paul@199 | 1 | Concepts
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paul@199 | 2 | ========
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paul@199 | 3 |
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paul@201 | 4 | This document describes the underlying concepts employed in micropython.
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paul@201 | 5 |
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paul@201 | 6 | * Namespaces and attribute definition
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paul@199 | 7 | * Contexts and values
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paul@200 | 8 | * Tables, attributes and lookups
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paul@199 | 9 | * Objects and structures
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paul@200 | 10 | * Parameters and lookups
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paul@200 | 11 | * Instantiation
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paul@222 | 12 | * Register usage
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paul@245 | 13 | * List and tuple representations
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paul@199 | 14 |
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paul@201 | 15 | Namespaces and Attribute Definition
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paul@201 | 16 | ===================================
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paul@201 | 17 |
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paul@201 | 18 | Namespaces are any objects which can retain attributes.
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paul@201 | 19 |
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paul@201 | 20 | * Module attributes are defined either at the module level or by global
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paul@201 | 21 | statements.
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paul@201 | 22 | * Class attributes are defined only within class statements.
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paul@534 | 23 | * Instance attributes are defined only by assignments to attributes of self
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paul@534 | 24 | or tentatively as references to attributes of self.
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paul@201 | 25 |
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paul@201 | 26 | These restrictions apply because such attributes are thus explicitly declared,
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paul@201 | 27 | permitting the use of tables (described below). Module and class attributes
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paul@201 | 28 | can also be finalised in this way in order to permit certain optimisations.
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paul@201 | 29 |
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paul@243 | 30 | An additional restriction required for the current implementation of tables
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paul@243 | 31 | (as described below) applies to class definitions: each class must be defined
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paul@243 | 32 | using a unique name; repeated definition of classes having the same name is
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paul@243 | 33 | thus not permitted. This restriction arises from the use of the "full name" of
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paul@243 | 34 | a class as a key to the object table, where the full name is a qualified path
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paul@243 | 35 | via the module hierarchy ending with the name of the class.
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paul@243 | 36 |
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paul@519 | 37 | Rebinding of attributes outside classes and modules can be allowed if
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paul@519 | 38 | attribute usage observations are being used to detect such external
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paul@519 | 39 | modifications to such objects. Without such observations, such rebinding
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paul@519 | 40 | should be forbidden since apparently constant attributes might be modified in
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paul@519 | 41 | a running program, but code may have been generated that provides specific
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paul@519 | 42 | objects for those attributes under the assumption that they will not be
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paul@519 | 43 | changed.
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paul@519 | 44 |
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paul@201 | 45 | See rejected.txt for complicating mechanisms which could be applied to
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paul@201 | 46 | mitigate the effects of these restrictions on optimisations.
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paul@201 | 47 |
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paul@199 | 48 | Contexts and Values
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paul@199 | 49 | ===================
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paul@199 | 50 |
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paul@199 | 51 | Values are used as the common reference representation in micropython: as
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paul@199 | 52 | stored representations of attributes (of classes, instances, modules, and
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paul@199 | 53 | other objects supporting attribute-like entities) as well as the stored values
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paul@199 | 54 | associated with names in functions and methods.
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paul@199 | 55 |
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paul@199 | 56 | Unlike other implementations, micropython does not create things like bound
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paul@199 | 57 | method objects for individual instances. Instead, all objects are referenced
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paul@199 | 58 | using a context, reference pair:
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paul@199 | 59 |
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paul@199 | 60 | Value Layout
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paul@199 | 61 | ------------
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paul@199 | 62 |
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paul@199 | 63 | 0 1
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paul@199 | 64 | context object
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paul@199 | 65 | reference reference
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paul@199 | 66 |
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paul@199 | 67 | Specific implementations might reverse this ordering for optimisation
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paul@199 | 68 | purposes.
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paul@199 | 69 |
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paul@199 | 70 | Rationale
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paul@199 | 71 | ---------
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paul@199 | 72 |
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paul@199 | 73 | To reduce the number of created objects whilst retaining the ability to
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paul@199 | 74 | support bound method invocations. The context indicates the context in which
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paul@199 | 75 | an invocation is performed, typically the owner of the method.
