Program 12: Ok, Be Negative

Program 12: Ok, Be Negative

Using XOR for sign detection #

With signed integers, the most significant bit will always be 1 if the number is negative. Try XOR-ing two negative numbers in our imaginary 8-bit registers.

    0b11110110  -10
XOR 0b11011110  -35
      ^--- First bit 0 

If you take a negative and a positive number, this happens.

    0b11110110  -10
XOR 0b00000010    2
      ^--- First bit 1

This is helpful because the N flag of the CPSR is a mirror of the most significant bit.

TEQ operation #

As the CMP operation compares two numbers by subtraction and then sets the CPSR, there are a couple of other comparison operations that perform and operation and discard the result. Of of those is the TEQ operation which compares a EOR (XOR) on two operands and sets the CPSR based on the result.

Program 12: Ok, Be Negative Video

Description of Program

I’ve realized that it’s okay if more of my friends have negative happiness scores. I mean, I want them to be happy. But I want to make sure that they are surrounded by people who feel the same way as them. And actually I decided to include everyone on my Facebook account and increase the happiness score to a 2-byte number.

In the list of numbers, count the number of consecutive pairs of numbers where the happiness score is either both positive or both negative.


Input: 2, 3, -2, -3, -4, 2 Output: 2 – (2, 3) and (-2, -3) would match the pattern

Completed Code
Expected Output