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
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
serialstream.cc
serialstream.h	Serial output stream
textstream.cc
textstream.h	TextStream outputs text onto the screen in TextMode
exercises
interrupt
handler.cc
handler.h	interrupt_handler() Interrupt handler
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)
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
gdt.cc
gdt.h	The Global Descriptor Table (GDT)
idt.cc
idt.h	Interrupt Descriptor Table (IDT) containing the entry points for interrupt handling
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
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
stringbuffer.cc
stringbuffer.h	Stringbuffer composes single characters into a buffer
reference
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