M6: user runtime library rt + C-convention heap

Add lib/ — the shared user-space runtime every user binary links against
(init now, servers/drivers later): syscall wrappers, the heap, IPC stub,
and the process start shim.

- lib/heap.zig: the kernel first-fit free-list ported to user space, grown
  via the mmap syscall instead of pmm+mapPage. Dual API over one global free
  list: extern "C" malloc/free/calloc/realloc (C ABI for future C code) and a
  std.mem.Allocator adapter (with in-place resize) for Zig std containers.
- lib/syscall.zig + sys.zig: raw syscall0..5 (arg3 in r10) and typed
  yield/write/sleep/exit/mmap/munmap over danos.Syscall.
- lib/start.zig: naked _start -> rt_start -> root.main() (SysV realign via
  call), panic -> exit(127).
- lib/user.ld: the user link script, moved from sbin/linker.ld (shared by all
  user binaries).
- build.zig: register the `rt` module; add an addUserBinary() helper that is
  the one recipe for every user binary (freestanding, .large, use_lld,
  user.ld, image_base), replacing the bespoke init block.
- sbin/init.zig: migrated onto rt; drops its hand-rolled syscall2/shims. Now
  proves the heap (alloc -> write from a heap pointer -> free) before the
  heartbeat loop. Serial shows "init: heap ok". Suite 28/28.

build.zig

@@ -48,6 +48,43 @@ fn timestamp(b: *std.Build) []const u8 {
     });
 }
 
+/// Build one user-space binary the same way for every program (init, and later
+/// the VFS server + drivers): freestanding, ReleaseSmall, `.large` code model
+/// (the image base is above 4 GiB — smaller models emit 32-bit relocations that
+/// can't reach), linked against the `rt` runtime library with the shared user
+/// link script. Pinned to LLVM + LLD so the script's PHDRS (segment permissions)
+/// are authoritative — the kernel's W^X user-ELF loader requires exact perms.
+fn addUserBinary(
+    b: *std.Build,
+    target: std.Build.ResolvedTarget,
+    rt_mod: *std.Build.Module,
+    name: []const u8,
+    root: []const u8,
+) *std.Build.Step.Compile {
+    const exe = b.addExecutable(.{
+        .name = name,
+        .root_module = b.createModule(.{
+            .root_source_file = b.path(root),
+            .target = target,
+            .optimize = .ReleaseSmall,
+            .code_model = .large,
+            .single_threaded = true,
+            .sanitize_c = .off,
+            .stack_check = false,
+            .stack_protector = false,
+            .imports = &.{
+                .{ .name = "rt", .module = rt_mod },
+            },
+        }),
+    });
+    exe.setLinkerScript(b.path("lib/user.ld"));
+    exe.entry = .{ .symbol_name = "_start" };
+    exe.image_base = 0x7000_0000_0000;
+    exe.use_llvm = true;
+    exe.use_lld = true;
+    return exe;
+}
+
 pub fn build(b: *std.Build) void {
     ensureZigVersion();
 
@@ -98,6 +135,18 @@ pub fn build(b: *std.Build) void {
         },
     });
 
+    // The user-space runtime library (a nascent libc): syscall wrappers, the
+    // C-convention heap, IPC helpers, the process start shim. Compiled into every
+    // user binary (see addUserBinary), so it inherits each exe's `.large` code
+    // model — do NOT set a target/code_model here. It imports `danos` for the
+    // shared Syscall numbers.
+    const rt_mod = b.addModule("rt", .{
+        .root_source_file = b.path("lib/rt.zig"),
+        .imports = &.{
+            .{ .name = "danos", .module = mod },
+        },
+    });
+
     // Compile-time config the kernel reads as `@import("build_options")`. The
     // QEMU test harness sets -Dtest-case=<name> to run one self-test at boot.
     const test_case = b.option([]const u8, "test-case", "Kernel self-test case to run at boot (see src/kernel/tests.zig)");
@@ -151,33 +200,10 @@ pub fn build(b: *std.Build) void {
     b.installArtifact(exe);
 
