1 Chapter 4: Arithmetic for Computers (Part 1) CS 447 Jason Bakos

2 Notes on Project 1 There are two different ways the following two words can be stored in a computer memory… –word1.byte0,1,2,3 –word2.half0,1 One way is big-endian, where the word is stored in memory in its original order… –word1: –word2: Another way is little-endian, where the word is stored in memory in reverse order… –word1: –word2: Of course, this affects the way in which the lw instruction works…

3 Notes on Project 1 MIPS uses the endian-style that the architecture underneath it uses –Intel uses little-endian, so we need to deal with that –This affects assignment 1 because the input data is stored as a series of bytes –If you use lw’s on your data set, the values will be loaded into your dest. register in reverse order –Hint: Try the lb/sb instruction This instruction will load/store a byte from an unaligned address and perform the translation for you

4 Notes on Project 1 Hint: Use SPIM’s breakpoint and single- step features to help debug your program –Also, make sure you use the registers and memory/stack displays Hint: You may want to temporarily store your input set into a word array for sorting Make sure you check Appendix A for additional useful instructions that I didn’t cover in class Make sure you comment your code!

5 Goals of Chapter 4 Data representation Hardware mechanisms for performing arithmetic on data Hardware implications on the instruction set design

6 Review of Binary Representation Binary/Hex -> Decimal conversion Decimal -> Binary/Hex conversion Least/Most significant bits Highest representable number/maximum number of unique representable symbols Two’s compliment representation –One’s compliment –Finding signed number ranges (-2 n-1 to 2 n-1 -1) –Doing arithmetic with two’s compliment Sign extending with load half/byte –Unsigned loads Signed/unsigned comparison

7 Binary Addition/Subtraction Binary subtraction works exactly like addition, except the second operand is converted to two’s compliment Overflow in signed arithmetic occurs under the following conditions: OperationOperand AOperand BResult A+BPositive Negative A+BNegative Positive A-BPositiveNegative A-BNegativePositive

8 What Happens When Overflow Occurs? MIPS detects overflow with an exception/interrupt When an interrupt occurs, a branch occurs to code in the kernel at address where special registers (BadVAddr, Status, Cause, and EPC) are used to handle the interrupt SPIM has a simple interrupt handler built-in that deals with interrupts We may come back to interrupts later

9 Review of Shift and Logical Operations MIPS has operations for SLL, SRL, and SRA –We covered this in the last chapter MIPS implements bit-wise AND, OR, and XOR logical operations –These operations perform a bit-by-bit parallel logical operation on two registers –In C, use > for arithmetic shifts, and &, |, ^, and ~ for bitwise and, or, xor, and NOT, respectively

10 Review of Logic Operations The three main parts of a CPU –ALU (Arithmetic and Logic Unit) Performs all logical, arithmetic, and shift operations –CU (Control Unit) Controls the CPU – performs load/store, branch, and instruction fetch –Registers Physical storage locations for data

11 Review of Logic Operations In this chapter, our goal is to learn how the ALU is implemented The ALU is entirely constructed using boolean functions as hardware building blocks –The 3 basic digital logic building blocks can be used to construct any digital logic system: AND, OR, and NOT –These functions can be directly implemented using electric circuits (wires and transistors)

12 Review of Logic Operations These “combinational” logic devices can be assembled to create a much more complex digital logic system ABA AND B ABA OR B Anot A 01 10

13 Review of Logic Operations We need another device to build an ALU… This is called a multiplexor… it implements an if-then-else in hardware ABDC (out) 0000 (a) 0010 (b) 0100 (a) 0111 (b) 1001 (a) 1010 (b) 1101 (a) 1111 (b)

14 A 1-bit ALU Perform logic operations in parellel and mux the output Next, we want to include addition, so let’s build a single-bit adder –Called a full adder

15 Full Adder From the following table, we can construct the circuit for a full adder and link multiple full adders together to form a multi-bit adder –We can also add this input to our ALU –How do we give subtraction ability to our adder? –How do we detect overflow and zero results? InputsOutputsComments ABCarryInCarryOutSum = = = = = = = =11