diff --git a/CHANGELOG.md b/CHANGELOG.md
index 63a7105b..288fcc19 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,6 +55,7 @@ See [extension/CHANGELOG.md](extension/CHANGELOG.md).
 ### Build tools
 
 - Many toolchains are now WASI builds: cc65 (pinned to 2021 6ac4aa4), DASM 2.20.17, acme, zmac, yasm and merlin32.
+- SDCC 4 mos6502 backend: C64 presets (`hello-sdcc.c`, `screen_ram-sdcc.c`, `joymove-sdcc.c`, `siegegame-sdcc.c`, `sprite_collision-sdcc.c`); `load_header` platform param wraps the image as a DOS 3.3 binary (apple2) or a PRG with a `SYS` stub (c64).
 - Updated batari BASIC; sdcc now parses `#symbol` directives and fixes lost link errors.
 - New build directives replace the legacy CFGFILE-style ones; build parameters no longer leak between builds.
 - `#pragma compile("...")` and oscar64 symbol/listing parsing.
@@ -64,7 +65,9 @@ See [extension/CHANGELOG.md](extension/CHANGELOG.md).
 - cc65 `a2.lo.s` (Apple II lo-res TGI) patched in `8bitworkshop-compilers/patches/cc65-apple2-lo-tgi.patch`: a stray `bpl :+` in `INSTALL` made `tgi_install(a2_lo_tgi)` crash on a non-enhanced Apple II.
 - Presets adjusted for the newer cc65: NES `chase` closes its zero-page `#pragma` region, C64 `side_scroller` uses an unsigned fixed-point constant, and the Atari vector presets no longer recurse into `main()`. `neslib.h` no longer redefines `NULL` if `stddef.h` already did.
 - SDCC: both 3.6.5 and 4.x are bundled; 3.6.5 is the default (several times faster to build) and `//#tooldef c sdcc=4` opts in to 4.x (the default is `SDCC_DEFAULT_VERSION` in `toolmeta.ts`). Programs without an `opt_code*` pragma now build with `--no-peep --nolospre --max-allocs-per-node 500`, including Game Boy, which previously always got the full optimizer. See *SDCC versions* in the build directives docs.
-- SDCC 4 mos6502 backend (experimental): a `-sdcc.c` file on `devel-6502` (see `presets/devel-6502/hello-sdcc.c`) builds with sdcc, `sdas6500` and `sdld6808`. Apple II and other 6502 platforms need their own crt0 and memory map.
+- SDCC 4 mos6502 backend (experimental): a `.sdcc` or `-sdcc.c` file on `devel-6502` or `apple2` builds with sdcc, `sdas6500` and `sdld6808` (see `hello-sdcc.c` in those presets). Apple II programs link as a DOS 3.3 binary at `$803`. `.sdcc` now selects SDCC on every Z80 and 6502 platform.
+- `//#tooldef ld code_start=ADDR` and `data_start=ADDR` move SDCC's code and RAM data. The Apple II preset `cosmic-sdcc.c` builds the Cosmic Impalas game with SDCC by including `cosmic.c`, which now has `__SDCC` alternatives for `conio` and inline assembly.
+- `devel-6502` runs headless in the `8bws` CLI, and the new `serial` run-script command prints what the program sent to its serial port.
 - Build is now only `esbuild` through `scripts/build.mjs`; code split into a headless `src/common/projectcore.ts` / `toolselect.ts`.
 - New `buildpresets` script that builds and runs every preset.
 
diff --git a/presets/apple2/cosmic-sdcc.c b/presets/apple2/cosmic-sdcc.c
new file mode 100644
index 00000000..cad60a1a
--- /dev/null
+++ b/presets/apple2/cosmic-sdcc.c
@@ -0,0 +1,7 @@
+// Cosmic Impalas built with SDCC's mos6502 backend instead of cc65.
+// The game is cosmic.c; this file picks the compiler and memory layout.
+
+// the hires page 1 buffer is $2000-$3FFF, so the program loads above it
+//#tooldef ld code_start=0x4000
+
+#include "cosmic.c"
diff --git a/presets/apple2/cosmic.c b/presets/apple2/cosmic.c
index ce8ffe28..9e70827e 100644
--- a/presets/apple2/cosmic.c
+++ b/presets/apple2/cosmic.c
@@ -1,8 +1,10 @@
 
 // reserve space for the HGR1 screen buffer
 //#tooldef ld cfgfile=apple2-hgr.cfg
+#ifndef __SDCC
 #pragma data-name(push,"HGR")
 #pragma data-name(pop)
+#endif
 
 /*
  * An Apple ][ port of the Cosmic Impalas game 
@@ -11,9 +13,11 @@
  */
 
 #include <string.h>
+#ifndef __SDCC
 #include <conio.h>
 #include <apple2.h>
 #include <peekpoke.h>
+#endif
 
 // type aliases for byte/signed byte/unsigned 16-bit
 typedef unsigned char byte;
@@ -21,7 +25,18 @@ typedef signed char sbyte;
 typedef unsigned short word;
 
 // peeks, pokes, and strobes
+#ifdef __SDCC
+// (building with sdcc, see cosmic-sdcc.c: no conio, so read the keyboard directly)
+#define STROBE(addr)       ((void)*(volatile byte*)(addr))
+byte kbhit(void) { return *(volatile byte*)0xc000 & 0x80; }
+char cgetc(void) {
+  char key = *(volatile byte*)0xc000 & 0x7f;
+  STROBE(0xc010);	// clear the keyboard strobe
+  return key;
+}
+#else
 #define STROBE(addr)       __asm__ ("sta %w", addr)
+#endif
 
 // speaker click
 #define CLICK		   STROBE(0xc030)
@@ -232,11 +247,24 @@ void draw_bcd_word(word bcd, byte col, byte row, byte vert) {
 
 // add two 4-digit BCD words
 word bcd_add(word a, word b) {
+#ifdef __SDCC
+  // add one digit at a time, in plain C
+  word result = 0;
+  byte shift, carry = 0;
+  for (shift = 0; shift < 16; shift += 4) {
+    byte d = ((a >> shift) & 0xf) + ((b >> shift) & 0xf) + carry;
+    carry = d > 9;
+    if (carry) d -= 10;
+    result |= (word)d << shift;
+  }
+  return result;
+#else
   word result;
   __asm__ ("sed"); // set decimal (BCD) mode
   result = a+b;
   __asm__ ("cld"); // clear BCD mode
   return result;
+#endif
 }
 
