RASPBERRY PI ASSEMBLER Roger Ferrer Ib´a~nez Cambridge, Cambridgeshire, U.K. William J. Pervin Dallas, Texas, U.S.A. December 18, 2018 i Contents 1 Raspberry Pi Assembler 1 1.1 Writing Assembler . 1 1.2 Our first program . 2 1.3 First program results . 3 2 ARM Registers 7 2.1 Basic arithmetic . 7 3 Memory 9 3.1 Memory . 9 3.2 Addresses . 9 3.3 Data . 10 3.4 Sections . 11 3.5 Load . 11 3.6 Store . 14 3.7 Programming style . 15 4 Debugging 19 4.1 gdb . 19 5 Branching 25 5.1 A special register . 25 5.2 Unconditional branches . 26 5.3 Conditional branches . 26 6 Control structures 31 6.1 If, then, else . 31 6.2 Loops . 32 6.3 1 + 2 + 3 + 4 + ··· +22 .......................... 32 6.4 3n + 1 . 35 7 Addressing modes 39 7.1 Indexing modes . 39 7.2 Shifted operand . 40 ii Contents 8 Arrays and structures 43 8.1 Arrays and structures . 43 8.2 Defining arrays and structs . 44 8.3 Naive approach without indexing modes . 44 8.4 Indexing modes . 45 8.4.1 Non-updating indexing modes . 45 8.4.2 Updating indexing modes . 46 8.4.3 Post-indexing modes . 47 8.4.4 Pre-indexing modes . 48 8.5 Back to structures . 49 8.6 Strings . 49 9 Functions 51 9.1 Do's and don'ts of a function . 51 9.1.1 New specially named registers . 51 9.1.2 Passing parameters . 52 9.1.3 Well behaved functions . 52 9.1.4 Calling a function . 52 9.1.5 Leaving a function . 52 9.1.6 Returning data from functions . 53 9.2 Hello world . 53 9.3 Real interaction! . 55 9.4 Our first function . 56 9.5 Unified Assembler Language . 59 10 Searching and Sorting 61 10.1 Binary Search . 61 10.2 Insertion Sort . 64 10.3 Random Numbers . 67 10.4 More Debugging . 69 11 Recursion and the Stack 71 11.1 Dynamic activation . 71 11.2 The stack . 72 11.3 Factorial . 74 11.4 Load and Store Multiple . 77 11.5 Factorial again . 78 11.6 Tail-recursion . 80 11.7 Dynamic Programming . 82 12 Conditional Execution 87 12.1 Predication . 87 12.2 The pipe line of instructions . 88 12.3 Predication in ARM . 90 iii Contents 12.4 Collatz conjecture revisited . 90 12.5 Adding predication . 92 12.6 Does it make any difference in performance? . 92 12.7 The s suffix . 93 13 Floating-point Numbers 97 13.1 IEEE-754 Standard . 97 13.2 Examples . 99 13.3 Extremes . 100 13.4 Exceptions . 101 13.5 Accuracy . 101 13.6 *Fixed-point Numbers . 103 14 Real Computations 105 14.1 VFPv2 Registers . 105 14.2 Arithmetic operations . 106 14.3 Load and Store . 108 14.4 Movements between registers . 109 14.5 Conversions . 110 14.6 Modifying the fpscr . 111 14.7 Function call convention and floating-point registers . 111 14.8 Printing Floating-point Numbers . 112 15 Pointers 113 15.1 Passing data to functions . 113 15.2 What is a pointer? . 113 15.3 Passing large amounts of data . 116 15.4 Passing a big array by value . 117 15.5 Passing a big array by reference . 121 15.6 Modifying data through pointers . 123 15.7 Returning more than one piece of data . 126 16 System Calls 127 16.1 File I/O . 128 16.2 lseek . 131 17 Local data 133 17.1 The frame pointer . 133 17.2 Dynamic link of the activation record . 134 17.3 What about parameters passed in the stack? . 137 17.4 Indexing through the frame pointer . 137 18 Inline Assembler in C Code 143 18.1 The asm Statement . 143 18.2 Simple Example . 144 iv Contents 19 Thumb 147 19.1 The Thumb instruction set . 147 19.2 Support of Thumb in Raspbian . 147 19.3 Instructions . 147 19.4 From ARM to Thumb . 148 19.5 Calling functions in Thumb . 149 19.6 From Thumb to ARM . 151 19.7 To know more . 152 20 Additional Topics 153 20.1 ARM Instruction Set . 153 20.2 Interrupt Handling . 154 20.3 To know more . 155 A ASCII Standard Character Set 157 B Integers 159 B.1 Unsigned Integers . 159 B.2 Signed-Magnitude Integers . 159 B.3 One's Complement . 160 B.4 Two's Complement . 161 B.5.