     // --- /sbin/init: the first user-space program ---
-    // Its own tiny freestanding binary, linked at a fixed address inside the
-    // kernel's user region (process.zig) and started in ring 3 by the kernel's
-    // user-ELF loader. `.large` because the image base is above 4 GiB — small/
-    // medium code models emit 32-bit absolute relocations that can't reach.
-    // Pinned to ReleaseSmall: the user region gives it a 2 MiB budget and its
-    // size has no reason to track the kernel's optimize mode.
-    const init_exe = b.addExecutable(.{
-        .name = "init",
-        .root_module = b.createModule(.{
-            .root_source_file = b.path("sbin/init.zig"),
-            .target = kernel_target,
-            .optimize = .ReleaseSmall,
-            .code_model = .large,
-            .single_threaded = true,
-            .sanitize_c = .off,
-            .stack_check = false,
-            .stack_protector = false,
-        }),
-    });
-    init_exe.setLinkerScript(b.path("sbin/linker.ld"));
-    init_exe.entry = .{ .symbol_name = "_start" };
-    init_exe.image_base = 0x7000_0000_0000;
-    // The self-hosted linker ignores the script's PHDRS (segment permissions),
-    // which the kernel's W^X user-ELF loader requires (code must be R+X). Pin to
-    // LLVM + LLD so the script is authoritative.
-    init_exe.use_llvm = true;
-    init_exe.use_lld = true;
+    // Built by the shared user-binary recipe (see addUserBinary): freestanding,
+    // linked into the kernel's user region against the `rt` runtime library, and
+    // started in ring 3 by the kernel's user-ELF loader.
+    const init_exe = addUserBinary(b, kernel_target, rt_mod, "init", "sbin/init.zig");
     b.installArtifact(init_exe);
 
     // Boot methods live in src/boot/, one per way of getting the kernel running.