 //
diff --git a/presets/apple2/hello-sdcc.c b/presets/apple2/hello-sdcc.c
new file mode 100644
index 00000000..f4eee7a4
--- /dev/null
+++ b/presets/apple2/hello-sdcc.c
@@ -0,0 +1,31 @@
+// Hello World in C for the Apple ][ using SDCC's mos6502 backend.
+// (The -sdcc.c suffix selects SDCC instead of cc65.) The program is linked
+// as a DOS 3.3 binary at $803; there is no C library or conio, so we poke
+// the text screen directly.
+
+#include <stdint.h>
+
+#define TEXT_PAGE1 ((volatile uint8_t *)0x400)
+
+const char message[] = "HELLO, SDCC 6502!";
+
+uint8_t row = 10;       // initialized data is copied to RAM at startup
+
+// text rows are interleaved: three groups of 8 rows, 40 bytes apart,
+// each row of a group $80 bytes after the previous
+volatile uint8_t *row_address(uint8_t r) {
+    return TEXT_PAGE1 + (r & 7) * 0x80 + (r >> 3) * 40;
+}
+
+void main(void) {
+    uint8_t i, r;
+    volatile uint8_t *p;
+    for (r = 0; r < 24; r++) {          // clear the screen
+        p = row_address(r);
+        for (i = 0; i < 40; i++) p[i] = 0xa0;
+    }
+    p = row_address(row) + 11;
+    for (i = 0; message[i]; i++)
+        p[i] = message[i] | 0x80;       // normal video: high bit set
+    while (1);
+}
diff --git a/presets/c64/c64-sdcc.h b/presets/c64/c64-sdcc.h
new file mode 100644
index 00000000..837bc048
--- /dev/null
+++ b/presets/c64/c64-sdcc.h
@@ -0,0 +1,106 @@
+// Minimal C64 definitions for SDCC's mos6502 backend, which has no c64.h,
+// conio.h or joystick.h like cc65 does. Included by common.h under __SDCC.
+
+#ifndef _C64_SDCC_H
+#define _C64_SDCC_H
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define PEEK(addr)         (*(volatile uint8_t*)(addr))
+#define POKE(addr,val)     (*(volatile uint8_t*)(addr) = (val))
+
+#define COLOR_BLACK      0
+#define COLOR_WHITE      1
+#define COLOR_RED        2
+#define COLOR_CYAN       3
+#define COLOR_PURPLE     4
+#define COLOR_GREEN      5
+#define COLOR_BLUE       6
+#define COLOR_YELLOW     7
+#define COLOR_ORANGE     8
+#define COLOR_BROWN      9
+#define COLOR_LIGHTRED   10
+#define COLOR_GRAY1      11
+#define COLOR_GRAY2      12
+#define COLOR_LIGHTGREEN 13
+#define COLOR_LIGHTBLUE  14
+#define COLOR_GRAY3      15
+
+// VIC-II registers, same field names as cc65's struct __vic2
+struct __vic2 {
+  struct { uint8_t x, y; } spr_pos[8];
+  uint8_t spr_hi_x, ctrl1, rasterline, strobe_x, strobe_y;
+  uint8_t spr_ena, ctrl2, spr_exp_y, addr, irr, imr;
+  uint8_t spr_bg_prio, spr_mcolor, spr_exp_x, spr_coll, spr_bg_coll;
+  uint8_t bordercolor, bgcolor0, bgcolor1, bgcolor2, bgcolor3;
+  uint8_t spr_mcolor0, spr_mcolor1;
+  uint8_t spr_color[8];
+};
+
+// 6526 CIA registers
+struct __6526 {
+  uint8_t pra, prb, ddra, ddrb, ta_lo, ta_hi, tb_lo, tb_hi;
+  uint8_t tod_10, tod_sec, tod_min, tod_hour, sdr, icr, cra, crb;
+};
+
+#define VIC  (*(volatile struct __vic2*)0xd000)
+#define CIA1 (*(volatile struct __6526*)0xdc00)
+#define CIA2 (*(volatile struct __6526*)0xdd00)
+#define COLOR_RAM ((volatile uint8_t*)0xd800)
+
+// joystick: bits are active low on CIA1 (port 2 = pra, port 1 = prb)
+#define JOY_UP_MASK     0x01
+#define JOY_DOWN_MASK   0x02
+#define JOY_LEFT_MASK   0x04
+#define JOY_RIGHT_MASK  0x08
+#define JOY_BTN_1_MASK  0x10
+#define JOY_UP(v)       ((v) & JOY_UP_MASK)
+#define JOY_DOWN(v)     ((v) & JOY_DOWN_MASK)
+#define JOY_LEFT(v)     ((v) & JOY_LEFT_MASK)
+#define JOY_RIGHT(v)    ((v) & JOY_RIGHT_MASK)
+#define JOY_BTN_1(v)    ((v) & JOY_BTN_1_MASK)
+#define joy_install(drv)
+#define joy_static_stddrv 0
+// (the KERNAL's keyboard scan also drives these ports, so stop it while reading)
+static uint8_t joy_read(uint8_t port) {
+  uint8_t v;
+  __asm__("sei");
+  if (port) { CIA1.ddra = 0; v = CIA1.pra; CIA1.ddra = 0xff; }  // joystick 2
+  else      { CIA1.ddrb = 0; v = CIA1.prb; }                    // joystick 1
+  __asm__("cli");
+  return ~v & 0x1f;
+}
+
+// conio-like text output straight to screen RAM (screen codes, not PETSCII)
+static uint8_t _cx, _cy, _ccolor = COLOR_LIGHTBLUE, _crev;
+
+static void clrscr(void) {
+  uint16_t i;
+  for (i = 0; i < 1000; i++) POKE(0x400 + i, 32);
+  _cx = _cy = 0;
+}
+
+static uint8_t _screencode(char c) {
+  if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) return c & 0x1f;
+  if (c == '#') return 0xa3;  // the uppercase character set has no plain '#' glyph; use reverse video
+  return c;     // digits, punctuation and space match ASCII
+}
+
+static void textcolor(uint8_t c) { _ccolor = c; }
+static uint8_t revers(uint8_t on) { uint8_t old = _crev; _crev = on ? 0x80 : 0; return old; }
+
+static void cputcxy(uint8_t x, uint8_t y, char c) {
+  uint16_t ofs = y * 40 + x;
+  POKE(0x400 + ofs, _screencode(c) | _crev);
+  POKE(0xd800 + ofs, _ccolor);
+}
+
+static void cputsxy(uint8_t x, uint8_t y, const char* s) {
+  while (*s) cputcxy(x++, y, *s++);
+}
+
+static uint8_t kbhit(void) { return PEEK(0xc6); }  // KERNAL keyboard buffer count
+
+#endif
diff --git a/presets/c64/common.h b/presets/c64/common.h
index 43b876a3..cbeaf115 100644
--- a/presets/c64/common.h
+++ b/presets/c64/common.h
@@ -1,12 +1,17 @@
 #ifndef _COMMON_H
 #define _COMMON_H
 
