fix(sdk): add eater example target

Author: kassane

build.zig

@@ -30,6 +30,7 @@ const SdkLibs = struct {
     atari5200: ?sdk_mod.Libs = null,
     a8megacart: ?sdk_mod.Libs = null,
     a8xegs: ?sdk_mod.Libs = null,
+    eater: ?sdk_mod.Libs = null,
 };
 
 pub fn build(b: *std.Build) void {
@@ -78,6 +79,7 @@ pub fn build(b: *std.Build) void {
         .{ .name = "atari5200-supercart", .query = .{ .cpu_arch = .mos, .os_tag = .atari5200 } },
         .{ .name = "atari8-cart-megacart", .query = .{ .cpu_arch = .mos, .os_tag = .atari8 } },
         .{ .name = "atari8-cart-xegs", .query = .{ .cpu_arch = .mos, .os_tag = .atari8 } },
+        .{ .name = "eater", .query = .{ .cpu_arch = .mos, .os_tag = .eater } },
     }) |pd| {
         const libs = sdk_mod.buildPlatform(b, sdk_src_raw, pd, optimize);
         const dest = b.fmt("mos-platform/{s}/lib", .{pd.name});
@@ -114,6 +116,7 @@ pub fn build(b: *std.Build) void {
         if (std.mem.eql(u8, pd.name, "atari5200-supercart")) sdk_libs.atari5200 = libs;
         if (std.mem.eql(u8, pd.name, "atari8-cart-megacart")) sdk_libs.a8megacart = libs;
         if (std.mem.eql(u8, pd.name, "atari8-cart-xegs")) sdk_libs.a8xegs = libs;
+        if (std.mem.eql(u8, pd.name, "eater")) sdk_libs.eater = libs;
     }
 
     // Translate neslib.h and nesdoug.h from the MOS SDK into Zig modules.
@@ -620,6 +623,16 @@ pub fn build(b: *std.Build) void {
         run_bininfo.addFileArg(exe.getEmittedBin());
     }
 
+    // ---- Ben Eater 6502 hello-lcd ----
+    {
+        const step = b.step("eater-hello-lcd", "Build Ben Eater 6502 LCD hello example");
+        const exe = addEaterExe(b, sdk_dep, sdk_src, sdk_libs.eater orelse @panic("eater libs not built"), optimize, "eater-hello-lcd", "eater/hello-lcd/hello-lcd.zig");
+        const install = b.addInstallArtifact(exe, .{ .dest_sub_path = "eater-hello-lcd.rom" });
+        step.dependOn(&install.step);
+        b.getInstallStep().dependOn(&install.step);
+        run_bininfo.addFileArg(exe.getEmittedBin());
+    }
+
     // ---- PC Engine color-cycle ----
     {
         const step = b.step("pce-color-cycle", "Build PC Engine color-cycle example");
@@ -1854,6 +1867,49 @@ fn addLynxBllExe(
     return exe;
 }
 
+fn addEaterExe(
+    b: *std.Build,
+    sdk_dep: *std.Build.Dependency,
+    sdk_src: []const u8,
+    libs: sdk_mod.Libs,
+    opt: std.builtin.OptimizeMode,
+    name: []const u8,
+    root_src: []const u8,
+) *std.Build.Step.Compile {
+    const target = b.resolveTargetQuery(.{ .cpu_arch = .mos, .os_tag = .eater });
+
+    const wf = b.addWriteFiles();
+    const wrapper_ld = wf.add("eater-wrapper.ld", b.fmt(
+        \\SEARCH_DIR("{s}/mos-platform/eater");
+        \\SEARCH_DIR("{s}/mos-platform/common/ldscripts");
+        \\INCLUDE "{s}/mos-platform/eater/link.ld"
+    , .{ sdk_src, sdk_src, sdk_src }));
+
+    const exe = b.addExecutable(.{
+        .name = name,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path(root_src),
+            .target = target,
+            .optimize = opt,
+            .sanitize_c = .off,
+        }),
+    });
+    exe.bundle_compiler_rt = false;
+    exe.lto = .full;
+    exe.root_module.addIncludePath(sdk_dep.path("mos-platform/eater"));
+    exe.root_module.addIncludePath(sdk_dep.path("mos-platform/common/include"));
+    exe.root_module.linkLibrary(libs.crt);
+    exe.root_module.linkLibrary(libs.crt0);
+    exe.root_module.linkLibrary(libs.c);
+    if (libs.crt0_obj) |obj| exe.root_module.addObject(obj);
+    exe.forceUndefinedSymbol("__zig_call_main_section");
+    exe.forceUndefinedSymbol("main");
+    exe.setLinkerScript(wrapper_ld);
+    addMosPanicImport(b, exe, target, opt);
+
+    return exe;
+}
+
 fn addPceExe(
     b: *std.Build,
     sdk_dep: *std.Build.Dependency,

