Preserve DL except for the rather few calls for which DL is part of the

return value.  This is required for DOS not to go ballistic.

diff --git a/memdisk/memdisk.asm b/memdisk/memdisk.asm
index c18545a..220db47 100644 (file)
--- a/memdisk/memdisk.asm
+++ b/memdisk/memdisk.asm
@@ -74,8 +74,18 @@ Pointers:    dw Int13Start
                dw TotalSize
 
 Int13Start:
+               ; Swap stack
+               mov [cs:Stack],esp
+               mov [cs:SavedAX],ax
+               mov ax,ss
+               mov [cs:Stack+4],ax
+               mov ax,cs
+               mov ss,ax
+               mov sp,[cs:MyStack]
+
                ; See if DL points to our class of device (FD, HD)
                push dx
+               push dx
                xor dl,[cs:DriveNo]
                pop dx
                js .nomatch             ; If SF=0, we have a class match here
@@ -84,23 +94,41 @@ Int13Start:
                jb .nomatch             ; Drive < Our drive
                dec dl                  ; Drive > Our drive, adjust drive #
 .nomatch:
-               jmp far [cs:OldInt13]
+               mov ax,[cs:SavedAX]
+               pushf
+               call far [cs:OldInt13]
+               pushf
+               push bp
+               mov bp,sp
+               cmp byte [cs:SavedAX+1],08h
+               je .norestoredl
+               cmp byte [cs:SavedAX+1],15h
+               jne .restoredl
+               test byte [bp+4],80h    ; Hard disk?
+               jnz .norestoredl
+.restoredl:
+               mov dl,[bp+4]
+.norestoredl:
+               push ax
+               push ebx
+               push ds
+               mov ax,[bp+2]           ; Flags
+               lds ebx,[cs:Stack]
+               mov [bx+4],al           ; Arithmetric flags
+               pop ds
+               pop ebx
+               pop ax
+               pop bp
+               lss esp,[cs:Stack]
+               iret
 
 .our_drive:
-               ; Swap stack
-               mov [cs:Stack],esp
-               mov [cs:SavedAX],ax
-               mov ax,ss
-               mov [cs:Stack+4],ax
-               mov ax,cs
-               mov ss,ax
-               mov sp,[cs:MyStack]
+               ; Set up standard entry frame
                push ds
                push es
                mov ds,ax
                mov es,ax
                mov ax,[SavedAX]
-
                pushad
                mov bp,sp               ; Point BP to the entry stack frame
                TRACER 'F'