+#ifdef __SDCC
+// SDCC has no cc65 headers, so use our own subset (see c64-sdcc.h)
+#include "c64-sdcc.h"
+#else
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
 #include <peekpoke.h>
 #include <string.h>
 #include <c64.h>
+#endif
 
 #ifdef __CC65__
 #include <conio.h>
@@ -16,7 +21,13 @@
 typedef uint8_t byte;	// 8-bit unsigned
 typedef int8_t sbyte;	// 8-bit signed
 typedef uint16_t word;	// 16-bit unsigned
+#ifdef __SDCC
+typedef uint8_t bool;
+#define false 0
+#define true 1
+#else
 typedef enum { false, true } bool; // boolean
+#endif
 
 #define COLS 40		// total # of columns
 #define ROWS 25		// total # of rows
@@ -127,7 +138,7 @@ char* get_screen_memory();
 char __fastcall__ poll_keyboard();
 #endif
 
-#ifndef __CC65__
+#if !defined(__CC65__) && !defined(__SDCC)
 inline void clrscr() {
     __asm__ volatile ("jsr $E544" : : : "a","x","y"); // regs clobbered
 }
@@ -135,6 +146,9 @@ inline void waitvsync() {
   raster_wait(255);
 }
 #endif
+#ifdef __SDCC
+static void waitvsync(void) { raster_wait(255); }
+#endif
 
 // for use with set_irq()
 // sets up the VIC to send raster interrupts
diff --git a/presets/c64/hello-sdcc.c b/presets/c64/hello-sdcc.c
new file mode 100644
index 00000000..d49133a3
--- /dev/null
+++ b/presets/c64/hello-sdcc.c
@@ -0,0 +1,32 @@
+// Hello World in C for the Commodore 64 using SDCC's mos6502 backend.
+// (The -sdcc.c suffix selects SDCC instead of cc65.) The program is a PRG
+// with a BASIC "SYS 2061" stub; there is no C library or conio, so we poke
+// screen RAM and color RAM directly.
+
+#include <stdint.h>
+
+#define SCREEN    ((volatile uint8_t *)0x400)
+#define COLOR_RAM ((volatile uint8_t *)0xd800)
+#define BORDER    (*(volatile uint8_t *)0xd020)
+#define BACKGROUND (*(volatile uint8_t *)0xd021)
+
+// screen codes (not ASCII): letters are 1-26, space is 32
+const uint8_t message[] = { 8,5,12,12,15, 32, 19,4,3,3, 32, 54,53,48,50 };
+
+uint8_t color = 7;      // initialized data is copied to RAM at startup
+
+void main(void) {
+    uint16_t i;
+    uint8_t c;
+    BORDER = 6;
+    BACKGROUND = 0;
+    for (i = 0; i < 1000; i++) {        // clear the screen
+        SCREEN[i] = 32;
+        COLOR_RAM[i] = 1;
+    }
+    for (c = 0; c < sizeof(message); c++) {
+        SCREEN[12*40 + 12 + c] = message[c];
+        COLOR_RAM[12*40 + 12 + c] = color;
+    }
+    while (1);
+}
diff --git a/presets/c64/joymove.c b/presets/c64/joymove.c
index 743bee66..38e4c4f3 100644
--- a/presets/c64/joymove.c
+++ b/presets/c64/joymove.c
@@ -20,6 +20,9 @@ void main(void) {
   byte bgcoll;	// sprite background collision flags
   byte joy;	// joystick flags
 
+#ifdef __SDCC
+  clrscr();	// (cc65's startup code clears the screen, SDCC's does not)
+#endif
   // copy sprite pattern to RAM address 0x3800
   memcpy((char*)0x3800, SPRITE_DATA, sizeof(SPRITE_DATA));
   // set sprite #0 shape entry (224)
diff --git a/presets/c64/screen_ram.c b/presets/c64/screen_ram.c
index 1b3aa4aa..fce1403d 100644
--- a/presets/c64/screen_ram.c
+++ b/presets/c64/screen_ram.c
@@ -2,7 +2,9 @@
 #include "common.h"
 //#link "common.c"
  
+#ifndef __SDCC
 #include <cbm_screen_charmap.h>
+#endif
 
 void main(void) {
   unsigned int i;
diff --git a/presets/c64/sprite_collision-sdcc.c b/presets/c64/sprite_collision-sdcc.c
new file mode 100644
index 00000000..f9a9e3d9
--- /dev/null
+++ b/presets/c64/sprite_collision-sdcc.c
@@ -0,0 +1,11 @@
+// sprite_collision.c built with SDCC's mos6502 backend instead of cc65.
+// The program is sprite_collision.c; common.h supplies the C64 definitions SDCC lacks.
+// (common.c and sprites.c are #included, since cc65 and SDCC objects can't be linked together.)
+
+#include "common.h"
+#include "common.c"
+
+#include "sprites.h"
+#include "sprites.c"
+
+#include "sprite_collision.c"
diff --git a/presets/devel-6502/hello.dasm b/presets/devel-6502/hello.dasm
new file mode 100644
index 00000000..5a85898d
--- /dev/null
+++ b/presets/devel-6502/hello.dasm
@@ -0,0 +1,22 @@
+ processor 6502
+ org $8000
+
+Start
+; ADD YOUR CODE HERE
+ rts
+ 
+ org $ffe0
+NMIVec
+RESVec
+BRKVec
+ sei
+ cld
+ ldx #$ff
+ txs
+ jsr Start
+ jmp .
+ 
+ org $fffa
+ word NMIVec	; NMI
+ word RESVec	; reset
+ word BRKVec	; BRK
diff --git a/src/common/baseplatform.ts b/src/common/baseplatform.ts
index 8d0a2f17..e93d7687 100644
--- a/src/common/baseplatform.ts
+++ b/src/common/baseplatform.ts
@@ -523,7 +523,7 @@ export abstract class Base6502Platform extends BaseDebugPlatform {
     return disassemble6502(pc, read(pc), read(pc + 1), read(pc + 2));
   }
   getToolForFilename = getToolForFilename_6502;
-  getDefaultExtensions() { return [".c", ".cpp", ".o64", ".acme", ".xa", ".ca65", ".dasm", ".ecs", ".wiz"]; };
+  getDefaultExtensions() { return [".c", ".sdcc", ".cpp", ".o64", ".acme", ".xa", ".ca65", ".dasm", ".ecs", ".wiz"]; };
 
