int2f: fix call interface around int2F_12_handler( )

This changes the interface between reloc_call_int2f_handler
(int2f.asm) and int2F_12_handler( ) (inthndlr.c) so that

  - int2F_12_handler( ) will not try to output values to its
    caller by passing "back" input parameter values
  - reloc_call_int2f_handler will switch to an internal
    stack before calling int2F_12_handler( ), so that SS ==
    DGROUP as expected by the latter.

This partly addresses https://github.com/FDOS/kernel/issues/11 .

(The call to syscall_MUX14( ) in int2f.asm seems to have
similar issues, and should probably also be updated.)

kernel/int2f.asm

@@ -31,6 +31,53 @@
         %include "segs.inc"
         %include "stacks.inc"
 
+; macro to switch to an internal stack (if necessary), set DS == SS == DGROUP,
+; and push the old SS:SP onto the internal stack
+;
+; destroys AX, SI, BP; turns on IRQs
+;
+; int2f does not really need to switch to a separate stack for MS-DOS
+; compatibility; this is mainly to work around the C code's assumption
+; that SS == DGROUP
+;
+; TODO: remove the need for this hackery  -- tkchia
+%macro SwitchToInt2fStack 0
+    mov ax,[cs:_DGROUP_]
+    mov ds,ax
+    mov si,ss
+    mov bp,sp
+    cmp ax,si
+    jz %%already
+    cli
+    mov ss,ax
+    extern int2f_stk_top
+    mov sp,int2f_stk_top
+    sti
+%%already:
+; well, GCC does not currently clobber function parameters passed on the
+; stack; but just in case it decides to do that in the future, we push _two_
+; copies of the old SS:SP:
+;   - the second copy can be passed as a pointer parameter to a C function
+;   - the first copy is used to actually restore the user stack later
+    push si
+    push bp
+    push si
+    push bp
+%endmacro
+
+; macro to switch back from an internal stack, i.e. undo SwitchToInt2fStack
+;
+; destroys BP; turns on IRQs  -- tkchia
+%macro DoneInt2fStack 0
+    pop bp
+    pop bp
+    pop bp
+    cli
+    pop ss
+    mov sp,bp
+    sti
+%endmacro
+
 segment	HMA_TEXT
             extern _cu_psp
             extern _HaltCpuWhileIdle
@@ -135,7 +182,7 @@ DriverSysCal:
 ; internal dos calls INT2F/12xx and INT2F/4A01,4A02 - handled through C 
 ;**********************************************************************
 IntDosCal:                
-                        ; set up register frame
+                        ; set up register structure
 ;struct int2f12regs
 ;{
 ;  [space for 386 regs]
@@ -165,9 +212,10 @@ IntDosCal:
   %endif
 %endif          
 
-    mov ds,[cs:_DGROUP_]
+    SwitchToInt2fStack
     extern   _int2F_12_handler
     call _int2F_12_handler
+    DoneInt2fStack
 
 %if XCPU >= 386
   %ifdef WATCOM

kernel/inthndlr.c

@@ -1734,14 +1734,15 @@ struct int2f12regs {
   UWORD callerARG1;             /* used if called from INT2F/12 */
 };
 
-/* WARNING: modifications in `r' are used outside of int2F_12_handler()
+/* On input pr->AX==0x12xx, 0x4A01 or 0x4A02
- * On input r.AX==0x12xx, 0x4A01 or 0x4A02
  */
-VOID ASMCFUNC int2F_12_handler(struct int2f12regs r)
+VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *pr)
 {
   COUNT rc;
   long lrc;
 
+#define r (*pr)
+
   if (r.AH == 0x4a)
   {
     size_t size = 0, offs = 0xffff;
@@ -2111,6 +2112,8 @@ error_exit:
     CritErrCode = r.AX;      /* Maybe set */
 error_carry:
   r.FLAGS |= FLG_CARRY;
+
+#undef r
 }
 
 /*

kernel/kernel.asm

@@ -812,6 +812,11 @@ blk_stk_top:
                 times 128 dw 0
 clk_stk_top:
 
+; int2fh private stack
+                global  int2f_stk_top
+                times 128 dw 0
+int2f_stk_top:
+
 ; Dynamic data:
 ; member of the DOS DATA GROUP
 ; and marks definitive end of all used data in kernel data segment