$make qemu
+ as kern/entry.S
+ cc kern/entrypgdir.c
+ cc kern/init.c
+ cc kern/console.c
+ cc kern/monitor.c
+ cc kern/printf.c
+ cc kern/kdebug.c
+ cc lib/printfmt.c
+ cc lib/readline.c
+ cc lib/string.c
+ ld obj/kern/kernel
obj/kern/printfmt.o: In function `printnum':
lib/printfmt.c:41: undefined reference to `__udivdi3'
lib/printfmt.c:49: undefined reference to `__umoddi3'
kern/Makefrag:70: recipe for target 'obj/kern/kernel' failed
make: *** [obj/kern/kernel] Error 1
解决：
sudo apt-get install gcc-4.8-multilib
$make clean; make
+ as kern/entry.S
+ cc kern/entrypgdir.c
+ cc kern/init.c
+ cc kern/console.c
+ cc kern/monitor.c
+ cc kern/printf.c
+ cc kern/kdebug.c
+ cc lib/printfmt.c
+ cc lib/readline.c
+ cc lib/string.c
+ ld obj/kern/kernel
+ as boot/boot.S
+ cc -Os boot/main.c
+ ld boot/boot
boot block is 377 bytes (max 510)
+ mk obj/kern/kernel.img