   getDebugCategories() {
     return ['CPU', 'ZPRAM', 'Stack'];
@@ -643,7 +643,7 @@ export abstract class BaseZ80Platform extends BaseDebugPlatform {
   }
 
   getToolForFilename = getToolForFilename_z80;
-  getDefaultExtensions() { return [".c", ".ns", ".s", ".scc", ".sgb", ".z", ".wiz"]; };
+  getDefaultExtensions() { return [".c", ".sdcc", ".ns", ".s", ".scc", ".sgb", ".z", ".wiz"]; };
   // TODO: Z80 opcode metadata
   //this.getOpcodeMetadata = function() { }
 
diff --git a/src/common/emucore.ts b/src/common/emucore.ts
index 9b3411ec..040e677e 100644
--- a/src/common/emucore.ts
+++ b/src/common/emucore.ts
@@ -421,6 +421,12 @@ export class EmuCore {
   get frameRate(): number { return this.video ? this.video.frameRate : 60; }
   saveState(): EmuState | null { return this.platform.saveState ? this.platform.saveState() : null; }
 
+  /** Text a platform's program has sent to a serial port, or null if it has none. */
+  getSerialOutput(): string | null {
+    const p: any = this.platform;
+    return typeof p.getSerialOutput === 'function' ? p.getSerialOutput() : null;
+  }
+
   getDebugInfo(): DebugSection[] {
     const state = this.saveState();
     const p: any = this.platform;
diff --git a/src/common/toolmeta.ts b/src/common/toolmeta.ts
index 05e566f3..aae60ab3 100644
--- a/src/common/toolmeta.ts
+++ b/src/common/toolmeta.ts
@@ -410,7 +410,7 @@ export const TOOL_META: { [id: string]: ToolMeta } = {
 
   sdcc: {
     id: 'sdcc', name: 'SDCC', kind: 'compiler', arch: 'z80',
-    extensions: ['.c', '.h'],
+    extensions: ['.c', '.h', '.sdcc'],
     includeDirs: ['/share/sdcc/include', '/include'],
     editorStyle: 'text/x-csrc',
     helpURL: 'http://sdcc.sourceforge.net/doc/sdccman.pdf',
diff --git a/src/common/toolselect.ts b/src/common/toolselect.ts
index 74252ed2..4b08d40e 100644
--- a/src/common/toolselect.ts
+++ b/src/common/toolselect.ts
@@ -12,7 +12,7 @@ export type ToolSelector = (fn: string) => string;
 
 export function getToolForFilename_6502(fn: string): string {
   if (fn.endsWith("-llvm.c")) return "remote:llvm-mos";
-  if (fn.endsWith("-sdcc.c")) return "sdcc";   // SDCC 4.x mos6502 backend
+  if (fn.endsWith("-sdcc.c") || fn.endsWith(".sdcc")) return "sdcc";   // SDCC 4.x mos6502 backend
   if (fn.endsWith(".c")) return "cc65";
   if (fn.endsWith(".h")) return "cc65";
   if (fn.endsWith(".s")) return "ca65";
@@ -30,7 +30,7 @@ export function getToolForFilename_6502(fn: string): string {
 }
 
 export function getToolForFilename_z80(fn: string): string {
-  if (fn.endsWith(".c")) return "sdcc";
+  if (fn.endsWith(".c") || fn.endsWith(".sdcc")) return "sdcc";
   if (fn.endsWith(".h")) return "sdcc";
   if (fn.endsWith(".s")) return "sdasz80";
   if (fn.endsWith(".sgb")) return "sdasgb";
@@ -61,7 +61,6 @@ export function getToolForFilename_arm32(fn: string): string {
 export function getToolForFilename_vcs(fn: string): string {
   if (fn.endsWith(".cc2600")) return "cc2600";
   if (fn.endsWith("-llvm.c")) return "remote:llvm-mos";
-  if (fn.endsWith("-sdcc.c")) return "sdcc";   // SDCC 4.x mos6502 backend
   if (fn.endsWith(".wiz")) return "wiz";
   if (fn.endsWith(".bb") || fn.endsWith(".bas")) return "bataribasic";
   if (fn.endsWith(".ca65")) return "ca65";
diff --git a/src/docs/build-directives.md b/src/docs/build-directives.md
index 7b20b552..a5cdea07 100644
--- a/src/docs/build-directives.md
+++ b/src/docs/build-directives.md
@@ -135,11 +135,13 @@ The supported parameters are:
 | --- | --- | --- |
 | `ld` | `cfgfile` | linker configuration file to use (e.g. for CC65's ld65) |
 | `ld` | `libargs` | comma-separated list of library/symbol arguments |
+| `ld` | `code_start`, `data_start` | where SDCC's linker places the code and the RAM data (e.g. `0x4000`) |
 | `c` | `sdcc` | SDCC version, `3` or `4` (see *SDCC versions* below) |
 
 ~~~c
 //#tooldef ld cfgfile=apple2-hgr2.cfg
 //#tooldef ld libargs=,nes.lib
+//#tooldef ld code_start=0x4000
 //#tooldef c sdcc=3
 ~~~
 
diff --git a/src/platform/apple2.ts b/src/platform/apple2.ts
index 89922372..264e0b8a 100644
--- a/src/platform/apple2.ts
+++ b/src/platform/apple2.ts
@@ -28,6 +28,8 @@ const APPLE2_PRESETS: Preset[] = [
   //  {id:'tb_6502.s', name:'Tom Bombem (assembler game)'},
   { id: 'dos33bin.a', name: "DOS 3.3 Binary" },
   { id: 'applesinglebin.a', name: "AppleSingle Binary" },
+  { id: 'hello-sdcc.c', name: 'Hello World (SDCC)', category: "C (SDCC)" },
+  { id: 'cosmic-sdcc.c', name: 'Cosmic Impalas (SDCC)' },
 ];
 
