Task: Multiplying Two Numbers

Multiplying Two Numbers represented as Bytes

You will solve this exercise starting from the multiply.asm file located in the tasks/mul/support directory.

Go through, run, and test the code from the file multiply.asm. In this program, we multiply two numbers defined as bytes. To access them, we use a construction like byte [register].

When performing multiplication, the process is as follows, as described here:

  1. We place the multiplicand in the multiplicand register, meaning:
    • if we’re operating on a byte (8 bits, one byte), we place the multiplicand in the al register;
    • if we’re operating on a word (16 bits, 2 bytes), we place the multiplicand in the ax register;
    • if we’re operating on a double word (32 bits, 4 bytes), we place the multiplicand in the eax register.
  2. The multiplier is passed as an argument to the mul mnemonic. The multiplier must have the same size as the multiplicand.
  3. The result is placed in two registers (the high part and the low part).

Multiplying Two Numbers represented as Words / Double Words

Update the area marked with TODO in the file multiply.asm to allow multiplication of word and dword numbers, namely num1_dw with num2_dw, and num1_dd with num2_dd.

TIP: For multiplying word numbers (16 bits), the components are arranged as follows:

  • Place the multiplicand in the ax register.
  • The argument of the mul instruction, the multiplier (possibly another register), is 16 bits (either a value or a register such as bx, cx, dx).
  • The result of the multiplication is arranged in the pair dx:ax, where the high part of the result is in the dx register, and the low part of the result is in the ax register.

For multiplying dword numbers (32 bits), the components are arranged as follows:

  • Place the multiplicand in the eax register.
  • The argument of the mul instruction, the multiplier (possibly another register), is 32 bits (either a value or a register such as ebx, ecx, edx).
  • The result of the multiplication is arranged in the pair edx:eax, where the high part of the result is in the edx register, and the low part of the result is in the eax register.

NOTE: When displaying the result, use the PRINTF32 macro to display the two registers containing the result:

  • Registers dx and ax for multiplying word numbers.
  • Registers edx and eax for multiplying dword numbers.

Testing

To test the implementation, enter the tests/ directory and run:

make check

In case of a correct solution, you will get an output such as:

test_byte_mul                    ........................ passed ...  33
test_short_mul                   ........................ passed ...  33
test_int_mul                     ........................ passed ...  34

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Total:                                                           100/100

If you’re having difficulties solving this exercise, go through this reading material.