eater/hello-lcd/hello-lcd.zig

@@ -0,0 +1,40 @@
+// Copyright (c) 2024 Matheus C. França
+// SPDX-License-Identifier: Apache-2.0
+//! Scrolls "Hello, World!" across a 16x2 HD44780 LCD on Ben Eater's 6502 computer.
+//! LCD driver (lcd_init / lcd_instruction / lcd_puts) lives in mos-platform/eater/lcd.c;
+//! delay() lives in mos-platform/eater/delay.c — both compiled into libc by the SDK.
+pub const panic = @import("mos_panic");
+
+extern fn lcd_init() void;
+extern fn lcd_instruction(insn: u8) void;
+extern fn lcd_puts(str: [*:0]const u8) void;
+extern fn delay(ms: c_uint) void;
+
+const LCD_I_DDRAM: u8 = 0x80; // Set DDRAM address
+const LCD_I_HOME: u8 = 0x02; // Move cursor to home position
+const LCD_I_SHIFT_L: u8 = 0x18; // Shift display left
+
+const message: [*:0]const u8 = "Hello, World!";
+const message_len: c_int = 13;
+
+pub export fn main() callconv(.c) void {
+    lcd_init();
+
+    // Write message just off-screen (DDRAM address 16 = second half of row 1)
+    lcd_instruction(LCD_I_DDRAM | 16);
+    lcd_puts(message);
+
+    // Scroll the message across the visible 16-character window
+    const count: c_int = 16 + message_len;
+    var x: c_int = count;
+    while (true) {
+        delay(350);
+        if (x <= 0) {
+            lcd_instruction(LCD_I_HOME);
+            x = count;
+        } else {
+            lcd_instruction(LCD_I_SHIFT_L);
+            x -= 1;
+        }
+    }
+}

sdk/build.zig

@@ -1090,13 +1090,27 @@ fn buildEater(
     });
     libcrt0.root_module.addCSourceFiles(.{
         .root = .{ .cwd_relative = crt0_dir },
-        .files = &.{ "crt0.S", "init-stack.S", "copy-data.c", "zero-bss.c" },
+        .files = &.{ "init-stack.S", "copy-data.c", "zero-bss.c" },
     });
     libcrt0.root_module.addCSourceFiles(.{
         .root = .{ .cwd_relative = b.fmt("{s}/exit", .{crt0_dir}) },
         .files = &.{"exit-loop.c"},
     });
 
+    // crt0.S provides .call_main (jsr main). Must be a TRUE object — section-only
+    // contribution with no exported symbol is silently skipped by ld.lld from archives.
+    const crt0_obj = b.addObject(.{
+        .name = "crt0",
+        .root_module = b.createModule(.{ .target = target, .optimize = opt }),
+    });
+    crt0_obj.root_module.addIncludePath(.{ .cwd_relative = com_asm });
+    crt0_obj.root_module.addIncludePath(.{ .cwd_relative = com_inc });
+    crt0_obj.root_module.addCSourceFiles(.{
+        .root = .{ .cwd_relative = crt0_dir },
+        .files = &.{"crt0.S"},
+    });
+    crt0_obj.lto = .none;
+
     const libc = addLib(b, "c", target, opt);
     libc.root_module.addIncludePath(.{ .cwd_relative = plat_dir });
     libc.root_module.addIncludePath(.{ .cwd_relative = com_inc });
@@ -1105,7 +1119,7 @@ fn buildEater(
         .files = &.{ "delay.c", "getchar.c", "lcd.c", "putchar.c" },
     });
 
-    return .{ .crt = libcrt, .crt0 = libcrt0, .c = libc };
+    return .{ .crt = libcrt, .crt0 = libcrt0, .c = libc, .crt0_obj = crt0_obj };
 }
 
 fn buildGeosCbm(