 /// MAME support
@@ -51,7 +53,7 @@ class Apple2MAMEPlatform extends BaseMAME6502Platform implements Platform {
   }
 
   getOpcodeMetadata = getOpcodeMetadata_6502;
-  getDefaultExtensions() { return [".c", ".cpp", ".acme", ".xa", ".ca65", ".dasm", ".ecs", ".wiz"]; }
+  getDefaultExtensions() { return [".c", ".sdcc", ".cpp", ".acme", ".xa", ".ca65", ".dasm", ".ecs", ".wiz"]; }
   getToolForFilename = getToolForFilename_apple2;
 
   getPresets() { return APPLE2_PRESETS; }
diff --git a/src/platform/c64.ts b/src/platform/c64.ts
index aefc7adb..26a3d221 100644
--- a/src/platform/c64.ts
+++ b/src/platform/c64.ts
@@ -46,6 +46,8 @@ const C64_PRESETS : Preset[] = [
   {id:'hello.cpp', name:'Hello Oscar64', category:'C++ (Oscar64)'},
   {id:'particles.cpp', name:'Fireworks'},
   {id:'lander.cpp', name:'Lunar Lander'},
+  {id:'hello-sdcc.c', name:'Hello World (SDCC)', category:'C (SDCC)'},
+  {id:'sprite_collision-sdcc.c', name:'Sprite Collision (SDCC)'},
 ];
 
 const C64_MEMORY_MAP = { main:[
diff --git a/src/platform/devel.ts b/src/platform/devel.ts
index be27e5f4..c2117d86 100644
--- a/src/platform/devel.ts
+++ b/src/platform/devel.ts
@@ -12,7 +12,26 @@ var DEVEL_6502_PRESETS = [
   {id:'hello-sdcc.c', name:'Hello World (SDCC C)'},
 ];
 
-class SerialInOutViewer {
+/** What the serial harness draws on: the TeleType viewer, or nothing (headless). */
+interface SerialViewer {
+  tty : { addtext(s: string, flags: number): void, newline(): void, clear(): void,
+          saveState(): any, loadState(state: any): void };
+  start(): void;
+  reset(): void;
+  saveState(): any;
+  loadState(state: any): void;
+}
+
+/** For the CLI and tests, which have no DOM: the output is kept by the harness. */
+class HeadlessSerialViewer implements SerialViewer {
+  tty = { addtext() {}, newline() {}, clear() {}, saveState() { return {}; }, loadState() {} };
+  start() {}
+  reset() {}
+  saveState() { return {}; }
+  loadState() {}
+}
+
+class SerialInOutViewer implements SerialViewer {
   div : HTMLElement;
   tty : TeleType;
   
@@ -48,7 +67,7 @@ function byteToASCII(b: number) : string {
 
 export class SerialTestHarness implements SerialIOInterface {
 
-  viewer : SerialInOutViewer;
+  viewer : SerialViewer;
   bufferedRead : boolean = true;
   cyclesPerByte = 1000000/(57600/8); // 138.88888 cycles
   maxOutputBytes = 4096;
@@ -117,11 +136,12 @@ export class SerialTestHarness implements SerialIOInterface {
 class Devel6502Platform extends Base6502MachinePlatform<Devel6502> implements Platform {
 
   serial : SerialTestHarness;
-  serview : SerialInOutViewer;
+  serview : SerialViewer;
 
   constructor(mainElement: HTMLElement) {
     super(mainElement);
-    this.serview = new SerialInOutViewer(mainElement);
+    // no element: running headless (the 8bws CLI)
+    this.serview = mainElement ? new SerialInOutViewer(mainElement) : new HeadlessSerialViewer();
   }
 
   async start() {
@@ -165,6 +185,9 @@ class Devel6502Platform extends Base6502MachinePlatform<Devel6502> implements Pl
     // TODO: reload tty UI
   }
 
+  /** Everything the program has sent to the serial port (for the CLI's `serial` command). */
+  getSerialOutput()     { return byteArrayToString(this.serial.outputBytes); }
+
   newMachine()          { return new Devel6502(); }
   getPresets()          { return DEVEL_6502_PRESETS; }
   readAddress(a)        { return this.machine.readConst(a); }
diff --git a/src/tools/buildpresets.ts b/src/tools/buildpresets.ts
index c2fb9953..af3b5ec1 100644
--- a/src/tools/buildpresets.ts
+++ b/src/tools/buildpresets.ts
@@ -103,6 +103,8 @@ const SKIP_PLATFORMS = [
     'astrocade-arcade',
     'mcr',
     'vectrex',
+    'devel',
+    'devel-6502',
 ];
 
 // build these, but don't run them: the emulator needs a real browser
diff --git a/src/tools/runscript.ts b/src/tools/runscript.ts
index 13efb0d0..ea8e1057 100644
--- a/src/tools/runscript.ts
+++ b/src/tools/runscript.ts
@@ -43,6 +43,7 @@ export const RUN_SCRIPT_HELP = [
   '  keydown KEY / keyup KEY     - raw key down/up events',
   '  mem START [LEN]             - hexdump memory (default 16 bytes)',
   '  screen [START] [COLS] [ROWS]- decode screen RAM to text (default $0400 40x25)',
+  '  serial                      - print what the program sent to its serial port (devel-6502)',
   '  pc [N]                      - print PC + disassembly of N instructions',
   '  info                        - platform/machine debug info',
   '  reset                       - reset the emulator',
@@ -485,6 +486,13 @@ export class RunScript {
     }
   }
 
