From 49733547f194a0af8b589dcae5925d5f5668e252 Mon Sep 17 00:00:00 2001
From: "C. Masloch" <pushbx@ulukai.org>
Date: Sat, 21 May 2022 15:22:48 +0200
Subject: [PATCH] kernel, main: add debugger_present variable and skip ints 0,
 1, 3, 6

Also adds a CheckDebugger byte to the CONFIG block and
support for reading and writing this setting in the
SYS CONFIG program. Default is 0 (no check assume absent).
---
 hdr/kconfig.h     |  4 ++++
 kernel/kernel.asm | 20 ++++++++++++++++++++
 kernel/main.c     | 14 +++++++++-----
 sys/fdkrncfg.c    | 17 +++++++++++++++--
 4 files changed, 48 insertions(+), 7 deletions(-)

diff --git a/hdr/kconfig.h b/hdr/kconfig.h
index 74ac683..160a34a 100644
--- a/hdr/kconfig.h
+++ b/hdr/kconfig.h
@@ -46,4 +46,8 @@ typedef struct _KernelConfig {
   unsigned short Version_Revision;
   unsigned short Version_Release;
 
+  unsigned char CheckDebugger;
+	/* 0 = do not check (assume absent),
+	   1 = do check by running breakpoint,
+	   2 = assume present */
 } KernelConfig;
diff --git a/kernel/kernel.asm b/kernel/kernel.asm
index aefa6eb..29a0173 100644
--- a/kernel/kernel.asm
+++ b/kernel/kernel.asm
@@ -76,6 +76,7 @@ Version_Major               db 2
 Version_Revision            dw 41       ; REVISION_SEQ
 Version_Release             dw 1        ; 0=release build, >0=svn#
 
+CheckDebugger:		db 0	; 0 = no check, 1 = check, 2 = assume present
 configend:
 kernel_config_size: equ configend - config_signature
 	; must be below-or-equal the size of struct _KernelConfig
@@ -253,6 +254,25 @@ extern _InitKernelConfig
 %endif
 	mov ds, ax			; => init data segment
 
+check_debugger_present:
+extern _debugger_present
+
+	mov al, 1			; assume debugger present
+	cmp byte [di - kernel_config_size + (CheckDebugger - config_signature)], 1
+	ja .skip_ints_00_06		; 2 means assume present
+	jb .absent			; 0 means assume absent
+	clc				; 1 means check
+	int3				; break to debugger
+	jc .skip_ints_00_06
+		; The debugger should set CY here to indicate its
+		;  presence. The flag set is checked later to skip
+		;  overwriting the interrupt vectors 00h, 01h, 03h,
+		;  and 06h. This logic is taken from lDOS init.
+.absent:
+	xor ax, ax			; no debugger present
+.skip_ints_00_06:
+	mov byte [_debugger_present], al
+
            jmp     _FreeDOSmain
 
 %if XCPU != 86
diff --git a/kernel/main.c b/kernel/main.c
index 4b5cdb9..ffd14e8 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -69,6 +69,9 @@ __segment DosTextSeg = 0;
 struct lol FAR *LoL = &DATASTART;
 
 struct _KernelConfig InitKernelConfig = { 0xFF };
+UBYTE debugger_present = 0xFF;	/* initialised in kernel.asm
+				   do NOT set 0 here or compiler may
+				   move it into bss that we zero out */
 
 VOID ASMCFUNC FreeDOSmain(void)
 {
@@ -235,10 +238,10 @@ STATIC void setup_int_vectors(void)
   } vectors[] =
     {
       /* all of these are in the DOS DS */
-      { 0x0, FP_OFF(int0_handler) },   /* zero divide */
-      { 0x1, FP_OFF(empty_handler) },  /* single step */
-      { 0x3, FP_OFF(empty_handler) },  /* debug breakpoint */
-      { 0x6, FP_OFF(int6_handler) },   /* invalid opcode */
+      { 0x80 | 0x0, FP_OFF(int0_handler) },   /* zero divide */
+      { 0x80 | 0x1, FP_OFF(empty_handler) },  /* single step */
+      { 0x80 | 0x3, FP_OFF(empty_handler) },  /* debug breakpoint */
+      { 0x80 | 0x6, FP_OFF(int6_handler) },   /* invalid opcode */
       { 0x19, FP_OFF(int19_handler) }, /* BIOS bootstrap loader, vdisk */
       { 0x20, FP_OFF(int20_handler) },
       { 0x21, FP_OFF(int21_handler) }, /* primary DOS API */
@@ -264,7 +267,8 @@ STATIC void setup_int_vectors(void)
     setvec(i, empty_handler); /* note: int 31h segment should be DOS DS */
   HaltCpuWhileIdle = 0;
   for (pvec = vectors; pvec < vectors + (sizeof vectors/sizeof *pvec); pvec++)
-    setvec(pvec->intno, (intvec)MK_FP(FP_SEG(empty_handler), pvec->handleroff));
+    if ((pvec->intno & 0x80) == 0 || debugger_present == 0)
+      setvec(pvec->intno & 0x7F, (intvec)MK_FP(FP_SEG(empty_handler), pvec->handleroff));
   pokeb(0, 0x30 * 4, 0xea);
   pokel(0, 0x30 * 4 + 1, (ULONG)cpm_entry);
 
diff --git a/sys/fdkrncfg.c b/sys/fdkrncfg.c
index 3c32782..7bec735 100644
--- a/sys/fdkrncfg.c
+++ b/sys/fdkrncfg.c
@@ -13,7 +13,7 @@
 *  merged into SYS by tom ehlert                                                                        *
 ***************************************************************************/
 
-char VERSION[] = "v1.02";
+char VERSION[] = "v1.03";
 char PROGRAM[] = "SYS CONFIG";
 char KERNEL[] = "KERNEL.SYS";
 
@@ -93,7 +93,8 @@ void showUsage(void)
   printf("  Current Options are: DLASORT=0|1, SHOWDRIVEASSIGNMENT=0|1\n"
          "                       SKIPCONFIGSECONDS=#, FORCELBA=0|1\n"
          "                       GLOBALENABLELBASUPPORT=0|1\n"
-         "                       BootHarddiskSeconds=0|seconds to wait\n");
+         "                       BootHarddiskSeconds=0|seconds to wait\n"
+         "                       CheckDebugger=0|1|2\n");
 }
 
 /* simply reads in current configuration values, exiting program
@@ -215,6 +216,13 @@ void displayConfigSettings(KernelConfig * cfg)
          cfg->BootHarddiskSeconds);
   }
 
+  if (cfg->ConfigSize >= 13)
+  {
+    printf
+        ("CheckDebugger=%d :            *0=no, 1=check, 2=assume\n",
+         cfg->CheckDebugger);
+  }
+
 #if 0                           /* we assume that SYS is as current as the kernel */
 
   /* Print value any options added that are unknown as hex dump */
@@ -482,6 +490,11 @@ int FDKrnConfigMain(int argc, char **argv)
       setSByteOption(&(cfg.BootHarddiskSeconds),
                      cptr, 0, 127, &updates, "BootHarddiskSeconds");
     }
+    else if (memicmp(argptr, "CheckDebugger", 5) == 0)
+    {
+      setByteOption(&(cfg.CheckDebugger),
+                     cptr, 2, &updates, "CheckDebugger");
+    }
     else
     {
     illegal_arg: