Here is a list of all files with brief descriptions:

boot
multiboot
data.cc
data.h	Multiboot Interface
startup.cc
startup.h	Startup of the first core, also known as bootstrap processor (BSP)
startup_ap.cc
startup_ap.h	Startup of additional cores, the application processors (APs)
compiler
fix.h	Compiler-dependent fixes & idiosyncrasies
libc.cc
libc.h	Initialization functions for global objects required by the compiler
libcxx.cc	C++ runtime support functions
debug
gdb
handler.cc
handler.h	For GDB_Stub Interrupt handler and its context
protocol.cc
state.cc
state.h	GDB_Stub State of the current core
stub.h	GDB_Stub contains the necessary functionality for remote debugging
assert.cc
assert.h	Contains several macros usable for making assertions
copystream.h	CopyStream duplicates output streams
kernelpanic.h	Macro to print an error message and stop the current core
nullstream.cc
nullstream.h	NullStream is a stream discarding everything
output.h	Debug macros enabling debug output on a separate window for each core
device
keyboard.cc
keyboard.h	The Keyboard device handles keystrokes
mouse.cc
mouse.h	The Mouse device handles mouse movements and clicks
panic.cc
panic.h	Default interrupt handling device Panic
serialstream.cc
serialstream.h	Serial output stream
textstream.cc
textstream.h	TextStream outputs text onto the screen in TextMode
exercises
interrupt
gate.h	Class Gate (Device interrupt handling)
handler.cc
handler.h	interrupt_handler() Interrupt handler
plugbox.cc
plugbox.h	Plugbox allows assigning devices to interrupt vectors
machine
acpi.cc
acpi.h	Structs and methods related to the Advanced Configuration and Power Interface (ACPI)
apic.cc
apic.h	Gather system information from the ACPI about the Advanced Programmable Interrupt Controller (APIC)
cache.h	Helper for cache alignment
cmos.cc
cmos.h	Controlling the complementary metal oxide semiconductor (CMOS)
core.cc
core.h	Access to internals of a CPU Core
core_cr.h	Access to Control Register of a CPU core
core_interrupt.h	Interrupt control and interrupt vector list
core_msr.h	Identifiers for Model-Specific Register
fpu.cc
fpu.h	Helper for floating point unit (x87), Multi Media Extension (MMX) and Streaming SIMD Extensions (SSE)
gdt.cc
gdt.h	The Global Descriptor Table (GDT)
idt.cc
idt.h	Interrupt Descriptor Table (IDT) containing the entry points for interrupt handling
ioapic.cc
ioapic.h	IOAPIC abstracts the access to the I/O APIC
ioapic_registers.h	Helper structures for interacting with the I/O APIC
ioport.h	IOPort provides access to the x86 IO address space
keydecoder.cc
keydecoder.h	KeyDecoder decodes a keystroke to the corresponding Key object
lapic.cc
lapic.h	LAPIC abstracts access to the Local APIC
lapic_ipi.cc
lapic_registers.h	Structures and macros for accessing the local APIC
lapic_timer.cc
mousedecoder.cc
mousedecoder.h	MouseDecoder decodes a mouse movement
pic.cc
pic.h	Handle (disable) the old Programmable Interrupt Controller (PIC)
pit.cc
pit.h	The old/historical Programmable Interval Timer (PIT)
ps2controller.cc
ps2controller.h	PS/2 Controller (Intel 8042, also known as Keyboard Controller)
serial.cc
serial.h	Communication via the Serial interface (RS-232)
system.cc
system.h	General System functionality (reboot)
textmode.cc
textmode.h	TextMode provides a basic interface to display a character in VGA-compatible text mode
textwindow.cc
textwindow.h	TextWindow provides virtual output windows in text mode
object
key.cc
key.h	Key, an abstraction for handling pressed keys and their modifiers
outputstream.cc
outputstream.h	This file contains the OutputStream
pointer.h	Pointer, the Mouse state
stringbuffer.cc
stringbuffer.h	Stringbuffer composes single characters into a buffer
reference
sync
spinlock.h	Contains the class Spinlock
ticketlock.h	Contains the class Ticketlock
test-stream
console_out.cc
console_out.h	Console output (for the voluntary C++ exercise only)
file_out.cc
file_out.h	File output (for the voluntary C++ exercise only)
test.cc
user
app1
appl.cc
appl.h
app2
kappl.cc
kappl.h	KeyboardApplication to test the input
utils
math.h	General purpose math functions
size.h	Template function to determine the length of an array
string.cc
string.h	General purpose String functions
main.cc
types.h	Definition of standard integer types with specified widths and their limits