diff --git a/defs.h b/defs.h
index 262da72..0e8cb52 100644
--- a/defs.h
+++ b/defs.h
@@ -179,7 +179,6 @@ int             uartgetc(void);
 
 // vm.c
 void            kvminit(void);
-void            kvmswitch(void);
 pagetable_t     uvmcreate(void);
 void            uvminit(pagetable_t, char *, uint);
 uint64          uvmalloc(pagetable_t, uint64, uint64);
diff --git a/kalloc.c b/kalloc.c
index cfbbae4..1ed1c49 100644
--- a/kalloc.c
+++ b/kalloc.c
@@ -27,8 +27,6 @@ void
 kinit()
 {
   initlock(&kmem.lock, "kmem");
-  if(PHYSTOP > RAMDISK)
-    panic("kinit");
   freerange(end, (void*)PHYSTOP);
 }
 
@@ -76,7 +74,7 @@ kalloc(void)
   if(r)
     kmem.freelist = r->next;
   release(&kmem.lock);
-  memset((char*)r, 5, PGSIZE); // fill with junk
+  if(r)
+    memset((char*)r, 5, PGSIZE); // fill with junk
   return (void*)r;
 }
-
diff --git a/kernel.ld b/kernel.ld
index 08fc280..53c9b90 100644
--- a/kernel.ld
+++ b/kernel.ld
@@ -26,8 +26,6 @@ SECTIONS
   }
   bss : {
     *(.bss)
+    PROVIDE(end = .);
   }
-
-  . = ALIGN(0x1000);
-  PROVIDE(end = .);
 }
diff --git a/memlayout.h b/memlayout.h
index 1a6b200..db233f7 100644
--- a/memlayout.h
+++ b/memlayout.h
@@ -2,6 +2,7 @@
 
 // qemu -machine virt is set up like this:
 // 00001000 -- boot ROM, provided by qemu
+// 0C000000 -- PLIC
 // 10000000 -- uart0 registers
 // 80000000 -- boot ROM jumps here in machine mode
 //             -kernel loads the kernel here
@@ -26,7 +27,7 @@
 // for use by the kernel and user pages
 // from physical address 0x80000000 to PHYSTOP.
 #define KERNBASE 0x80000000L
-#define PHYSTOP (KERNBASE + 64*1024*1024)
+#define PHYSTOP (KERNBASE + 128*1024*1024)
 
 // map the trampoline page to the highest address,
 // in both user and kernel space.
diff --git a/proc.c b/proc.c
index e9aec5d..b696d0c 100644
--- a/proc.c
+++ b/proc.c
@@ -360,7 +360,7 @@ scheduler(void)
   for(;;){
     // Enable interrupts on this processor.
     intr_on();
-    
+
     // Loop over process table looking for process to run.
     acquire(&ptable.lock);
     for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
diff --git a/riscv.h b/riscv.h
index 6ecee84..d59503c 100644
--- a/riscv.h
+++ b/riscv.h
@@ -132,6 +132,14 @@ w_stvec(uint64 x)
   asm("csrw stvec, %0" : : "r" (x));
 }
 
+static inline uint64
+r_stvec()
+{
+  uint64 x;
+  asm("csrr %0, stvec" : "=r" (x) );
+  return x;
+}
+
 // use riscv's sv39 page table scheme.
 #define SATP_SV39 (8L << 60)
 
@@ -201,6 +209,14 @@ intr_get()
   return (x & SSTATUS_SIE) != 0;
 }
 
+static inline uint64
+r_sp()
+{
+  uint64 x;
+  asm("mv %0, sp" : "=r" (x) );
+  return x;
+}
+
 #define PGSIZE 4096 // bytes per page
 #define PGSHIFT 12  // bits of offset within a page
 
diff --git a/start.c b/start.c
index 5167ab4..b7af38a 100644
--- a/start.c
+++ b/start.c
@@ -6,7 +6,7 @@
 void main();
 
 // entry.S uses this as the initial stack.
-char stack0[4096];
+__attribute__ ((aligned (16))) char stack0[4096];
 
 // entry.S jumps here in machine mode on stack0.
 void
diff --git a/sysproc.c b/sysproc.c
index 329feee..82ad884 100644
--- a/sysproc.c
+++ b/sysproc.c
@@ -39,6 +39,7 @@ sys_sbrk(void)
 
   if(argint(0, &n) < 0)
     return -1;
+  printf("sbrk(%d), sz was %d\n", n, (int)myproc()->sz);
   addr = myproc()->sz;
   if(growproc(n) < 0)
     return -1;
diff --git a/trap.c b/trap.c
index 98ab143..39ff61d 100644
--- a/trap.c
+++ b/trap.c
@@ -108,7 +108,7 @@ usertrapret(void)
 // interrupts and exceptions from kernel code go here,
 // on whatever the current kernel stack is.
 // must be 4-byte aligned to fit in stvec.
-void __attribute__ ((aligned (4)))
+void 
 kerneltrap()
 {
   uint64 sstatus = r_sstatus();
diff --git a/uart.c b/uart.c
index 29f6df4..35fac1b 100644
--- a/uart.c
+++ b/uart.c
@@ -46,6 +46,9 @@ uartinit(void)
 void
 uartputc(int c)
 {
+  // wait for Transmit Holding Empty to be set in LSR.
+  while((*R(5) & (1 << 5)) == 0)
+    ;
   *R(0) = c;
 }
 
diff --git a/vm.c b/vm.c
index 89b1aa2..1e014a0 100644
--- a/vm.c
+++ b/vm.c
@@ -33,12 +33,11 @@ kvminit()
   // PLIC
   mappages(kernel_pagetable, PLIC, 0x4000000,
            PLIC, PTE_R | PTE_W);
-  
 
   // map kernel text executable and read-only.
   mappages(kernel_pagetable, KERNBASE, (uint64)etext-KERNBASE,
            KERNBASE, PTE_R | PTE_X);
-  
+
   // map kernel data and the physical RAM we'll make use of.
   mappages(kernel_pagetable, (uint64)etext, PHYSTOP-(uint64)etext,
            (uint64)etext, PTE_R | PTE_W);
@@ -52,14 +51,8 @@ kvminit()
   mappages(kernel_pagetable, TRAMPOLINE, PGSIZE,
            (uint64)trampout, PTE_R | PTE_X);
 
-  kvmswitch();
-}
-
-// Switch h/w page table register to the kernel's page table,
-// and enable paging.
-void
-kvmswitch(void)
-{
+  // Switch h/w page table register to the kernel's page table,
+  // and enable paging.
   w_satp(MAKE_SATP(kernel_pagetable));
 }
 
@@ -214,7 +207,7 @@ uvmalloc(pagetable_t pagetable, uint64 oldsz, uint64 newsz)
   for(; a < newsz; a += PGSIZE){
     mem = kalloc();
     if(mem == 0){
-      uvmdealloc(pagetable, newsz, oldsz);
+      uvmdealloc(pagetable, a, oldsz);
       return 0;
     }
     memset(mem, 0, PGSIZE);