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paul@199 | 76 |
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paul@199 | 77 | Usage
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paul@199 | 78 | -----
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paul@199 | 79 |
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paul@199 | 80 | The context may be inserted as the first argument when a value is involved in
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paul@199 | 81 | an invocation. This argument may then be omitted from the invocation if its
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paul@199 | 82 | usage is not appropriate.
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paul@199 | 83 |
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paul@199 | 84 | See invocation.txt for details.
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paul@199 | 85 |
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paul@237 | 86 | Context Value Types
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paul@237 | 87 | -------------------
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paul@237 | 88 |
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paul@237 | 89 | The following types of context value exist:
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paul@237 | 90 |
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paul@237 | 91 | Type Usage Transformations
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paul@237 | 92 | ---- ----- ---------------
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paul@237 | 93 |
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paul@237 | 94 | Replaceable With functions (not methods) May be replaced with an
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paul@237 | 95 | instance or a class when a
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paul@237 | 96 | value is stored on an
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paul@237 | 97 | instance or class
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paul@237 | 98 |
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paul@237 | 99 | Placeholder With classes May not be replaced
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paul@237 | 100 |
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paul@237 | 101 | Instance With instances (and constants) May not be replaced
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paul@237 | 102 | or functions as methods
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paul@237 | 103 |
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paul@237 | 104 | Class With functions as methods May be replaced when a
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paul@237 | 105 | value is loaded from a
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paul@237 | 106 | class attribute via an
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paul@237 | 107 | instance
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paul@237 | 108 |
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paul@199 | 109 | Contexts in Acquired Values
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paul@199 | 110 | ---------------------------
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paul@199 | 111 |
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paul@237 | 112 | There are four classes of instructions which provide values:
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paul@199 | 113 |
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paul@199 | 114 | Instruction Purpose Context Operations
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paul@199 | 115 | ----------- ------- ------------------
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paul@199 | 116 |
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paul@237 | 117 | 1) LoadConst Load module, constant Use loaded object with itself
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paul@237 | 118 | as context
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paul@199 | 119 |
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paul@237 | 120 | 2) LoadFunction Load function Combine replaceable context
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paul@237 | 121 | with loaded object
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paul@223 | 122 |
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paul@237 | 123 | 3) LoadClass Load class Combine placeholder context
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paul@237 | 124 | with loaded object
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paul@237 | 125 |
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paul@237 | 126 | 4) LoadAddress* Load attribute from Preserve or override stored
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paul@201 | 127 | LoadAttr* class, module, context (as described in
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paul@201 | 128 | instance assignment.txt)
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paul@199 | 129 |
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paul@199 | 130 | In order to comply with traditional Python behaviour, contexts may or may not
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paul@199 | 131 | represent the object from which an attribute has been acquired.
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paul@199 | 132 |
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paul@199 | 133 | See assignment.txt for details.
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paul@199 | 134 |
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paul@199 | 135 | Contexts in Stored Values
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paul@199 | 136 | -------------------------
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paul@199 | 137 |
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paul@223 | 138 | There are two classes of instruction for storing values:
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paul@199 | 139 |
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paul@223 | 140 | Instruction Purpose Context Operations
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paul@223 | 141 | ----------- ------- ------------------
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paul@199 | 142 |
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paul@223 | 143 | 1) StoreAddress Store attribute in a Preserve context; note that no
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paul@223 | 144 | known object test for class attribute
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paul@223 | 145 | assignment should be necessary
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paul@223 | 146 | since this instruction should only
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paul@223 | 147 | be generated for module globals
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paul@199 | 148 |
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paul@223 | 149 | StoreAttr Store attribute in an Preserve context; note that no
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paul@223 | 150 | instance test for class attribute
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paul@223 | 151 | assignment should be necessary
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paul@223 | 152 | since this instruction should only
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paul@223 | 153 | be generated for self accesses
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paul@199 | 154 |
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paul@223 | 155 | StoreAttrIndex Store attribute in an Preserve context; since the index
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paul@223 | 156 | unknown object lookup could yield a class
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paul@223 | 157 | attribute, a test of the nature of
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paul@223 | 158 | the nature of the structure is
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paul@223 | 159 | necessary in order to prevent
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paul@223 | 160 | assignments to classes
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paul@199 | 161 |
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paul@223 | 162 | 2) StoreAddressContext Store attribute in a Override context if appropriate;
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paul@237 | 163 | known object if the value has a replaceable
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paul@237 | 164 | context, permit the target to
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paul@237 | 165 | take ownership of the value
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paul@199 | 166 |
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paul@199 | 167 | See assignment.txt for details.
