CS 208 s21 — Review Worksheet

Consider the binary value 0b110101:

1. Interpreting this value as an unsigned 6-bit integer, what is its value in decimal?
   
   
   
   
   
2. If we instead interpret it as a signed (two's complement) 6-bit integer, what would its value be in decimal?
   
   
   
   
   
3. Assuming these are all signed two's complement 6-bit integers, compute the result (leaving it in binary is fine) of each of the following additions. For each, indicate if it resulted in overflow.

Overflow?

Overflow?

Overflow?

Overflow?

1. What happens when we fix/set one of the inputs to the 2-input operators? Let x be the other 1-bit input. Fill in the following blanks with either 0, 1, x, or NOT x:
   • x & 0 =
     
   • x & 1 =
     
   • x | 0 =
     
   • x | 1 =
     
   • x ^ 0 =
     
   • x ^ 1 =
     
2. Bit Extraction: Returns the value (0 or 1) of the 19th bit (counting from LSB). Use >> and &.

   int extract19(int x) {
       return ____________________________________________________________;
   }
   

For this problem we are using a 64-bit x86-64 machine (little endian). The current state of memory (values in hex) is shown below:

Write the value in hexadecimal of each expression within the commented lines at their respective state in the execution of the given program. Write UNKNOWN in the blank if the value cannot be determined.

int main() {
    char *charP;
    short *shortP;
    int *intP = 0x00;
    long *longP = 0x28;

    // The value of intP is:                   0x_____________________________

    // *intP                                   0x_____________________________

    // &intP                                   0x_____________________________

    // longP[-2]                               0x_____________________________

    charP = 0x20;
    shortP = (short *) intP;
    intP++;
    longP--;

    // *shortP                                 0x_____________________________

    // *intP                                   0x_____________________________

    // *((int*) longP)                         0x_____________________________

    // (short*) (((long*) charP) - 2)          0x_____________________________
}

1. Write an English description for each of the following instructions:

   x86 instruction	English equivalent
   movq $351, %rax	Move the number 351 into 8-byte (quad) register "rax"
   addq %rdi, %rsi
   movq (%rdi), %r8
   leaq (%rax,%rax,8), %rax

2. Consider the following x86-64, (partially blank) C code, and memory diagram. Addresses and values are 64-bit. Fill in the C code based on the given assembly.

   foo:
       movl $0, %eax
   L1:
       testq %rdi, %rdi
       je L2
       movq (%rdi), %rdi
       addl $1, %eax
       jmp L1
   L2:
       ret
   

   int foo(long* p) {
       int result = ____;
       while (________) {
           p = ___________;
           _____ = __________;
       }
       return result;
   }
   

3. Follow the execution of foo in assembly, where 0x1000 is passed in to %rdi. Write the values of %rdi and %eax in the columns. If the value doesn't change, you can leave it blank. Continue writing the next instruction and any changed values until you have traced the full execution (i.e., reached ret).

   instruction	%rdi (in hex)	%eax (decimal)
   movl	0x1000	0
   testq
   je