase),y    ; $C000 because Y is 0 from above
          bpl   -
 Close
          jsr   MLI                   ; close all files