RISC-V ALE Exercise Book v1.0.5

Bit Masking and Shift Operations

Instructions

Write a C program that reads 5 numbers from the user input in decimal format and packs its bits to a single 32 bit value that will be written to the standard output as a hexadecimal value. The program should handle both positive and negative numbers.

A function with the signature void pack(int input, int start_bit, int end_bit, int *val) might be a good start for this exercise.

The previously presented functions read and write can be used for reading/writing information from/to the standard input/output. The code snippet below can be used to write the resulting hexadecimal value to STDOUT (note that it uses the write function).

void hex_code(int val){
    char hex[11];
    unsigned int uval = (unsigned int) val, aux;

    hex[0] = '0';
    hex[1] = 'x';
    hex[10] = '\n';

    for (int i = 9; i > 1; i--){
        aux = uval % 16;
        if (aux >= 10)
            hex[i] = aux - 10 + 'A';
        else
            hex[i] = aux + '0';
        uval = uval / 16;
    }
    write(1, hex, 11);
}

Input

• 5 signed 4-digit decimal numbers separated by spaces (' '), followed by a newline character ('\n'). The whole input takes up 30 bytes.
  • String Format - "SDDDD SDDDD SDDDD SDDDD SDDDD\n"
  • S: sign, can be either '+' for positive numbers and '-' for negative.
  • D: a decimal digit, (0-9)

Output

After reading all 5 numbers, you must pack their least significant bits (LSB) following the rules listed below:

• 1^st number: 3 LSB => Bits 0 - 2
• 2^nd number: 8 LSB => Bits 3 - 10
• 3^rd number: 5 LSB => Bits 11 - 15
• 4^th number: 5 LSB => Bits 16 - 20
• 5^th number: 11 LSB => Bits 21 - 31

Figure 5.1.1: Bit extraction from 5 32-bit inputs to constitute a single 32-bit output

Examples

Notes and Tips

• You may use the C operator for bit manipulation (<<, >>, |, &, ^, …) to implement your function. Suggested readings:
  • Bit Operations
  • Bitwise Operators
• You can test your code using the simulator's assistant from this link.