Lab attendance is your responsibility. Before you leave this and all future labs you must ensure that your name is on one of the check-in sheets. You will no longer be given credit if your name can not be found on one of those sheets.

lab08.zip contains all of the starter template code for constructing your programs. Implement the functions push_bits() and pop_bits() for the first checkpoint, bit_twiddling() for the second, and decode() for the final checkpoint.

Pushing and popping bits

Using the shift left operator (<<), we can “append” bits onto our values by shifting and then applying an OR operation onto the new value. Left shifting a 1 onto a 0 will yield 1. Left shifting a 1 onto a 4 (100) will yield 9 (1001). Likewise, the right shift operator (>>) will shift values off the right (least significant) side, turning 1011 into 101 after a single right shift. Change the values you pass into push_bits(): do you notice any patterns?

Bit Twiddling

Let’s say you’re interested in a specific range of 10 bits inside another value. How do you access them? Bit masks. A bit mask allows you to eliminate all irrelevant bits leaving behind only the bits that you care about. A very common way to create a bit mask for, say 10 bits would be to left shift 1 by 10 and then subtract one. Why? Because all that will remain will be 1’s which will allow us to AND away the uninteresting values. (Try it: 1 « 10 = 1024, 1024 - 1 = 1023 and in binary, 0b1111111111). Now we can shift our values around and apply a mask to arrive at our values of interest.

Instruction decode (sort of)

Using the bit_twiddling() function you implemented above as well as this page of MIPS instruction formats, write the decode() function to test the appropriate bits of the instruction argument to determine whether it is an R-type instruction or not.