entry.asm: fix Critical Error caller 386 registers

Fixes #210, fixes #212

Author: ecm-pushbx

hdr/stacks.inc

@@ -135,6 +135,8 @@ irp_hi    equ 26
 ;	error 1 2 3
 	%macro  Protect386Registers 0
 	%endmacro
+
+	%assign Size386Registers 0
 	
 	%macro  RestoreSP 0
 		mov sp, bp  
@@ -151,8 +153,10 @@ irp_hi    equ 26
 		push gs
 	%endmacro
 
+	%assign Size386Registers 4
+
 	%macro  RestoreSP 0
-		lea sp, [bp-4]
+		lea sp, [bp - Size386Registers]
 	%endmacro
 
 	%macro  Restore386Registers  0
@@ -176,8 +180,10 @@ irp_hi    equ 26
 		pop dx
 	%endmacro
 
+	%assign Size386Registers 6
+
 	%macro  RestoreSP 0
-		lea sp, [bp-6]
+		lea sp, [bp - Size386Registers]
 	%endmacro
 
 	%macro  Restore386Registers  0

kernel/entry.asm

@@ -611,6 +611,8 @@ CritErr05:
                 mov     bp,sp
                 push    si
                 push    di
+                Protect386Registers
+
                 ;
                 ; Get parameters
                 ;
@@ -659,6 +661,13 @@ CritErr05:
                 ;
                 cld
                 cli
+                Protect386Registers
+                ; ecm: The extended stack frame must be restored here
+                ;  in case the response isn't Abort. The int 21h handler
+                ;  will expect the extended stack frame to be still
+                ;  intact, but the stack written by the int 24h (even
+                ;  only the int instruction) will have overwritten it.
+
                 mov     bp, [cs:_DGROUP_]
                 mov     ds,bp
                 mov     ss,bp
@@ -672,7 +681,13 @@ CritErr05:
                 pop     word [es:PSP_USERSP]
                 pop     word [es:PSP_USERSS]
                 mov     bp, sp
-                mov     ah, byte [bp+4+4]       ; restore old AH from nFlags
+                mov     ah, byte [bp + 4 + 4 + Size386Registers]
+                ; restore old AH from nFlags
+                ; ecm: One 4 is the displacement of nFlags from the
+                ;  usual bp, the other 4 accounts for the si and di
+                ;  on the stack, the Size386Registers is added to
+                ;  skip the fs and gs (OpenWatcom 386 build) or high
+                ;  words that are a part of the stack frame, if any.
                 sti                             ; Enable interrupts
                 ;
                 ; clear flags
@@ -715,6 +730,8 @@ CritErr30:
 
 CritErrExit:
                 xor     ah,ah                   ; clear out top for return
+
+                Restore386Registers
                 pop     di
                 pop     si
                 pop     bp