how to shift right with only + – & | and tests for >, < ==

byMR

April 18, 2024

Asked before but got closed due to lack of detail. So here is more info

I am working on cp without shift or rotate.

It supports

add & subtract
logical & and logical or
conditional jump on > < == or any combination of those (>=…)

I can see how to shift left using repeated add to self. I can do the rotate by testing the leftmost bit before shift and oring it onto the rightmost after shift.

I cant figure out a right shift mechanism though

>Solution :

Here’s an example in C++ using only + and -:

template<class T>
T rightshift(T value, int steps) {
    T div = 1;
    while(steps--)  div += div; // steps=1 => 2, 2=>4 etc.
    
    T res = 0;
    while(value >= div) { // subtract div until it's less than div
        value -= div;
        ++res;            // and count the number of times we subtract
    }

    return res;
}