+  cmdSerial() {
+    const text = this.target.getSerialOutput();
+    if (text == null) throw new Error(`'${this.target.id}' has no serial output`);
+    this.log(`serial output (${text.length} bytes):`);
+    this.out(text.endsWith('\n') ? text : text + '\n');
+  }
+
   cmdPC(tokens: string[]) {
     this.target.settle();
     const pc = this.target.getPC();
@@ -534,6 +542,7 @@ const COMMANDS: { [name: string]: Command } = {
   'keyup': RunScript.prototype.cmdKeyUp,
   'mem': RunScript.prototype.cmdMem,
   'screen': RunScript.prototype.cmdScreen,
+  'serial': RunScript.prototype.cmdSerial,
   'pc': RunScript.prototype.cmdPC,
   'info': RunScript.prototype.cmdInfo,
   'reset': RunScript.prototype.cmdReset,
diff --git a/src/worker/builder.ts b/src/worker/builder.ts
index df9a86e0..9c7b608e 100644
--- a/src/worker/builder.ts
+++ b/src/worker/builder.ts
@@ -466,8 +466,8 @@ export function applyAsmProjectParams(params) {
  *
  *   //#symbol [<phase>] NAME[=VALUE]   phase in c|as|ld (default c)
  *   //#flag   <phase> <args...>        raw argv for one phase
- *   //#tooldef <phase> NAME=VALUE      typed knob (linker: cfgfile, libargs;
- *                                      compiler: sdcc=3|4)
+ *   //#tooldef <phase> NAME=VALUE      typed knob (linker: cfgfile, libargs,
+ *                                      code_start, data_start; compiler: sdcc=3|4)
  *
  * Symbols are phase-scoped because they are different things: a preprocessor
  * macro (`c`) takes text, while an assembler/linker global (`as`/`ld`) is an
@@ -733,6 +733,11 @@ export function applyBuildDirectives(dir: SourceDirectives, params) {
       params.cfgfile = td.value;
     } else if (td.phase === 'linker' && td.name === 'libargs') {
       params.libargs = td.value.split(',').filter((s) => s !== '');
+    } else if (td.phase === 'linker' && (td.name === 'code_start' || td.name === 'data_start')) {
+      // where sdld puts the code (and the DOS 3.3 header's load address) and the RAM data
+      const addr = Number(td.value);
+      if (Number.isInteger(addr) && addr >= 0 && addr < 0x10000) params[td.name] = addr;
+      else dir.errors.push(`#tooldef ld ${td.name}: expected an address, got '${td.value}'`);
     } else if (td.phase === 'compiler' && td.name === 'sdcc') {
       // SDCC major version; every sdcc/sdas/sdld step of the build reads it
       if (td.value === '3' || td.value === '4') params.sdcc_version = parseInt(td.value);
diff --git a/src/worker/platforms.ts b/src/worker/platforms.ts
index 887188b4..d7eaee47 100644
--- a/src/worker/platforms.ts
+++ b/src/worker/platforms.ts
@@ -221,6 +221,11 @@ export var PLATFORM_PARAMS = {
       libargs: [ '--lib-path', '/share/target/apple2/drv', 'apple2.lib'],
       __CODE_RUN__: 16384,
       code_start: 0x803,
+      // for SDCC (-sdcc.c): a DOS 3.3 binary at $803 with data above the program
+      rom_size: 0x9400 - 0x803,
+      data_start: 0x9400,
+      zp_start: 0x60,
+      load_header: 'dos33',
       acmeargs: ['-f', 'apple'],
     },
     'apple2-e': {
@@ -301,6 +306,12 @@ export var PLATFORM_PARAMS = {
       cfgfile: 'c64.cfg', // SYS 2061
       libargs: ['openroms-compat.o', 'c64.lib'],
       extra_link_files: ['openroms-compat.o'],
+      // for SDCC (-sdcc.c): a PRG with a BASIC SYS stub, code at $80D, data in upper RAM
+      code_start: 0x80d,
+      rom_size: 0xa000 - 0x80d,
+      data_start: 0xc000,
+      zp_start: 0x2,
+      load_header: 'prg',
       acmeargs: ['-f', 'cbm'],
       //extra_link_files: ['c64-cart.cfg'],
     },
diff --git a/src/worker/tools/sdcc.ts b/src/worker/tools/sdcc.ts
index 6fb124ba..6613438a 100644
--- a/src/worker/tools/sdcc.ts
+++ b/src/worker/tools/sdcc.ts
@@ -209,6 +209,53 @@ export function parseIHX(ihx: string, rom_start: number, rom_size: number, error
     return output;
 }
 
