diff --git a/kernel/syscall.c b/kernel/syscall.c
index ca34f2c..ff10f9c 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -116,29 +116,29 @@ argstr(int n, char *buf, int max)
   return fetchstr(addr, buf, max);
 }
 
-extern int sys_chdir(void);
-extern int sys_close(void);
-extern int sys_dup(void);
-extern int sys_exec(void);
-extern int sys_exit(void);
-extern int sys_fork(void);
-extern int sys_fstat(void);
-extern int sys_getpid(void);
-extern int sys_kill(void);
-extern int sys_link(void);
-extern int sys_mkdir(void);
-extern int sys_mknod(void);
-extern int sys_open(void);
-extern int sys_pipe(void);
-extern int sys_read(void);
-extern int sys_sbrk(void);
-extern int sys_sleep(void);
-extern int sys_unlink(void);
-extern int sys_wait(void);
-extern int sys_write(void);
-extern int sys_uptime(void);
+extern uint64 sys_chdir(void);
+extern uint64 sys_close(void);
+extern uint64 sys_dup(void);
+extern uint64 sys_exec(void);
+extern uint64 sys_exit(void);
+extern uint64 sys_fork(void);
+extern uint64 sys_fstat(void);
+extern uint64 sys_getpid(void);
+extern uint64 sys_kill(void);
+extern uint64 sys_link(void);
+extern uint64 sys_mkdir(void);
+extern uint64 sys_mknod(void);
+extern uint64 sys_open(void);
+extern uint64 sys_pipe(void);
+extern uint64 sys_read(void);
+extern uint64 sys_sbrk(void);
+extern uint64 sys_sleep(void);
+extern uint64 sys_unlink(void);
+extern uint64 sys_wait(void);
+extern uint64 sys_write(void);
+extern uint64 sys_uptime(void);
 
-static int (*syscalls[])(void) = {
+static uint64 (*syscalls[])(void) = {
 [SYS_fork]    sys_fork,
 [SYS_exit]    sys_exit,
 [SYS_wait]    sys_wait,
diff --git a/kernel/sysfile.c b/kernel/sysfile.c
index 7788de3..77e273f 100644
--- a/kernel/sysfile.c
+++ b/kernel/sysfile.c
@@ -52,7 +52,7 @@ fdalloc(struct file *f)
   return -1;
 }
 
-int
+uint64
 sys_dup(void)
 {
   struct file *f;
@@ -66,7 +66,7 @@ sys_dup(void)
   return fd;
 }
 
-int
+uint64
 sys_read(void)
 {
   struct file *f;
@@ -78,7 +78,7 @@ sys_read(void)
   return fileread(f, p, n);
 }
 
-int
+uint64
 sys_write(void)
 {
   struct file *f;
@@ -91,7 +91,7 @@ sys_write(void)
   return filewrite(f, p, n);
 }
 
-int
+uint64
 sys_close(void)
 {
   int fd;
@@ -104,7 +104,7 @@ sys_close(void)
   return 0;
 }
 
-int
+uint64
 sys_fstat(void)
 {
   struct file *f;
@@ -116,7 +116,7 @@ sys_fstat(void)
 }
 
 // Create the path new as a link to the same inode as old.
-int
+uint64
 sys_link(void)
 {
   char name[DIRSIZ], new[MAXPATH], old[MAXPATH];
@@ -182,7 +182,7 @@ isdirempty(struct inode *dp)
 }
 
 //PAGEBREAK!
-int
+uint64
 sys_unlink(void)
 {
   struct inode *ip, *dp;
@@ -284,7 +284,7 @@ create(char *path, short type, short major, short minor)
   return ip;
 }
 
-int
+uint64
 sys_open(void)
 {
   char path[MAXPATH];
@@ -347,7 +347,7 @@ sys_open(void)
   return fd;
 }
 
-int
+uint64
 sys_mkdir(void)
 {
   char path[MAXPATH];
@@ -363,7 +363,7 @@ sys_mkdir(void)
   return 0;
 }
 
-int
+uint64
 sys_mknod(void)
 {
   struct inode *ip;
@@ -383,7 +383,7 @@ sys_mknod(void)
   return 0;
 }
 
-int
+uint64
 sys_chdir(void)
 {
   char path[MAXPATH];
@@ -408,7 +408,7 @@ sys_chdir(void)
   return 0;
 }
 
-int
+uint64
 sys_exec(void)
 {
   char path[MAXPATH], *argv[MAXARG];
@@ -446,7 +446,7 @@ sys_exec(void)
   return ret;
 }
 
-int
+uint64
 sys_pipe(void)
 {
   uint64 fdarray; // user pointer to array of two integers
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index e57e045..93ea9bc 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -6,32 +6,32 @@
 #include "memlayout.h"
 #include "proc.h"
 
-int
+uint64
 sys_exit(void)
 {
   exit();
   return 0;  // not reached
 }
 
-int
+uint64
 sys_getpid(void)
 {
   return myproc()->pid;
 }
 
-int
+uint64
 sys_fork(void)
 {
   return fork();
 }
 
-int
+uint64
 sys_wait(void)
 {
   return wait();
 }
 
-int
+uint64
 sys_sbrk(void)
 {
   int addr;
@@ -45,7 +45,7 @@ sys_sbrk(void)
   return addr;
 }
 
-int
+uint64
 sys_sleep(void)
 {
   int n;
@@ -66,7 +66,7 @@ sys_sleep(void)
   return 0;
 }
 
-int
+uint64
 sys_kill(void)
 {
   int pid;
@@ -78,7 +78,7 @@ sys_kill(void)
 
 // return how many clock tick interrupts have occurred
 // since start.
-int
+uint64
 sys_uptime(void)
 {
   uint xticks;
diff --git a/user/usertests.c b/user/usertests.c
index beca8f9..0aafe73 100644
--- a/user/usertests.c
+++ b/user/usertests.c
@@ -1438,6 +1438,13 @@ sbrktest(void)
   printf(stdout, "sbrk test\n");
   oldbrk = sbrk(0);
 
+  // does sbrk() return the expected failure value?
+  a = sbrk(1024*1024*1024);
+  if(a != (char*)0xffffffffffffffffL){
+    printf(stdout, "sbrk(<toomuch>) returned %p\n", a);
+    exit();
+  }
+
   // can one sbrk() less than a page?
   a = sbrk(0);
   for(i = 0; i < 5000; i++){
@@ -1478,7 +1485,7 @@ sbrktest(void)
   // can one de-allocate?
   a = sbrk(0);
   c = sbrk(-4096);
-  if(c == (char*)0xffffffff){
+  if(c == (char*)0xffffffffffffffffL){
     printf(stdout, "sbrk could not deallocate\n");
     exit();
   }
@@ -1551,7 +1558,7 @@ sbrktest(void)
     kill(pids[i]);
     wait();
   }
-  if(c == (char*)0xffffffff){
+  if(c == (char*)0xffffffffffffffffL){
     printf(stdout, "failed sbrk leaked memory\n");
     exit();
   }