diff --git a/sys/test.cpp b/sys/test.cpp
index b31ae99..822e269 100644
--- a/sys/test.cpp
+++ b/sys/test.cpp
@@ -6,7 +6,7 @@ constexpr uint64_t prog2 = 0x00000020'00000000;
 
 int main() {
   ZDebug("Testing");
-  uint64_t err = ZProcessSpawn(0x100, prog2, 0x1000);
+  uint64_t err = ZProcessSpawnElf(0x100, prog2, 0x1000);
   if (err != Z_OK) {
     ZDebug("Error");
   } else {
diff --git a/zion/include/cap_types.h b/zion/include/cap_types.h
deleted file mode 100644
index a0a3192..0000000
--- a/zion/include/cap_types.h
+++ /dev/null
@@ -1,3 +0,0 @@
-#pragma once
-
-#define ZC_PROC_SPAWN_CHILD 0x1
diff --git a/zion/include/zcall.h b/zion/include/zcall.h
index 84f9e46..bdd8f9a 100644
--- a/zion/include/zcall.h
+++ b/zion/include/zcall.h
@@ -2,19 +2,34 @@
 
 #include <stdint.h>
 
-#define Z_THREAD_EXIT 0x01
+#define Z_INVALID 0x0
 
-#define Z_PROCESS_SPAWN 0x10
+#define ZC_WRITE 0x01
+#define ZC_READ 0x02
 
-#define Z_DEBUG_PRINT 0x100
+#define Z_PROCESS_EXIT 0x01
+#define Z_PROCESS_SPAWN 0x02
+#define Z_PROCESS_START 0x03
+
+#define Z_PROCESS_SPAWN_ELF 0x1'00000002
+
+#define ZC_PROC_SPAWN_PROC 0x100
+#define ZC_PROC_SPAWN_THREAD 0x101
+
+#define Z_THREAD_CREATE 0x10
+#define Z_THREAD_START 0x11
+#define Z_THREAD_EXIT 0x12
+
+#define Z_DEBUG_PRINT 0x10000000
 
 uint64_t ZDebug(const char* message);
 
 // TODO: Move structs into an internal header.
-struct ZProcessSpawnReq {
+struct ZProcessSpawnElfReq {
   uint64_t cap_id;
   uint64_t elf_base;
   uint64_t elf_size;
 };
 
-uint64_t ZProcessSpawn(uint64_t cap_id, uint64_t elf_base, uint64_t elf_size);
+uint64_t ZProcessSpawnElf(uint64_t cap_id, uint64_t elf_base,
+                          uint64_t elf_size);
diff --git a/zion/scheduler/process.cpp b/zion/scheduler/process.cpp
index fedb9ba..890fd82 100644
--- a/zion/scheduler/process.cpp
+++ b/zion/scheduler/process.cpp
@@ -1,7 +1,7 @@
 #include "scheduler/process.h"
 
 #include "debug/debug.h"
-#include "include/cap_types.h"
+#include "include/zcall.h"
 #include "memory/paging_util.h"
 #include "memory/physical_memory.h"
 #include "scheduler/scheduler.h"
@@ -27,7 +27,7 @@ void Process::CreateThread(uint64_t entry) {
   Thread* thread = new Thread(*this, next_thread_id_++, entry);
   threads_.PushBack(thread);
   caps_.PushBack(new Capability(this, Capability::PROCESS, next_cap_id_++,
-                                ZC_PROC_SPAWN_CHILD));
+                                ZC_PROC_SPAWN_PROC));
   gScheduler->Enqueue(thread);
 }
 
diff --git a/zion/syscall/syscall.cpp b/zion/syscall/syscall.cpp
index 26f97a1..76fa85b 100644
--- a/zion/syscall/syscall.cpp
+++ b/zion/syscall/syscall.cpp
@@ -3,7 +3,6 @@
 #include <stdint.h>
 
 #include "debug/debug.h"
-#include "include/cap_types.h"
 #include "include/zcall.h"
 #include "include/zerrors.h"
 #include "loader/elf_loader.h"
@@ -58,7 +57,7 @@ void InitSyscall() {
   SetMSR(LSTAR, reinterpret_cast<uint64_t>(syscall_enter));
 }
 
-uint64_t ProcessSpawn(ZProcessSpawnReq* req) {
+uint64_t ProcessSpawnElf(ZProcessSpawnElfReq* req) {
   auto& curr_proc = gScheduler->CurrentProcess();
   auto cap = curr_proc.GetCapability(req->cap_id);
   if (cap.empty()) {
@@ -68,7 +67,7 @@ uint64_t ProcessSpawn(ZProcessSpawnReq* req) {
     return ZE_INVALID;
   }
 
-  if (!cap->HasPermissions(ZC_PROC_SPAWN_CHILD)) {
+  if (!cap->HasPermissions(ZC_PROC_SPAWN_PROC)) {
     return ZE_DENIED;
   }
   dbgln("Proc spawn: %u:%u", req->elf_base, req->elf_size);
@@ -82,7 +81,8 @@ uint64_t ProcessSpawn(ZProcessSpawnReq* req) {
 extern "C" uint64_t SyscallHandler(uint64_t call_id, char* message) {
   Thread& thread = gScheduler->CurrentThread();
   switch (call_id) {
-    case Z_THREAD_EXIT:
+    case Z_PROCESS_EXIT:
+      // FIXME: kill process here.
       thread.Exit();
       panic("Returned from thread exit");
       break;
@@ -90,7 +90,7 @@ extern "C" uint64_t SyscallHandler(uint64_t call_id, char* message) {
       dbgln("[Debug] %s", message);
       break;
     case Z_PROCESS_SPAWN:
-      return ProcessSpawn(reinterpret_cast<ZProcessSpawnReq*>(message));
+      return ProcessSpawnElf(reinterpret_cast<ZProcessSpawnElfReq*>(message));
     default:
       panic("Unhandled syscall number: %u", call_id);
   }
diff --git a/zion/usr/zcall.cpp b/zion/usr/zcall.cpp
index 19f67f2..8198ae5 100644
--- a/zion/usr/zcall.cpp
+++ b/zion/usr/zcall.cpp
@@ -12,8 +12,9 @@ uint64_t ZDebug(const char* message) {
   return SysCall1(Z_DEBUG_PRINT, message);
 }
 
-uint64_t ZProcessSpawn(uint64_t cap_id, uint64_t elf_base, uint64_t elf_size) {
-  ZProcessSpawnReq req{
+uint64_t ZProcessSpawnElf(uint64_t cap_id, uint64_t elf_base,
+                          uint64_t elf_size) {
+  ZProcessSpawnElfReq req{
       .cap_id = cap_id,
       .elf_base = elf_base,
       .elf_size = elf_size,