+/** End offset, within [rom_start, rom_start + rom_size), of the highest data record. */
+export function ihxExtent(ihx: string, rom_start: number, rom_size: number): number {
+    let end = 0, upper = 0;
+    for (const s of ihx.split("\n")) {
+        if (s[0] != ':') continue;
+        const arr = hexToArray(s, 1);
+        if (arr[3] == 0) {
+            const offset = upper + (arr[1] << 8) + arr[2] - rom_start;
+            if (offset >= 0 && offset + arr[0] <= rom_size) end = Math.max(end, offset + arr[0]);
+        } else if (arr[3] == 4) {
+            upper = ((arr[4] << 8) | arr[5]) << 16;
+        }
+    }
+    return end;
+}
+
+/**
+ * The `load_header` platform param: how a loadable file wraps the linked
+ * image (trimmed to the program's end, `length` bytes, linked at `start`).
+ *   dos33  Apple II binary: load address and length (2 bytes each, little
+ *          endian); the loader checks length == file size - 4
+ *   prg    Commodore PRG: load address $0801, then the BASIC line
+ *          `10 SYS <start>` that runs the program, which must be linked at
+ *          $080D (right after that stub)
+ */
+export function loadHeader(kind: string, image: Uint8Array, start: number, length: number): Uint8Array<ArrayBuffer> {
+    var header: number[];
+    switch (kind) {
+        case 'dos33':
+            header = [start & 0xff, start >> 8, length & 0xff, length >> 8];
+            break;
+        case 'prg': {
+            const sys = [...String(start)].map((c) => c.charCodeAt(0));
+            const end = 0x801 + 4 + 1 + sys.length + 1; // link, line number, SYS token, digits, NUL
+            header = [0x01, 0x08, end & 0xff, end >> 8, 0x0a, 0x00, 0x9e, ...sys, 0, 0, 0];
+            if (0x801 + header.length - 2 !== start) throw new Error(`load_header prg: code must start at $${(0x801 + header.length - 2).toString(16)}, not $${start.toString(16)}`);
+            break;
+        }
+        default:
+            throw new Error(`unknown load_header '${kind}'`);
+    }
+    const out = new Uint8Array(new ArrayBuffer(header.length + length));
+    out.set(header);
+    out.set(image.subarray(0, length), header.length);
+    return out;
+}
+
 function errorMatcherSDASZ80(path: string, errors: WorkerError[]) {
     //?ASxxxx-Error-<o> in line 1 of main.asm null
     //              <o> .org in REL area or directive / mnemonic error
@@ -432,6 +479,10 @@ async function linkSDLD(step: BuildStep, ld: 'sdldz80' | 'sdld6808') {
         if (errors.length) {
             return { errors: errors };
         }
+        if (params.load_header) {
+            const start = params.rom_start !== undefined ? params.rom_start : params.code_start;
+            binout = loadHeader(params.load_header, binout, start, ihxExtent(hexout, start, params.rom_size));
+        }
         // parse listings
         var listings: CodeListingMap = {};
         for (var fn of step.files) {
@@ -499,13 +550,14 @@ async function linkSDLD(step: BuildStep, ld: 'sdldz80' | 'sdld6808') {
  * source comments, source lines. `srcprefix` is the source file name without
  * its extension: SDCC 3.6.5 writes `;<stdin>:N:` comments, SDCC 4.x writes
  * `;file.c:N: <source>` (with mcpp's `//` path prefix), and 4.x addresses
- * have 8 hex digits instead of 4.
+ * have 8 hex digits instead of 4. The mos6502 backend writes `;\tfile.c: N: <source>`.
  */
 export function parseRSTListing(rstout: string, srcprefix: string) {
     //   0000 21 02 00      [10]   52 	ld	hl, #2
     var asmlines = parseListing(rstout, /^\s*([0-9A-F]{4,8})\s+([0-9A-F][0-9A-F r]*[0-9A-F])\s+\[([0-9 ]+)\]?\s+(\d+) (.*)/i, 4, 1, 2, 3);
     const name = srcprefix.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
-    const srcre = new RegExp(`^\\s+\\d+ ;(?:<stdin>|/*${name}\\.[^:\\s]+):(\\d+):`, 'i');
+    // the mos6502 backend writes `;<tab>file.c: 17: source`, with a space after the colon
+    const srcre = new RegExp(`^\\s+\\d+ ;\\s*(?:<stdin>|/*${name}\\.[^:\\s]+):\\s*(\\d+):`, 'i');
     var srclines = parseSourceLines(rstout, srcre, /^\s*([0-9A-F]{4,8})/i);
     // TODO: you have to get rid of all source lines to get asm listing
     return {
@@ -597,9 +649,9 @@ export async function compileSDCC(step: BuildStep): Promise<BuildStepResult> {
         // if "#pragma opt_code" found do not disable optimziations
         if (!/^\s*#pragma\s+opt_code/m.exec(code)) {
             args.push.apply(args, [
-                '--max-allocs-per-node', '500',
                 '--no-peep',
-                '--nolospre'
+                '--nolospre',
+                '--max-allocs-per-node', '500',
             ]);
         }
         if (params.extra_compile_args) {
diff --git a/test/cli/testcli.js b/test/cli/testcli.js
index 72264081..dad9af08 100644
--- a/test/cli/testcli.js
+++ b/test/cli/testcli.js
@@ -124,6 +124,30 @@ describe('8bws CLI', function () {
             assert.ok(r.success, r.error);
             assert.equal(r.data.frames, 10);
         });
+        it('should build cosmic.c with SDCC from a header-only wrapper', function () {
+            var out = path.join(os.tmpdir(), '8bws-cosmic-sdcc.bin');
+            cli('build', '-p', 'apple2', 'presets/apple2/cosmic-sdcc.c', '-o', out);
+            var bin = fs.readFileSync(out);
+            assert.equal(bin[0] | bin[1] << 8, 0x4000);           // //#tooldef ld code_start=0x4000
+            assert.equal(bin[2] | bin[3] << 8, bin.length - 4);
+            assert.ok(bin.length > 4000);                          // the whole game, not just the wrapper
+        });
+        ['hello', 'screen_ram', 'joymove', 'siegegame', 'sprite_collision'].forEach(function (name) {
+            it('should build the c64 ' + name + ' SDCC preset as a PRG', function () {
+                var out = path.join(os.tmpdir(), '8bws-c64-' + name + '-sdcc.prg');
+                cli('build', '-p', 'c64', 'presets/c64/' + name + '-sdcc.c', '-o', out);
+                var bin = fs.readFileSync(out);
+                assert.equal(bin[0] | bin[1] << 8, 0x801);        // load address, then 10 SYS 2061
+                assert.equal(bin[6], 0x9e);
+            });
+        });
+        it('should print the serial output of a devel-6502 program', function () {
+            var out = cli('run', '-p', 'devel-6502', 'presets/devel-6502/hello-sdcc.c', '-e', 'run 5; serial');
+            assert.ok(out.includes('Hello, SDCC 6502!\nagain\nagain\nagain'));
+        });
+        it('should reject serial on a platform without a serial port', function () {
+            cliFails('run', '-p', 'nes', 'presets/nes/hello.c', '-e', 'run 1; serial');
+        });
         it('should build and run a directory', function () {
             var dir = fs.mkdtempSync(path.join(os.tmpdir(), '8bws-dir-'));
             fs.copyFileSync('presets/nes/hello.c', path.join(dir, 'hello.c'));
diff --git a/test/cli/testworker.js b/test/cli/testworker.js
index 5351ef04..313867ab 100644
--- a/test/cli/testworker.js
+++ b/test/cli/testworker.js
@@ -245,6 +245,25 @@ describe('Worker', function() {
       done();
     }, 32768, 0, 0);
   });
+  it('should link SDCC mos6502 as a DOS 3.3 binary on apple2', function(done) {
+    var csource = ab2str(fs.readFileSync('presets/apple2/hello-sdcc.c'));
+    compile('sdcc', csource, 'apple2', function(err, msg) {
+      var bin = msg.output;
+      assert.equal(bin[0] | bin[1] << 8, 0x803);            // load address
+      assert.equal(bin[2] | bin[3] << 8, bin.length - 4);   // length: what the loader checks
+      assert.ok(bin.length < 1024);                          // trimmed to the program
+      done();
+    }, 377, 0, 0);
+  });
+  it('should link SDCC mos6502 as a PRG with a BASIC stub on c64', function(done) {
+    var csource = ab2str(fs.readFileSync('presets/c64/hello-sdcc.c'));
+    compile('sdcc', csource, 'c64', function(err, msg) {
+      var bin = msg.output;
+      assert.deepEqual(Array.from(bin.slice(0, 12)), [1,8, 0x0b,8, 10,0, 0x9e, 0x32,0x30,0x36,0x31, 0]); // 10 SYS 2061
+      assert.ok(bin.length < 1024);                          // trimmed to the program
+      done();
+    }, 332, 0, 0);
+  });
   it('should compile oscar64 and return listings/symbols/segments', async function() {
     var msgs = [{code:'#include <stdio.h>\nint main() { printf("FOO"); return 0; }', platform:'c64', tool:'oscar64', path:'main.c', mainfile:true}];
     var result = await new Promise(function(resolve, reject) {
diff --git a/test/unit/testbuildparams.ts b/test/unit/testbuildparams.ts
index ba879cbf..1f07be57 100644
--- a/test/unit/testbuildparams.ts
+++ b/test/unit/testbuildparams.ts
@@ -257,6 +257,16 @@ describe("explicit build directives (//#symbol, //#flag, //#tooldef)", function
     assert.strictEqual(params.sdcc_version, 3);
   });
 
+  it("#tooldef ld code_start and data_start take addresses", function () {
+    const params: any = {};
+    applyDirectives("//#tooldef ld code_start=0x4000\n//#tooldef ld data_start=36864\n", params);
+    assert.strictEqual(params.code_start, 0x4000);
+    assert.strictEqual(params.data_start, 0x9000);
+    const dir = applyDirectives("//#tooldef ld code_start=nowhere\n//#tooldef ld data_start=0x10000\n", params);
+    assert.strictEqual(dir.errors.length, 2);
+    assert.strictEqual(params.code_start, 0x4000);
+  });
+
   it("#tooldef rejects unknown knobs", function () {
     const params = c64Params();
     const dir = applyDirectives("//#tooldef ld nonsense=1\n", params);
diff --git a/test/unit/testsdcclisting.ts b/test/unit/testsdcclisting.ts
index e44eaad9..3b93844b 100644
--- a/test/unit/testsdcclisting.ts
+++ b/test/unit/testsdcclisting.ts
@@ -18,8 +18,24 @@ const RST_SDCC4 = [
   "    0000056F CD 40 00         [17] 1278 \tcall\t_inline_fn",
 ].join("\n");
 
