backtrace

answers-traps.txt

@@ -0,0 +1,33 @@
+Q: Which registers contain arguments to functions? For example, which register holds 13 in main's call to printf?
+A: The base integer calling convention provides eight argument registers, a0-a7, the first two of which are also used to return values. Register a2 holds 13.
+
+Q: Where is the call to function f in the assembly code for main? Where is the call to g?
+A:  There is no call to function f for main, the value of f(8) (which is 12) is calculated at compile time and hard-coded into the executable.
+    The call to g is inlined into f, at address 0x14.
+
+Q: At what address is the function printf located?
+A:  The function printf is located at address 0x642.
+
+Q: What value is in the register ra just after the jalr to printf in main?
+A:  The address of the next instruction after return, aka 0x38.
+
+Q:
+---
+Run the following code.
+
+	unsigned int i = 0x00646c72;
+	printf("H%x Wo%s", 57616, &i);
+      
+What is the output? Here's an ASCII table that maps bytes to characters.
+The output depends on that fact that the RISC-V is little-endian. If the RISC-V were instead big-endian what would you set i to in order to yield the same output? Would you need to change 57616 to a different value?
+---
+A:  The output is, He110 World(with no whitespace or new line).
+    If big endian, the new i should be 0x726c6400; and there is no need to change 56716.
+
+Q:
+---
+In the following code, what is going to be printed after 'y='? (note: the answer is not a specific value.) Why does this happen?
+	printf("x=%d y=%d", 3);
+---
+A:  The value that register a2 holds when the caller calls printf. By default, the third argument passed to a procedure is stored in a2.
+

kernel/defs.h

@@ -80,6 +80,7 @@ int             pipewrite(struct pipe*, uint64, int);
 void            printf(char*, ...);
 void            panic(char*) __attribute__((noreturn));
 void            printfinit(void);
+void            backtrace(void);
 
 // proc.c
 int             cpuid(void);

kernel/printf.c

@@ -122,6 +122,7 @@ panic(char *s)
   printf("panic: ");
   printf(s);
   printf("\n");
+  backtrace();
   panicked = 1; // freeze uart output from other CPUs
   for(;;)
     ;
@@ -133,3 +134,14 @@ printfinit(void)
   initlock(&pr.lock, "pr");
   pr.locking = 1;
 }
+
+void backtrace(void)
+{
+  printf("backtrace:\n");
+  uint64 fp = r_fp();
+  uint64 fp_pg = PGROUNDDOWN(fp);
+  while (PGROUNDDOWN(fp) == fp_pg) {
+    printf("%p\n", (*(uint64*)(fp-8)));
+    fp = *(uint64*)(fp-16);
+  }
+}

kernel/riscv.h

@@ -319,6 +319,14 @@ r_ra()
   return x;
 }
 
+static inline uint64
+r_fp()
+{
+  uint64 x;
+  asm volatile("mv %0, s0" : "=r" (x) );
+  return x;
+}
+
 // flush the TLB.
 static inline void
 sfence_vma()

kernel/sysproc.c

@@ -57,6 +57,7 @@ sys_sleep(void)
   argint(0, &n);
   if(n < 0)
     n = 0;
+  backtrace();
   acquire(&tickslock);
   ticks0 = ticks;
   while(ticks - ticks0 < n){