diff --git a/abi/abi.zig b/abi/abi.zig
new file mode 100644
index 0000000..3b9e8f9
--- /dev/null
+++ b/abi/abi.zig
@@ -0,0 +1,3 @@
+pub const SyscallNumber = enum(usize) { SYS_debug_write = 1, SYS_exit = 2, _ };
+
+pub const MAX_SYSCALL: usize = 32;
diff --git a/build.zig b/build.zig
index ee10aff..2fd002e 100644
--- a/build.zig
+++ b/build.zig
@@ -201,6 +201,13 @@ pub fn build(b: *std.Build) anyerror!void {
     });
     kernel_module.addAnonymousImport("userspace", .{ .root_source_file = b.path("zig-out/bin/userspace.bin") });
 
+    const abi_module = b.addModule("abi", .{
+        .optimize = optimize,
+        .red_zone = false,
+        .target = kernel_target,
+        .code_model = kernel_code_model,
+        .root_source_file = b.path("abi/abi.zig"),
+    });
     const user_module = b.addModule("userspace", .{
         .optimize = optimize,
         .target = user_target,
@@ -216,6 +223,9 @@ pub fn build(b: *std.Build) anyerror!void {
     });
     kernel.pie = true;
 
+    user_module.addImport("abi", abi_module);
+    kernel_module.addImport("abi", abi_module);
+
     const user = b.addExecutable(.{
         .name = "userspace",
         .root_module = user_module,
diff --git a/src/syscall.zig b/src/syscall.zig
index 4a74a42..f58cc31 100644
--- a/src/syscall.zig
+++ b/src/syscall.zig
@@ -1,27 +1,43 @@
 const kernel = @import("kernel.zig");
+const abi = @import("abi");
 
 const thread = kernel.thread;
+const Thread = thread.Thread;
 const log = kernel.log;
 
-pub fn syscall_handler(func: usize, args: []usize) usize {
-    // TODO validation, etc
-    const cthread = thread.Thread.current();
-    switch (func) {
-        // debug_print(text_data: [*]const u8, text_len: usize)
-        1 => {
-            if (args[0] != 0 and args[1] != 0) {
-                const text = @as([*]u8, @ptrFromInt(args[0]))[0..args[1]];
-                log.debug("{*}: {s}", .{ cthread, text });
-            }
-            return 0;
-        },
-        // exit(code: u32)
-        2 => {
-            log.info("{*} exits with code {}", .{ cthread, args[0] });
-            thread.Thread.exit_current();
-        },
-        else => {
-            @panic("Undefined syscall");
-        },
-    }
+pub const SyscallFn = *const fn (*Thread, []usize) usize;
+
+pub const syscall_table: [abi.MAX_SYSCALL]?SyscallFn = make_syscall_table();
+
+fn make_syscall_table() [abi.MAX_SYSCALL]?SyscallFn {
+    const SC = abi.SyscallNumber;
+    var array = [_]?SyscallFn{undefined} ** abi.MAX_SYSCALL;
+    array[@intFromEnum(SC.SYS_debug_write)] = sys_debug_write;
+    array[@intFromEnum(SC.SYS_exit)] = sys_exit;
+    return array;
+}
+
+fn sys_exit(cthread: *Thread, args: []usize) usize {
+    log.info("{*} exits with code {}", .{ cthread, args[0] });
+    Thread.exit_current();
+}
+
+fn sys_debug_write(cthread: *Thread, args: []usize) usize {
+    const message = @as([*]const u8, @ptrFromInt(args[0]))[0..args[1]];
+    log.debug("{*}: {s}", .{ cthread, message });
+    return 0;
+}
+
+fn sys_undefined_syscall(cthread: *Thread, args: []usize) usize {
+    _ = args;
+    log.warn("{*} invoked an undefined syscall", .{cthread});
+    Thread.exit_current();
+}
+
+pub fn syscall_handler(func: usize, args: []usize) usize {
+    const cthread = thread.Thread.current();
+    const handler = if (func >= abi.MAX_SYSCALL) sys_undefined_syscall //
+        else syscall_table[func] //
+        orelse sys_undefined_syscall;
+    return handler(cthread, args);
 }
diff --git a/user/arch/aarch64.zig b/user/arch/aarch64.zig
index 69b0c24..c9a30c1 100644
--- a/user/arch/aarch64.zig
+++ b/user/arch/aarch64.zig
@@ -1,19 +1,21 @@
+const abi = @import("abi");
+
 pub const syscall = struct {
-    pub fn syscall1(func: usize, arg0: usize) usize {
+    pub fn syscall1(func: abi.SyscallNumber, arg0: usize) usize {
         return asm volatile ("svc #0"
             : [result] "={x0}" (-> usize),
             : [arg0] "{x0}" (arg0),
-              [func] "{x8}" (func),
+              [func] "{x8}" (@intFromEnum(func)),
             : "memory"
         );
     }
 
