add divzfix command

It is trying to fix divizion by zero by doing fnclex before
calling the realmode handler.
This prevents potential IGNNE stuck-ups, and also makes some
handlers to cleanly exit w/o terminating the program.

See dosemu2/dosemu2#2724

Author: stsp

32/makefile

@@ -15,7 +15,8 @@ LINK_OPT =
 SRCS = $(SRCDIR)/command.c $(SRCDIR)/cmdbuf.c $(SRCDIR)/ms.c \
   $(SRCDIR)/env.c $(SRCDIR)/psp.c $(SRCDIR)/umb.c $(SRCDIR)/ae0x.c \
    $(SRCDIR)/compl.c $(SRCDIR)/clip.c memmem.c fmemcpy.c findclos.c
-ASSRCS = $(SRCDIR)/asm.S $(SRCDIR)/int23.S $(SRCDIR)/int0.S $(SRCDIR)/mouse.S
+ASSRCS = $(SRCDIR)/asm.S $(SRCDIR)/int23.S $(SRCDIR)/int0.S $(SRCDIR)/int75.S \
+  $(SRCDIR)/mouse.S
 OBJS = $(notdir $(SRCS:.c=.o)) $(notdir $(ASSRCS:.S=.o))
 CMD = comcom32.exe
 REVISIONID := $(shell git describe --dirty=+)

src/asm.h

@@ -12,6 +12,7 @@
 
 int ASMCFUNC do_int23(void);
 void ASMCFUNC do_int0(void);
+int ASMCFUNC do_int75(void);
 void ASMCFUNC do_mouse(void);
 
 #else

src/command.c

@@ -142,6 +142,8 @@ static int mouseopt_enabled;
 
 static __dpmi_raddr int0_vec;
 static int int0_wa;
+static __dpmi_paddr int75_vec;
+static int int75_wa;
 
 /*
  * Command parser defines/variables
@@ -243,6 +245,7 @@ static void perform_popd(const char *arg);
 static void perform_prompt(const char *arg);
 static void perform_pushd(const char *arg);
 static void perform_r200fix(const char *arg);
+static void perform_divzfix(const char *arg);
 static void perform_rd(const char *arg);
 static void perform_rename(const char *arg);
 static void perform_shift(const char *arg);
@@ -303,6 +306,7 @@ struct built_in_cmd cmd_table[] =
     {"prompt", perform_prompt, "", "customize prompt string"},
     {"pushd", perform_pushd, "", "push cwd to stack and cd"},
     {"r200fix", perform_r200fix, "", "runtime error 200 fix"},
+    {"divzfix", perform_divzfix, "", "division by zero fix"},
     {"rd", perform_rd, "", "remove directory"},
     {"rmdir", perform_rd, "", "remove directory"},
     {"rename", perform_rename, "", "rename with wildcards"},
@@ -3065,6 +3069,15 @@ static int is_HMA_enabled(void)
   return !!(r.h.dh & 0x10);
   }
 
+static void activate_int75_handling(void)
+{
+  __dpmi_paddr pa;
+
+  pa.selector = _my_cs();
+  pa.offset32 = (uintptr_t)my_int75_handler;
+  __dpmi_set_protected_mode_interrupt_vector(0x75, &pa);
+}
+
 static void perform_external_cmd(int call, int lh, char *ext_cmd)
   {
   finddata_t ff;
@@ -3231,6 +3244,7 @@ static void perform_external_cmd(int call, int lh, char *ext_cmd)
 #ifdef __DJGPP__
     __djgpp_exception_toggle();
 #endif
+    activate_int75_handling();
     set_env_seg();
     /* prepend command tail with space */
     alen = strlen(cmd_args);
@@ -3709,6 +3723,21 @@ static void perform_r200fix(const char *arg)
   int0_wa = 0;
   }
 
+static void perform_divzfix(const char *arg)
+  {
+  if (!*arg)
+    {
+    cprintf("divzfix: command name missing\r\n");
+    reset_batfile_call_stack();
+    return;
+    }
+  shift_cmdline();
+
+  int75_wa = 1;
+  perform_external_cmd(false, false, cmd);
+  int75_wa = 0;
+  }
+
 static void perform_rd(const char *arg)
   {
   while (*cmd_switch)  // skip switches
@@ -4705,6 +4734,20 @@ void do_int0(void)
     __dpmi_set_real_mode_interrupt_vector(0, &int0_vec);
 }
 
+static void setup_int75_handling(void)
+{
+  __djgpp_exception_toggle();
+  __dpmi_get_protected_mode_interrupt_vector(0x75, &int75_vec);
+  __djgpp_exception_toggle();
+  _prev75_eip = int75_vec.offset32;
+  _prev75_cs = int75_vec.selector;
+}
+
+int do_int75(void)
+{
+  return int75_wa;
+}
+
 int main(int argc, const char *argv[], const char *envp[])
   {
   int a;
@@ -4729,6 +4772,7 @@ int main(int argc, const char *argv[], const char *envp[])
     _osmajor = 7;  // fake _osmajor to enable extended functionality
   setup_break_handling();
   setup_int0_handling();
+  setup_int75_handling();
 
   // unbuffer stdin and stdout
   setbuf(stdin, NULL);

src/glob_asm.h

@@ -1,6 +1,9 @@
 __ASM(unsigned short, _ds) SEMIC
 __ASM_FUNC(my_int23_handler) SEMIC
 __ASM_FUNC(my_int0_handler) SEMIC
+__ASM_FUNC(my_int75_handler) SEMIC
 __ASM_FUNC(my_mouse_handler) SEMIC
 __ASM(unsigned int, _prev0_eip) SEMIC
 __ASM(unsigned short, _prev0_cs) SEMIC
+__ASM(unsigned int, _prev75_eip) SEMIC
+__ASM(unsigned short, _prev75_cs) SEMIC

src/int75.S

@@ -0,0 +1,46 @@
+/*
+ *  comcom64 - 64bit command.com
+ *  Copyright (C) 2023-2024  @stsp
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "asm.h"
+.include "asm.inc"
+
+.bss
+.balign 2
+__prev75:
+.global __prev75_eip
+__prev75_eip: .long 0
+.global __prev75_cs
+__prev75_cs: .word 0
+
+.text
+.global _my_int75_handler
+_my_int75_handler:
+    pusha
+    handler_prolog SIGSTK_LEN
+    call _do_int75
+    restore_stack
+    testl %eax, %eax
+    jz 0f
+    fnclex
+0:
+    popa
+    ljmpl *__prev75
+
+#ifdef __ELF__
+.section .note.GNU-stack,"",%progbits
+#endif

src/makefile

@@ -22,7 +22,7 @@ HEADERS = $(addprefix $(srcdir)/,ae0x.h cmdbuf.h compl.h psp.h command.h env.h m
 PDHDR = $(srcdir)/asm.h
 GLOB_ASM = $(srcdir)/glob_asm.h
 OBJECTS = $(SOURCES:.c=.o)
-AS_SOURCES = int23.S int0.S asm.S ms.S
+AS_SOURCES = int23.S int0.S int75.S asm.S ms.S
 AS_OBJECTS = $(AS_SOURCES:.S=.o)
 CMD = $(if $(LINK),comcom64.exe,)
 DBG = $(if $(LINK),$(CMD).dbg,)