lib/heap.zig

@@ -0,0 +1,193 @@
+//! The user-space heap: C-convention dynamic allocation (`malloc`/`free`/…) plus
+//! a `std.mem.Allocator` adapter over the same free list, so both C-style code
+//! and Zig `std` containers share one heap.
+//!
+//! The algorithm is a straight port of the kernel's first-fit free list
+//! (src/kernel/heap.zig): an address-ordered singly linked list of free blocks,
+//! split on allocation and coalesced with neighbours on free. The only thing
+//! that changes on this side of the syscall boundary is where memory comes from
+//! — `grow` asks the kernel for pages via `mmap` instead of mapping frames
+//! itself, and the kernel picks the base address.
+//!
+//! Single-threaded and 16-byte max alignment, exactly like the kernel heap; a
+//! lock and larger alignments come when user programs gain threads.
+
+const std = @import("std");
+const danos = @import("danos");
+const sys = @import("sys.zig");
+
+const page_size = danos.page_size;
+
+/// A block header, at the start of every block; while free it also links the
+/// free list via `next`.
+const Block = extern struct {
+    size: usize, // total block size in bytes, including this header; a multiple of 16
+    next: ?*Block, // free-list link (only meaningful while free)
+};
+
+const header_size = @sizeOf(Block); // 16
+const min_block = header_size + 16; // smallest block worth splitting off
+/// Grow granularity: one `mmap` per 64 KiB amortises the syscall.
+const chunk = 64 * 1024;
+
+var free_list: ?*Block = null;
+
+fn alignUp(value: usize, alignment: usize) usize {
+    return (value + alignment - 1) & ~(alignment - 1);
+}
+
+fn payloadOf(block: *Block) [*]u8 {
+    return @ptrFromInt(@intFromPtr(block) + header_size);
+}
+
+/// Ask the kernel for more pages and add them as a free block. Because each
+/// `mmap` is an independent grant, cross-grant coalescing happens only when the
+/// kernel returns adjacent bases (its arena is a bump allocator, so consecutive
+/// grants usually are adjacent). Returns false if the kernel is out of memory.
+fn grow(min_bytes: usize) bool {
+    const bytes = alignUp(@max(min_bytes, chunk), page_size);
+    const ret = sys.mmap(bytes, sys.PROT_READ | sys.PROT_WRITE);
+    if (sys.mmapFailed(ret)) return false;
+
+    const block: *Block = @ptrFromInt(ret);
+    block.size = bytes;
+    insertFree(block); // coalesces if this grant is adjacent to a prior one
+    return true;
+}
+
+/// Insert a block into the address-ordered free list, coalescing with the
+/// physically adjacent free blocks on either side.
+fn insertFree(block: *Block) void {
+    var prev: ?*Block = null;
+    var cur = free_list;
+    while (cur) |c| : (cur = c.next) {
+        if (@intFromPtr(c) > @intFromPtr(block)) break;
+        prev = c;
+    }
+
+    block.next = cur;
+    if (prev) |p| p.next = block else free_list = block;
+
+    // Merge forward into `cur` if they're contiguous.
+    if (cur) |c| {
+        if (@intFromPtr(block) + block.size == @intFromPtr(c)) {
+            block.size += c.size;
+            block.next = c.next;
+        }
+    }
+    // Merge `prev` forward into `block` if they're contiguous.
+    if (prev) |p| {
+        if (@intFromPtr(p) + p.size == @intFromPtr(block)) {
+            p.size += block.size;
+            p.next = block.next;
+        }
+    }
+}
+
+/// Allocate `len` bytes (16-byte aligned), or null if out of memory.
+fn rawAlloc(len: usize) ?[*]u8 {
+    const need = alignUp(header_size + len, 16);
+
+    var attempts: u32 = 0;
+    while (attempts < 2) : (attempts += 1) {
+        var prev: ?*Block = null;
+        var cur = free_list;
+        while (cur) |block| : ({
+            prev = block;
+            cur = block.next;
+        }) {
+            if (block.size < need) continue;
+
+            if (block.size >= need + min_block) {
+                // Split: carve `need` off the front, leave the rest free.
+                const rest: *Block = @ptrFromInt(@intFromPtr(block) + need);
+                rest.size = block.size - need;
+                rest.next = block.next;
+                if (prev) |p| p.next = rest else free_list = rest;
+                block.size = need;
+            } else {
+                // Take the whole block.
+                if (prev) |p| p.next = block.next else free_list = block.next;
+            }
+            return payloadOf(block);
+        }
+
+        // Nothing fit: grow and try once more.
+        if (!grow(need)) return null;
+    }
+    return null;
+}
+
+fn rawFree(ptr: [*]u8) void {
+    const block: *Block = @ptrFromInt(@intFromPtr(ptr) - header_size);
+    insertFree(block);
+}
+
+// --- C ABI: the global implicit heap ---------------------------------------
+// `extern "C"` symbols so future C code links the same malloc/free directly.
+
+export fn malloc(size: usize) callconv(.c) ?*anyopaque {
+    if (size == 0) return null;
+    const p = rawAlloc(size) orelse return null;
+    return @ptrCast(p);
+}
+
+export fn free(ptr: ?*anyopaque) callconv(.c) void {
+    const p = ptr orelse return;
+    rawFree(@ptrCast(p));
+}
+
+export fn calloc(nmemb: usize, size: usize) callconv(.c) ?*anyopaque {
+    const total = std.math.mul(usize, nmemb, size) catch return null; // overflow-safe
+    if (total == 0) return null;
+    const p = rawAlloc(total) orelse return null;
+    @memset(p[0..total], 0);
+    return @ptrCast(p);
+}
+
+export fn realloc(ptr: ?*anyopaque, size: usize) callconv(.c) ?*anyopaque {
+    const p = ptr orelse return malloc(size);
+    if (size == 0) {
+        rawFree(@ptrCast(p));
+        return null;
+    }
+    const block: *Block = @ptrFromInt(@intFromPtr(p) - header_size);
+    const old_payload = block.size - header_size;
+    if (size <= old_payload) return p; // shrink/same: keep the block
+    const np = rawAlloc(size) orelse return null; // grow: alloc + copy + free
+    @memcpy(np[0..old_payload], @as([*]u8, @ptrCast(p))[0..old_payload]);
+    rawFree(@ptrCast(p));
+    return @ptrCast(np);
+}
+
+// --- std.mem.Allocator interface (same free list) --------------------------
+
+pub fn allocator() std.mem.Allocator {
+    return .{ .ptr = undefined, .vtable = &vtable };
+}
+
+const vtable = std.mem.Allocator.VTable{
+    .alloc = allocImpl,
+    .resize = resizeImpl,
+    .remap = remapImpl,
+    .free = freeImpl,
+};
+
+fn allocImpl(_: *anyopaque, len: usize, alignment: std.mem.Alignment, _: usize) ?[*]u8 {
+    if (alignment.toByteUnits() > 16) return null; // blocks are 16-byte aligned
+    return rawAlloc(len);
+}
+
+fn resizeImpl(_: *anyopaque, memory: []u8, _: std.mem.Alignment, new_len: usize, _: usize) bool {
+    // In-place iff the new payload still fits the current block.
+    const block: *Block = @ptrFromInt(@intFromPtr(memory.ptr) - header_size);
+    return new_len + header_size <= block.size;
+}
+
+fn remapImpl(_: *anyopaque, _: []u8, _: std.mem.Alignment, _: usize, _: usize) ?[*]u8 {
+    return null;
+}
+
+fn freeImpl(_: *anyopaque, memory: []u8, _: std.mem.Alignment, _: usize) void {
+    rawFree(memory.ptr);
+}

