Add process scheduler, pipes, and separate terminal from shell

meson.build

@@ -38,6 +38,13 @@ kernel_entry_o = custom_target(
   command: [nasm, '-f', 'elf64', '@INPUT@', '-o', '@OUTPUT@'],
 )
 
+timer_o = custom_target(
+  'timer',
+  input:  'src/kernel/timer.asm',
+  output: 'timer.o',
+  command: [nasm, '-f', 'elf64', '@INPUT@', '-o', '@OUTPUT@'],
+)
+
 process_o = custom_target(
   'process',
   input:  'src/kernel/process.asm',
@@ -71,15 +78,17 @@ kernel_sources = files([
   'src/kernel/panic.c',
   'src/kernel/path.c',
   'src/kernel/pic.c',
+  'src/kernel/pipe.c',
   'src/kernel/process.c',
   'src/kernel/syscall.c',
+  'src/kernel/timer.c',
   'src/kernel/tss.c',
   'src/kernel/vga.c',
 ])
 
 kernel_elf = executable(
   'kernel.elf',
-  sources: [kernel_entry_o, process_o, syscall_o, lib_sources, kernel_sources],
+  sources: [kernel_entry_o, timer_o, process_o, syscall_o, lib_sources, kernel_sources],
   c_args: [
     '-ffreestanding',
     '-nostdlib',
@@ -146,6 +155,45 @@ custom_target(
   build_by_default: true,
 )
 
+shell_start = custom_target(
+  'shell_start',
+  input:  'src/user/start.asm',
+  output: 'shell_start.o',
+  command: [nasm, '-f', 'elf64', '-o', '@OUTPUT@', '@INPUT@'],
+)
+
+shell_syscall = custom_target(
+  'shell_syscall',
+  input:  'src/user/syscall.asm',
+  output: 'shell_syscall.o',
+  command: [nasm, '-f', 'elf64', '-o', '@OUTPUT@', '@INPUT@'],
+)
+
+shell_elf = executable(
+  'shell.elf',
+  sources: [shell_start, shell_syscall, lib_sources, 'src/user/app/shell/shell.c'],
+  c_args: [
+    '-ffreestanding',
+    '-nostdlib',
+    '-fno-stack-protector',
+    '-mcmodel=large',
+    '-DVERSION="' + meson.project_version() + '"',
+  ],
+  link_args: [
+    '-T', meson.project_source_root() / 'src/user/linker.ld',
+    '-nostdlib',
+  ],
+  link_depends: 'src/user/linker.ld',
+)
+
+custom_target(
+  'shell',
+  input:  shell_elf,
+  output: 'shell',
+  command: ['objcopy', '-O', 'binary', '@INPUT@', '@OUTPUT@'],
+  build_by_default: true,
+)
+
 echo_start = custom_target(
   'echo_start',
   input:  'src/user/start.asm',