Add 16-bit real-mode DOS target -- 127KB standalone, no extender

New Makefile.dos16 builds with OpenWatcom wcc (16-bit, MEDIUM model)
producing a standard MZ executable that runs on any DOS without DOS/4GW.
All 24 source files compile clean; tested on FreeDOS 1.4 via QEMU.

Changes for 16-bit compatibility:
- hal_dos.c: INTX macro selects int86() vs int386() based on _M_I86
- sound.c: reduce stack buffer from 8192 to 512 samples on 16-bit
- tui.c: gracefully disable TUI if screen buffer allocation fails
  (near heap exhaustion common on 16-bit), batch mode still works
- .gitignore: add .obj/.exe/.err/.lib for OpenWatcom build artifacts

Size comparison:
- 32-bit DOS/4GW: 175KB + 265KB extender = 440KB total
- 16-bit real-mode: 127KB standalone

The 32-bit build (Makefile.dos) and Linux build are unaffected.
72/72 interpreter tests pass.

.gitignore

@@ -2,6 +2,10 @@ build/
 docs/_build/
 *.o
 *.d
+*.obj
+*.exe
+*.err
+*.lib
 gwbasic_*.txt
 gwbasic_*.dat
 gwbasic_*.bas

Makefile.dos16

@@ -0,0 +1,44 @@
+# Makefile for building GW-BASIC 2026 with OpenWatcom for 16-bit DOS
+#
+# Usage:
+#   wmake -f Makefile.dos16           (from OpenWatcom environment)
+#   wmake -f Makefile.dos16 clean
+#
+# Requires: OpenWatcom 2.0+ targeting 16-bit real-mode DOS
+# Produces: GWBASIC.EXE (interpreter, standalone MZ executable — no DOS extender)
+#
+# The 16-bit build uses the MEDIUM memory model (-mm):
+#   - Code: multiple segments (far calls), supports > 64KB total code
+#   - Data: single segment (near pointers), must fit in 64KB
+#
+# The compiler (GWBASCOM.EXE) is not built in 16-bit mode because it uses
+# open_memstream() which is not available in 16-bit DOS.  Use Makefile.dos
+# (32-bit) for the compiler.
+
+CC = wcc
+CFLAGS = -bt=dos -mm -ox -w4 -zq -za99 -Iinclude -D__MSDOS__
+LINKER = wlink
+LIBRARIAN = wlib
+
+# Interpreter sources (same as 32-bit minus compiler files)
+INTERP_OBJS = &
+    src/main.obj src/tokens.obj src/tokenizer.obj src/error.obj &
+    src/eval.obj src/interp.obj src/vars.obj src/arrays.obj &
+    src/input.obj src/math_int.obj src/math_float.obj &
+    src/math_transcend.obj src/strings.obj src/print.obj &
+    src/fileio.obj src/program_io.obj src/print_using.obj &
+    src/graphics.obj src/virmem.obj src/portio.obj src/strpool.obj &
+    src/sound.obj src/tui.obj platform/hal_dos.obj
+
+.c.obj:
+    $(CC) $(CFLAGS) -fo=$@ $<
+
+all: gwbasic.exe
+
+gwbasic.exe: $(INTERP_OBJS)
+    $(LINKER) system dos option stack=8192 name $@ file { $< }
+
+clean: .SYMBOLIC
+    del src\*.obj
+    del platform\*.obj
+    del gwbasic.exe

platform/hal_dos.c

@@ -5,7 +5,8 @@
  * and screen control.  Selected at compile time via __MSDOS__.
  * Linux HAL (hal_posix.c) is unchanged -- full backward compatibility.
  *
- * Build: wcc386 -bt=dos -mf -ox -za99 -D__MSDOS__ -Iinclude
+ * Build: wcc386 -bt=dos -mf -ox -za99 -D__MSDOS__ -Iinclude  (32-bit)
+ *        wcc   -bt=dos -mm -ox -za99 -D__MSDOS__ -Iinclude  (16-bit)
  */
 