lib/ipc.zig

@@ -0,0 +1,15 @@
+//! User-space IPC helpers. The kernel's synchronous IPC syscalls
+//! (create_endpoint, ipc_register/lookup, ipc_call, ipc_reply_wait) arrive in a
+//! later milestone; this reserves the module boundary now so drivers and servers
+//! can be written against `rt.ipc` without restructuring once they light up.
+
+/// A fixed-size, register-friendly message payload. The wire format for the VFS
+/// and driver protocols is layered on top of this by the servers themselves.
+pub const Message = extern struct {
+    tag: u64 = 0,
+    a: u64 = 0,
+    b: u64 = 0,
+    c: u64 = 0,
+};
+
+// call() / replyWait() / createEndpoint() land with the kernel IPC syscalls.

lib/rt.zig

@@ -0,0 +1,22 @@
+//! danos user-space runtime library — a nascent libc. Every user binary (init,
+//! and later the VFS server + device drivers) imports this as `@import("rt")`:
+//! syscall wrappers, the C-convention heap, IPC helpers, and the process start
+//! shim. It is compiled into each binary (inheriting its `.large` code model and
+//! freestanding target), so all user programs share one implementation.
+//!
+//! A user binary needs three lines:
+//!   const rt = @import("rt");
+//!   pub const panic = rt.panic;
+//!   comptime { _ = &rt.start._start; }   // pull the entry shim in
+//! and a `pub fn main() void`.
+
+pub const sys = @import("sys.zig");
+pub const heap = @import("heap.zig");
+pub const ipc = @import("ipc.zig");
+pub const start = @import("start.zig");
+
+/// Re-exported so a user binary can `pub const panic = rt.panic;`.
+pub const panic = start.panic;
+
+/// The heap as a `std.mem.Allocator`, for Zig `std` containers in user code.
+pub const allocator = heap.allocator;

lib/start.zig

@@ -0,0 +1,32 @@
+//! The user-space process entry shim. Every user binary roots `_start` here (via
+//! `entry = _start` in build.zig) and forces this file to be analysed with
+//! `comptime { _ = &rt.start._start; }`, so the whole runtime is linked in.
+
+const std = @import("std");
+const sys = @import("sys.zig");
+
+/// The kernel enters at `_start` with rsp 16-aligned, but a SysV function expects
+/// rsp ≡ 8 (mod 16) on entry (as if reached by `call`). The `call` below pushes
+/// the 8-byte return address, satisfying the ABI before any Zig frame runs; the
+/// `ud2` is a safety net if `rt_start` ever returns.
+pub export fn _start() callconv(.naked) noreturn {
+    asm volatile (
+        \\call rt_start
+        \\ud2
+    );
+}
+
+/// The first Zig frame. The heap is lazy (first alloc grows it), so there is no
+/// runtime init to order here — just hand control to the program's `main`.
+export fn rt_start() callconv(.c) noreturn {
+    const root = @import("root"); // the user binary's root source file
+    root.main();
+    sys.exit(0);
+}
+
+/// No runtime to unwind into — report a panic as a nonzero exit code.
+pub const panic = std.debug.FullPanic(struct {
+    fn panic(_: []const u8, _: ?usize) noreturn {
+        sys.exit(127);
+    }
+}.panic);

