asm: new --bits option to set the processor mode on the command line

This is really just a shorthand for the --before option, but it is
really convenient for quickly writing multimode tests.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

asm/nasm.c

@@ -912,7 +912,8 @@ enum text_options {
     OPT_NO_LINE,
     OPT_DEBUG,
     OPT_INFO,
-    OPT_REPRODUCIBLE
+    OPT_REPRODUCIBLE,
+    OPT_BITS
 };
 enum need_arg {
     ARG_NO,
@@ -947,6 +948,7 @@ static const struct textargs textopts[] = {
     {"verbose",  OPT_INFO , ARG_MAYBE, 0},
     {"debug",    OPT_DEBUG, ARG_MAYBE, 0},
     {"reproducible", OPT_REPRODUCIBLE, ARG_NO, 0},
+    {"bits",     OPT_BITS, ARG_YES, 0},
     {NULL, OPT_BOGUS, ARG_NO, 0}
 };
 
@@ -1301,6 +1303,10 @@ static bool process_arg(char *p, char *q, int pass)
                     if (pass == 2)
                         pp_pre_command("%pragma", param);
                     break;
+                case OPT_BITS:
+                    if (pass == 2)
+                        pp_pre_command("BITS", param);
+                    break;
                 case OPT_BEFORE:
                     if (pass == 2)
                         pp_pre_command(NULL, param);
@@ -2469,6 +2475,7 @@ static void help(FILE *out, const char *what)
         fputs(
             "    --pragma str   pre-executes a specific %pragma\n"
             "    --before str   add line (usually a preprocessor statement) before the input\n"
+            "    --bits nn      set bits to nn (equivalent to --before \"BITS nn\")\n"
             "    --no-line      ignore %line directives in input\n"
             "    --prefix str   prepend the given string to the names of all extern,\n"
             "                   common and global symbols (also --gprefix)\n"

doc/changes.src

@@ -33,6 +33,9 @@ It is the production version of NASM since 2025.
 
 \b A new \c{%selbits()} preprocessor function.  See \k{f_selbits}.
 
+\b A new \c{--bits} option as convenience shorthand for \c{--before
+"BITS ..."}.  See \k{opt-bits}.
+
 
 \S{cl-3.00} Version 3.00
 

doc/running.src

@@ -556,12 +556,26 @@ is equivalent to the example in \k{opt-pfix}. See \k{pragma}.
 
 \S{opt-before} The \i\c{--before} Option
 
-A preprocess statement can be accepted with this option. The example
+Insert a statement (usually, but not necessarily) a preprocess
-shown in \k{opt-pragma} is the same as running this:
+statement before the input file. The example shown in \k{opt-pragma}
+is the same as running this:
 
 \c nasm -f macho --before "%pragma macho gprefix _"
 
 
+\S{opt-bits} The \i\c{--bits} Option
+
+Set the processor mode by inserting a \c{BITS} directive (kee
+\k{bits}) before the input file. The following two statements are
+exactly equivalent:
+
+\c nasm -f bin --bits 16 file.asm
+\c nasm -f bin --before "BITS 16" file.asm
+
+The \c{--bits} option was introduced in NASM 3.01; the \c{--before}
+form can be used for compatibility with older versions of NASM.
+
+
 \S{opt-limit} The \i\c{--limit-X} Option
 
 This option allows user to setup various maximum values after which