-    pub fn syscall2(func: usize, arg0: usize, arg1: usize) usize {
+    pub fn syscall2(func: abi.SyscallNumber, arg0: usize, arg1: usize) usize {
         return asm volatile ("svc #0"
             : [result] "={x0}" (-> usize),
             : [arg0] "{x0}" (arg0),
               [arg1] "{x1}" (arg1),
-              [func] "{x8}" (func),
+              [func] "{x8}" (@intFromEnum(func)),
             : "memory"
         );
     }
diff --git a/user/arch/riscv64.zig b/user/arch/riscv64.zig
index dcfc0a2..c9c9dfb 100644
--- a/user/arch/riscv64.zig
+++ b/user/arch/riscv64.zig
@@ -1,17 +1,19 @@
+const abi = @import("abi");
+
 pub const syscall = struct {
-    pub fn syscall1(func: usize, arg0: usize) usize {
+    pub fn syscall1(func: abi.SyscallNumber, arg0: usize) usize {
         return asm volatile ("ecall"
             : [result] "={a0}" (-> usize),
-            : [func] "{a0}" (func),
+            : [func] "{a0}" (@intFromEnum(func)),
               [arg0] "{a1}" (arg0),
             : "memory"
         );
     }
 
-    pub fn syscall2(func: usize, arg0: usize, arg1: usize) usize {
+    pub fn syscall2(func: abi.SyscallNumber, arg0: usize, arg1: usize) usize {
         return asm volatile ("ecall"
             : [result] "={a0}" (-> usize),
-            : [func] "{a0}" (func),
+            : [func] "{a0}" (@intFromEnum(func)),
               [arg0] "{a1}" (arg0),
               [arg1] "{a2}" (arg1),
             : "memory"
diff --git a/user/log.zig b/user/log.zig
index 69209fe..b72de6b 100644
--- a/user/log.zig
+++ b/user/log.zig
@@ -23,6 +23,6 @@ pub fn println(comptime format: []const u8, args: anytype) void {
     var w = W{ .context = &context };
     w.print(format, args) catch return;
     if (context.position != 0) {
-        syscall.debug_print(context.buffer[0..context.position]);
+        syscall.debug_write(context.buffer[0..context.position]);
     }
 }
diff --git a/user/syscall.zig b/user/syscall.zig
index 7ac13d3..87d16de 100644
--- a/user/syscall.zig
+++ b/user/syscall.zig
@@ -1,12 +1,14 @@
 const arch = @import("arch.zig");
+const abi = @import("abi");
 
 const sc = arch.syscall;
+const SC = abi.SyscallNumber;
 
 pub fn exit(code: u32) noreturn {
-    _ = sc.syscall1(2, code);
+    _ = sc.syscall1(SC.SYS_exit, code);
     unreachable;
 }
 
-pub fn debug_print(text: []const u8) void {
-    _ = sc.syscall2(1, @intFromPtr(text.ptr), text.len);
+pub fn debug_write(text: []const u8) void {
+    _ = sc.syscall2(SC.SYS_debug_write, @intFromPtr(text.ptr), text.len);
 }