diff --git a/Mkfiles/openwcom.mak b/Mkfiles/openwcom.mak
index d0ba5128..631b0aaf 100644
--- a/Mkfiles/openwcom.mak
+++ b/Mkfiles/openwcom.mak
@@ -4,9 +4,11 @@
 # cross-compile on a DOS/Win32/OS2 platform host
 #
 
+.DEFAULT : what
+
 top_srcdir  = .
 srcdir      = .
-VPATH       = $(srcdir)\asm;$(srcdir)\x86;asm;x86;$(srcdir)\macros;macros;$(srcdir)\output;$(srcdir)\lib;$(srcdir)\common;$(srcdir)\stdlib;$(srcdir)\nasmlib;$(srcdir)\disasm
+VPATH       = $(srcdir)\asm;$(srcdir)\x86;asm;x86;$(srcdir)\macros;macros;$(srcdir)\output;$(srcdir)\lib;$(srcdir)\common;$(srcdir)\stdlib;$(srcdir)\nasmlib;$(srcdir)\disasm;$(srcdir)\zlib
 prefix      = C:\Program Files\NASM
 exec_prefix = $(prefix)
 bindir      = $(prefix)\bin
@@ -16,7 +18,7 @@ CC      = *wcl386
 DEBUG       =
 CFLAGS      = -zq -6 -ox -wx -wcd=124 -ze -fpi $(DEBUG)
 BUILD_CFLAGS    = $(CFLAGS) $(%TARGET_CFLAGS)
-INTERNAL_CFLAGS = -I$(srcdir) -I. -I$(srcdir)\include -I$(srcdir)\x86 -Ix86 -I$(srcdir)\asm -Iasm -I$(srcdir)\disasm -I$(srcdir)\output
+INTERNAL_CFLAGS = -I$(srcdir) -I. -I$(srcdir)\include -I$(srcdir)\x86 -Ix86 -I$(srcdir)\asm -Iasm -I$(srcdir)\disasm -I$(srcdir)\output -I$(srcdir)\zlib
 ALL_CFLAGS  = $(BUILD_CFLAGS) $(INTERNAL_CFLAGS)
 LD      = *wlink
 LDEBUG      =
@@ -31,9 +33,9 @@ RUNPERL         = $(PERL) $(PERLFLAGS)
 .BEFORE
 	set COPYCMD=/y
 
-RM_F		= -del /f
+RM_F		= -rm -f
 LN_S		= copy
-EMPTY		= copy nul:
+EMPTY		= %create
 SIDE		= %null Created by side effect
 
 MAKENSIS        = makensis
@@ -149,15 +151,23 @@ LIBOBJ_DIS = &
 
 # Objects for the local copy of zlib. The variable ZLIB is set to
 # $(ZLIBOBJ) if the internal version of zlib should be used.
+# zlib\crc32.obj is renamed to zlib\z_crc32.obj to avoid conflicts with
+# nasmlib\crc32.obj.
 ZLIBOBJ = &
 	zlib\adler32.obj &
-	zlib\crc32.obj &
+	zlib\z_crc32.obj &
 	zlib\infback.obj &
 	zlib\inffast.obj &
 	zlib\inflate.obj &
 	zlib\inftrees.obj &
 	zlib\zutil.obj
 
+# Special rule to avoid conflicts with nasmlib\crc32.obj because of stupid
+# behavior of implicit rules and VPATH of Open Watcom Make.
+zlib\z_crc32.obj : zlib\crc32.c
+    @set INCLUDE=
+    $(CC) -c $(ALL_CFLAGS) -fo=$^@ $[@
+
 LIBOBJ    = $(LIBOBJ_W) $(LIBOBJ_NW) $(ZLIB)
 ALLOBJ_W  = $(NASM) $(LIBOBJ_W)
 ALLOBJ    = $(PROGOBJ) $(LIBOBJ)
diff --git a/asm/assemble.c b/asm/assemble.c
index 4aeb2d43..da12940a 100644
--- a/asm/assemble.c
+++ b/asm/assemble.c
@@ -3103,6 +3103,7 @@ static enum match_result matches(const struct itemplate * const itemp,
              * If this is an *explicitly* sized immediate,
              * allow it to match an extending pattern.
              */
+#ifndef __WATCOMC__
             switch (isize[i]) {
             case BITS8:
                 if (ttype & BYTEEXTMASK) {
@@ -3117,6 +3118,18 @@ static enum match_result matches(const struct itemplate * const itemp,
             default:
                 break;
             }
+#else
+            /* Open Watcom does not support 64-bit constants at *case*. */
+            if (isize[i] == BITS8) {
+                if (ttype & BYTEEXTMASK) {
+                    isize[i]  = tsize[i];
+                    itype[i] |= BYTEEXTMASK;
+                }
+            } else if (isize[i] == BITS32) {
+                if (ttype & DWORDEXTMASK)
+                    isize[i]  = tsize[i];
+            }
+#endif
 
             /*
              * MOST instructions which take an sdword64 are the only form;
diff --git a/disasm/disasm.c b/disasm/disasm.c
index 0f421f80..c9e82e16 100644
--- a/disasm/disasm.c
+++ b/disasm/disasm.c
@@ -113,6 +113,7 @@ static enum reg_enum whichreg(opflags_t regflags, int regval, uint32_t rex)
  */
 static enum reg_enum implicit_reg(opflags_t regflags)
 {
+#ifndef __WATCOMC__
     switch (regflags) {
     case REG_AL:  return R_AL;
     case REG_AX:  return R_AX;
@@ -139,6 +140,56 @@ static enum reg_enum implicit_reg(opflags_t regflags)
     case OPMASK0: return R_K0;
     default:      return 0;
     }
+#else
+    /* Open Watcom does not support 64-bit constants at *case*. */
+    if (regflags == REG_AL)
+        return R_AL;
+    if (regflags == REG_AX)
+        return R_AX;
+    if (regflags == REG_EAX)
+        return R_EAX;
+    if (regflags == REG_RAX)
+        return R_RAX;
+    if (regflags == REG_DL)
+        return R_DL;
+    if (regflags == REG_DX)
+        return R_DX;
+    if (regflags == REG_EDX)
+        return R_EDX;
+    if (regflags == REG_RDX)
+        return R_RDX;
+    if (regflags == REG_CL)
+        return R_CL;
+    if (regflags == REG_CX)
+        return R_CX;
+    if (regflags == REG_ECX)
+        return R_ECX;
+    if (regflags == REG_RCX)
+        return R_RCX;
+    if (regflags == FPU0)
+        return R_ST0;
+    if (regflags == XMM0)
+        return R_XMM0;
+    if (regflags == YMM0)
+        return R_YMM0;
+    if (regflags == ZMM0)
+        return R_ZMM0;
+    if (regflags == REG_ES)
+        return R_ES;
+    if (regflags == REG_CS)
+        return R_CS;
+    if (regflags == REG_SS)
+        return R_SS;
+    if (regflags == REG_DS)
+        return R_DS;
+    if (regflags == REG_FS)
+        return R_FS;
+    if (regflags == REG_GS)
+        return R_GS;
+    if (regflags == OPMASK0)
+        return R_K0;
+    return 0;
+#endif
 }
 
 /*