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322
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1 OS
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2 ==
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272
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3
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407
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4 Implementation
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5 --------------
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6
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7 Arm
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8 ~~~
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9
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10 Vexpress-a9
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11
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12
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13 For the first implementation the qemu arm system vexpress-a9 machine was
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14 targeted.
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15
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16 To launch this machine with a kernel use:
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17
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18 .. code::
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19
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20 qemu-system-arm -M vexpress-a9 -m 128M -kernel kernel/kernel_arm.bin \
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21 -serial stdio
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22
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23 The memory layout of this image is as follows:
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24
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25 - 0x00000000
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26 - 0x10000000 : hardware.
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27 - 0x10009000 : pl011 --> the uart peripheral
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28 - 0x60000000 : bootloader of qemu itself.
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29 - 0x60010000 : main memory, where kernel is loaded by the bootloader.
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30
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31
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322
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32 Design
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33 ------
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273
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34
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35 Processes / threads
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36 ~~~~~~~~~~~~~~~~~~~
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272
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37
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38 Processes are completely seperated and fully pre-emptive.
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39 This means a process can be unscheduled at any moment.
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40
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41 Threads are co-operative. This means they yield control
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42 voluntary. This means that mutexes and locks are not required.
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43 This is done with the built-in language feature called tasks.
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44
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45 If some heavy duty task must be performed, either way spawn
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46 a new process, or yield frequently from this hard labour.
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47
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273
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48 tasks
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49 ~~~~~
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272
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50
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273
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51 Consider the following:
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52
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53 .. code::
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272
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54
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273
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55 function int insanemath(int a)
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272
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56 {
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273
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57 while (a > 0)
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58 {
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59 a = a -1;
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60 resume agent1;
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61 }
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62 return a - 1;
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272
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63 }
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64
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273
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65 task agent1()
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66 {
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67 start agent2;
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68 }
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272
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69
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273
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70 task agent2()
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71 {
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72 insanemath(55);
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73 insanemath(44);
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74 }
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272
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75
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273
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76 task main()
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77 {
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78 start agent1;
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79 join agent1;
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80 }
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81
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272
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82
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83 Say to tasks are running in concurrent / parallel.
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84
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85
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86
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87 Stack layout for tasks.
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88 ||
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89 ||
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90 \/
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91 +---------+
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92 | return address
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93 | locals
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94 |
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95 +------
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96 | return address
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97 | locals
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98 |
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99 +---
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100
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101 Assembly code for the functions above:
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102
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273
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103 .. code::
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104
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105 .code
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106 insanemath:
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107 L1:
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108 load r0, sp - 4
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109 cmp r0, 0
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110 jl L2
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111 dec r0
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112 store r0, sp - 4
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113 jmp L1
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114 L2:
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115 ret
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272
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116
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273
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117 agent1:
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118 hlt?
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272
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119
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273
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120 agent2:
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121 hlt?
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272
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122
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273
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123 main:
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124 jmp agent1
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272
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125
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273
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126 .data
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127 agent1_task:
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128 dd 0
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129 agent2_task:
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130 dd 0
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272
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131
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