+// the mos6502 backend: a tab and a space around the line number, [ 2] cycles
+const RST_MOS6502 = [
+  "                                    117 ;\thello.c: 17: void main(void) {",
+  "                                    118 ;\tgenLabel",
+  "    00000865                        126 _main:",
+  "                                    128 ;\thello.c: 18: putstr(message);",
+  "    00000865 A2 08            [ 2]  136 \tldx\t#>_message",
+  "    00000869 20 4E 08         [ 6]  138 \tjsr\t_putstr",
+].join("\n");
+
 describe("sdcc listings", function () {
 
+  it("maps mos6502 source comments", function () {
+    const l = parseRSTListing(RST_MOS6502, "hello");
+    assert.deepStrictEqual(l.lines.map((x) => [x.line, x.offset]), [[17, 0x865], [18, 0x865]]); // the function line maps to its label
+    assert.deepStrictEqual(l.asmlines.map((x) => x.offset), [0x865, 0x869]);
+  });
+
   it("maps SDCC 3.6.5 <stdin> source comments", function () {
     const l = parseRSTListing(RST_SDCC3, "skeleton");
     assert.deepStrictEqual(l.lines.map((x) => [x.line, x.offset]), [[172, 0x56c]]);
diff --git a/test/unit/testsdcctarget.ts b/test/unit/testsdcctarget.ts
index 95ebb1fd..284c86b9 100644
--- a/test/unit/testsdcctarget.ts
+++ b/test/unit/testsdcctarget.ts
@@ -1,7 +1,7 @@
 import assert from "assert";
 import { describe, it } from "mocha";
-import { sdccTarget } from "../../src/worker/tools/sdcc";
-import { getToolForFilename_6502 } from "../../src/common/toolselect";
+import { sdccTarget, loadHeader } from "../../src/worker/tools/sdcc";
+import { getToolForFilename_6502, getToolForFilename_z80, getToolForFilename_apple2 } from "../../src/common/toolselect";
 
 describe("sdcc targets", function () {
 
@@ -25,4 +25,20 @@ describe("sdcc targets", function () {
     assert.strictEqual(getToolForFilename_6502('hello.c'), 'cc65');
   });
 
+  it("selects SDCC for .sdcc files", function () {
+    assert.strictEqual(getToolForFilename_6502('hello.sdcc'), 'sdcc');
+    assert.strictEqual(getToolForFilename_apple2('hello.sdcc'), 'sdcc');
+    assert.strictEqual(getToolForFilename_z80('skeleton.sdcc'), 'sdcc');
+  });
+
+
+  it("wraps linked images in a load header", function () {
+    const img = Uint8Array.of(1, 2, 3, 4, 5);
+    assert.deepStrictEqual(Array.from(loadHeader('dos33', img, 0x803, 3)), [3, 8, 3, 0, 1, 2, 3]);
+    // load address, link pointer, line 10, SYS 2061, then the code
+    assert.deepStrictEqual(Array.from(loadHeader('prg', img, 0x80d, 2)),
+      [1, 8, 0x0b, 8, 10, 0, 0x9e, 0x32, 0x30, 0x36, 0x31, 0, 0, 0, 1, 2]);
+    assert.throws(() => loadHeader('prg', img, 0x900, 2), /\$80d/);
+    assert.throws(() => loadHeader('bogus', img, 0, 0), /unknown load_header/);
+  });
 });
diff --git a/test/unit/testtoolmeta.ts b/test/unit/testtoolmeta.ts
index 92474f0e..95066bcb 100644
--- a/test/unit/testtoolmeta.ts
+++ b/test/unit/testtoolmeta.ts
@@ -4,7 +4,7 @@ import {
   TOOL_META, getToolMeta, getToolMetaForFilename, getPreloadFSName, getSkeletonName,
   getIncludePatterns, getLinkPatterns, matchDependencyPatterns,
   getSystemIncludePatterns, SYSTEM_INCLUDE_PATTERNS, getSharedFileSystemName, getToolHelpURL,
-  getPlatformToolHelpURL
+  getPlatformToolHelpURL, SDCC_DEFAULT_VERSION
 } from "../../src/common/toolmeta";
 // Node-friendly worker entry point (re-exports TOOLS without Worker wiring)
 import { TOOLS } from "../../src/worker/workerlib";
@@ -51,12 +51,16 @@ describe('Tool metadata registry', function () {
     assert.strictEqual(getSharedFileSystemName('cc65', 'nes'), 'wasi:cc65-fs-nes.zip');
     assert.strictEqual(getSharedFileSystemName('cc65', 'c64'), 'wasi:cc65-fs-c64.zip');
     assert.strictEqual(getSharedFileSystemName('cc65', 'nonexistent'), undefined);
-    // SDCC 4.x (WASI zip) by default; 3.x-library platforms keep the 3.6.5 preloadFS
-    assert.strictEqual(getPreloadFSName('sdcc'), undefined);
-    assert.strictEqual(getSharedFileSystemName('sdcc', 'mw8080bw'), 'wasi:sdcc-fs.zip');
+    // the 'default' config leads with whichever SDCC the project defaults to
+    // (getSharedFileSystemName prefers preloadFS); platforms whose libraries
+    // were built by 3.x always keep the 3.6.5 preloadFS.
+    const defaultPreload = SDCC_DEFAULT_VERSION === 3 ? 'sdcc' : undefined;
+    const defaultShared = SDCC_DEFAULT_VERSION === 3 ? 'sdcc' : 'wasi:sdcc-fs.zip';
+    assert.strictEqual(getPreloadFSName('sdcc'), defaultPreload);
+    assert.strictEqual(getSharedFileSystemName('sdcc', 'mw8080bw'), defaultShared);
     assert.strictEqual(getPreloadFSName('sdcc', 'gb'), 'sdcc');
     assert.strictEqual(getPreloadFSName('sdcc', 'msx-libcv'), 'sdcc');
-    assert.strictEqual(getSharedFileSystemName('sdcc', 'msx'), 'wasi:sdcc-fs.zip');
+    assert.strictEqual(getSharedFileSystemName('sdcc', 'msx'), defaultShared);
     assert.strictEqual(getPreloadFSName('dasm'), undefined);
   });