 #ifdef __MSDOS__
@@ -18,6 +19,13 @@
 #include <i86.h>
 #include <dos.h>
 
+/* int86 (16-bit real mode) vs int386 (32-bit protected mode) */
+#ifdef _M_I86
+#define INTX(n, r_in, r_out) int86(n, r_in, r_out)
+#else
+#define INTX(n, r_in, r_out) int386(n, r_in, r_out)
+#endif
+
 static int cursor_row = 0;
 static int cursor_col = 0;
 static int screen_cols = 80;
@@ -33,7 +41,7 @@ static void bios_set_cursor(int row, int col)
     r.h.bh = 0;
     r.h.dh = (unsigned char)row;
     r.h.dl = (unsigned char)col;
-    int386(0x10, &r, &r);
+    INTX(0x10, &r, &r);
     cursor_row = row;
     cursor_col = col;
 }
@@ -44,7 +52,7 @@ static void bios_get_cursor(int *row, int *col)
     memset(&r, 0, sizeof(r));
     r.h.ah = 0x03;
     r.h.bh = 0;
-    int386(0x10, &r, &r);
+    INTX(0x10, &r, &r);
     *row = r.h.dh;
     *col = r.h.dl;
 }
@@ -60,7 +68,7 @@ static void bios_scroll_up(int lines, int attr, int r1, int c1, int r2, int c2)
     r.h.cl = (unsigned char)c1;
     r.h.dh = (unsigned char)r2;
     r.h.dl = (unsigned char)c2;
-    int386(0x10, &r, &r);
+    INTX(0x10, &r, &r);
 }
 
 static void bios_write_char(int ch, int attr)
@@ -72,7 +80,7 @@ static void bios_write_char(int ch, int attr)
     r.h.bh = 0;
     r.h.bl = (unsigned char)attr;
     r.w.cx = 1;
-    int386(0x10, &r, &r);
+    INTX(0x10, &r, &r);
 }
 
 /* --- Terminal I/O --- */
@@ -118,7 +126,7 @@ static int dos_getch(void)
     union REGS r;
     memset(&r, 0, sizeof(r));
     r.h.ah = 0x00;
-    int386(0x16, &r, &r);
+    INTX(0x16, &r, &r);
     return r.h.al ? r.h.al : (0x100 | r.h.ah);
 }
 
@@ -156,7 +164,7 @@ static void dos_init(void)
     union REGS r;
     memset(&r, 0, sizeof(r));
     r.h.ah = 0x0F;
-    int386(0x10, &r, &r);
+    INTX(0x10, &r, &r);
     screen_cols = r.h.ah;
     screen_rows = 25;  /* safe default; BIOS data area read needs far ptr */
     bios_get_cursor(&cursor_row, &cursor_col);

src/sound.c

@@ -91,8 +91,13 @@ void snd_tone(int freq_hz, int duration_ticks)
         return;
 
     /* Allocate buffer (or use stack for short tones) */
+#ifdef _M_I86
+    int16_t stack_buf[512];   /* 1KB — 16-bit DOS has ~4-8KB stack */
+    int16_t *buf = (total <= 512) ? stack_buf : malloc(total * sizeof(int16_t));
+#else
     int16_t stack_buf[8192];
     int16_t *buf = (total <= 8192) ? stack_buf : malloc(total * sizeof(int16_t));
+#endif
     if (!buf)
         return;
 

src/tui.c

@@ -665,8 +665,10 @@ void tui_init(bool fullscreen)
     /* Allocate screen buffer */
     tui.screen = calloc(tui.rows * tui.cols, sizeof(tui_cell_t));
     if (!tui.screen) {
-        fprintf(stderr, "Out of memory for screen buffer\n");
-        exit(1);
+        /* Not enough near heap (common on 16-bit DOS).
+         * Disable TUI — batch mode still works via HAL. */
+        tui.active = false;
+        return;
     }
 
     /* Set default F-key definitions */