lib/sys.zig

@@ -0,0 +1,52 @@
+//! Typed syscall surface for user space — thin wrappers over the raw `syscall`
+//! stubs, one per kernel call. Numbers come from `danos.Syscall`, the single
+//! source of truth shared with the kernel dispatcher.
+
+const danos = @import("danos");
+const sc = @import("syscall.zig");
+
+/// `mmap` protection flags (matching the usual C bit values). Grants are always
+/// readable+writable today; the kernel does not yet honour finer prot.
+pub const PROT_READ: usize = danos.prot_read;
+pub const PROT_WRITE: usize = danos.prot_write;
+pub const PROT_EXEC: usize = danos.prot_exec;
+
+/// Give up the rest of this quantum.
+pub fn yield() void {
+    _ = sc.syscall0(.yield);
+}
+
+/// Write raw bytes to the kernel log (a bring-up diagnostic; real output goes
+/// through the console/VFS later). Returns the byte count, or a wrapped -1.
+pub fn write(msg: []const u8) usize {
+    return sc.syscall2(.debug_write, @intFromPtr(msg.ptr), msg.len);
+}
+
+/// Block the caller for `ms` milliseconds.
+pub fn sleep(ms: usize) void {
+    _ = sc.syscall1(.sleep, ms);
+}
+
+/// End the process. Never returns.
+pub fn exit(code: usize) noreturn {
+    _ = sc.syscall1(.exit, code);
+    unreachable; // the kernel never returns from exit
+}
+
+/// Grant `len` bytes (rounded up to whole pages) of fresh, zeroed, writable
+/// memory and return the base virtual address. On failure returns a value in the
+/// top page (see `mmapFailed`). The user heap grows through this call.
+pub fn mmap(len: usize, prot: usize) usize {
+    return sc.syscall2(.mmap, len, prot);
+}
+
+/// Release a range previously handed out by `mmap`.
+pub fn munmap(base: usize, len: usize) usize {
+    return sc.syscall2(.munmap, base, len);
+}
+
+/// Whether an `mmap` return value is an error (the kernel returns a wrapped
+/// -errno, which lands in the top page — no real grant base is ever that high).
+pub inline fn mmapFailed(ret: usize) bool {
+    return ret > ~@as(usize, 0) - 4095;
+}

lib/syscall.zig

@@ -0,0 +1,52 @@
+//! Raw `syscall` instruction wrappers for user space — one per arity.
+//!
+//! ABI: number in rax, arguments in rdi, rsi, rdx, r10, r8, r9, result in rax.
+//! The `syscall` instruction itself clobbers rcx (it holds the return rip) and
+//! r11 (the saved rflags); the kernel entry stub preserves everything else.
+//! Note argument #3 goes in **r10, not rcx** — rcx is unavailable across the
+//! instruction, so the kernel reads the 4th argument from r10.
+
+const danos = @import("danos");
+const Syscall = danos.Syscall;
+
+pub inline fn syscall0(n: Syscall) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn syscall1(n: Syscall, a0: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn syscall2(n: Syscall, a0: usize, a1: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn syscall3(n: Syscall, a0: usize, a1: usize, a2: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1), [a2] "{rdx}" (a2),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn syscall4(n: Syscall, a0: usize, a1: usize, a2: usize, a3: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1), [a2] "{rdx}" (a2), [a3] "{r10}" (a3),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}
+
+pub inline fn syscall5(n: Syscall, a0: usize, a1: usize, a2: usize, a3: usize, a4: usize) usize {
+    return asm volatile ("syscall"
+        : [ret] "={rax}" (-> usize),
+        : [n] "{rax}" (@intFromEnum(n)), [a0] "{rdi}" (a0), [a1] "{rsi}" (a1), [a2] "{rdx}" (a2), [a3] "{r10}" (a3), [a4] "{r8}" (a4),
+        : .{ .rcx = true, .r11 = true, .memory = true });
+}

lib/user.ld

@@ -1,4 +1,4 @@
-/* /sbin/init link layout.
+/* Shared link layout for every user binary (init, servers, drivers).
  *
  * Linked at a fixed user-space virtual base (set by `image_base` in build.zig,
  * inside the kernel's user region). Same discipline as the kernel's script:

sbin/init.zig

@@ -1,65 +1,39 @@
 //! /sbin/init — the first user-space program, PID 1. Built as its own
 //! freestanding binary (see build.zig), shipped on the boot volume at sbin/init,
 //! loaded by the bootloader, and started in ring 3 as a scheduled process by the
-//! kernel (src/kernel/process.zig). It talks to the kernel only through the
-//! `syscall` instruction.
+//! kernel (src/kernel/process.zig). It links against the shared user runtime
+//! library `rt` and talks to the kernel only through `rt`'s syscall wrappers.
 //!
-//! Today it's a heartbeat: it prints a line and sleeps, forever — enough to show
-//! the system reaches user space and stays alive with a real process scheduled
-//! alongside the kernel's idle loop. It grows into the real init (service
-//! supervision) once there are other user programs to supervise.
+//! Today it proves the C-convention heap works, then settles into a heartbeat:
+//! it prints a line and sleeps, forever — enough to show the system reaches user
+//! space and stays alive with a real process scheduled alongside the kernel's
+//! idle loop. It grows into the real init (service supervision) once there are
+//! other user programs to supervise.
 
-const std = @import("std");
+const rt = @import("rt");
 
-// Syscall numbers (see src/kernel/process.zig):
-const sys_exit = 0;
-const sys_write = 2;
-const sys_sleep = 3;
+pub fn main() void {
+    // Prove the heap end to end: allocate through the runtime allocator (which
+    // mmaps pages from the kernel and carves them with the free list), write into
+    // that heap buffer (exercising the widened debug_write bounds check), and
+    // free it. A fault here would kill init before it heartbeats — so the init
+    // test doubles as the heap regression test. (C code links the same heap via
+    // the extern malloc/free symbols; Zig code uses this allocator.)
+    const gpa = rt.allocator();
+    if (gpa.alloc(u8, 64)) |buf| {
+        const msg = "init: heap ok\n";
+        @memcpy(buf[0..msg.len], msg);
+        _ = rt.sys.write(buf[0..msg.len]);
+        gpa.free(buf);
+    } else |_| {}
 
-fn syscall2(n: u64, a: u64, b: u64) u64 {
-    // The `syscall` instruction clobbers RCX (return RIP) and R11 (saved RFLAGS);
-    // the kernel entry stub preserves everything else.
-    return asm volatile ("syscall"
-        : [ret] "={rax}" (-> u64),
-        : [n] "{rax}" (n),
-          [a] "{rdi}" (a),
-          [b] "{rsi}" (b),
-        : .{ .rcx = true, .r11 = true, .memory = true });
-}
-
-fn write(msg: []const u8) void {
-    _ = syscall2(sys_write, @intFromPtr(msg.ptr), msg.len);
-}
-
-fn sleep(ms: u64) void {
-    _ = syscall2(sys_sleep, ms, 0);
-}
-
-fn exit(code: u64) noreturn {
-    _ = syscall2(sys_exit, code, 0);
-    unreachable; // the kernel never returns from exit
-}
-
-/// Entry. Naked: the kernel enters with rsp 16-aligned, but a SysV function
-/// expects rsp ≡ 8 (mod 16) on entry (as if reached by `call`) — so re-enter
-/// the ABI with an actual call. The trap after is a safety net.
-pub export fn _start() callconv(.naked) noreturn {
-    asm volatile (
-        \\call init_main
-        \\ud2
-    );
-}
-
-export fn init_main() callconv(.c) noreturn {
     while (true) {
-        write("init: heartbeat\n");
-        sleep(1000);
+        _ = rt.sys.write("init: heartbeat\n");
+        rt.sys.sleep(1000);
     }
 }
 
-/// No runtime to unwind into — report the panic as a nonzero exit code.
-pub const panic = std.debug.FullPanic(struct {
-    fn panic(_: []const u8, _: ?usize) noreturn {
-        exit(127);
+pub const panic = rt.panic;
+comptime {
+    _ = &rt.start._start; // pull the runtime entry shim into the image
 }
-}.panic);