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paul@199 | 168 |
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paul@200 | 169 | Tables, Attributes and Lookups
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paul@200 | 170 | ==============================
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paul@199 | 171 |
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paul@199 | 172 | Attribute lookups, where the exact location of an object attribute is deduced,
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paul@199 | 173 | are performed differently in micropython than in other implementations.
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paul@199 | 174 | Instead of providing attribute dictionaries, in which attributes are found,
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paul@199 | 175 | attributes are located at fixed places in object structures (described below)
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paul@199 | 176 | and their locations are stored using a special representation known as a
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paul@199 | 177 | table.
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paul@199 | 178 |
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paul@199 | 179 | For a given program, a table can be considered as being like a matrix mapping
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paul@199 | 180 | classes to attribute names. For example:
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paul@199 | 181 |
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paul@199 | 182 | class A:
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paul@200 | 183 | # instances have attributes x, y
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paul@199 | 184 |
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paul@199 | 185 | class B(A):
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paul@200 | 186 | # introduces attribute z for instances
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paul@199 | 187 |
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paul@199 | 188 | class C:
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paul@200 | 189 | # instances have attributes a, b, z
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paul@199 | 190 |
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paul@200 | 191 | This would provide the following table, referred to as an object table in the
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paul@200 | 192 | context of classes and instances:
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paul@199 | 193 |
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paul@199 | 194 | Class/attr a b x y z
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paul@199 | 195 |
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paul@199 | 196 | A 1 2
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paul@199 | 197 | B 1 2 3
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paul@199 | 198 | C 1 2 3
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paul@199 | 199 |
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paul@199 | 200 | A limitation of this representation is that instance attributes may not shadow
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paul@199 | 201 | class attributes: if an attribute with a given name is not defined on an
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paul@199 | 202 | instance, an attribute with the same name cannot be provided by the class of
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paul@401 | 203 | the instance or any superclass of the instance's class. This impacts the
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paul@401 | 204 | provision of the __class__ attribute, as described below.
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paul@199 | 205 |
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paul@199 | 206 | The table can be compacted using a representation known as a displacement
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paul@200 | 207 | list (referred to as an object list in this context):
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paul@199 | 208 |
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paul@199 | 209 | Classes with attribute offsets
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paul@199 | 210 |
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paul@199 | 211 | classcode A
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paul@199 | 212 | attrcode a b x y z
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paul@199 | 213 |
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paul@199 | 214 | B
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paul@199 | 215 | a b x y z
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paul@199 | 216 |
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paul@199 | 217 | C
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paul@199 | 218 | a b x y z
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paul@199 | 219 |
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paul@199 | 220 | List . . 1 2 1 2 3 1 2 . . 3
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paul@199 | 221 |
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paul@199 | 222 | Here, the classcode refers to the offset in the list at which a class's
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paul@199 | 223 | attributes are defined, whereas the attrcode defines the offset within a
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paul@199 | 224 | region of attributes corresponding to a single attribute of a given name.
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paul@199 | 225 |
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paul@200 | 226 | Attribute Locations
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paul@200 | 227 | -------------------
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paul@200 | 228 |
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paul@394 | 229 | The locations stored in table/list elements are generally for instance
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paul@394 | 230 | attributes relative to the location of the instance, whereas those for class
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paul@394 | 231 | attributes and module attributes are generally absolute addresses. Thus, each
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paul@394 | 232 | occupied table cell has the following structure:
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paul@242 | 233 |
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paul@242 | 234 | attrcode, uses-absolute-address, address (or location)
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paul@200 | 235 |
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paul@247 | 236 | This could be given instead as follows:
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paul@247 | 237 |
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paul@247 | 238 | attrcode, is-class-or-module, location
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paul@247 | 239 |
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paul@247 | 240 | Since uses-absolute-address corresponds to is-class-or-module, and since there
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paul@247 | 241 | is a need to test for classes and modules to prevent assignment to attributes
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paul@247 | 242 | of such objects, this particular information is always required.
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paul@247 | 243 |
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paul@394 | 244 | The __class__ Attribute
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paul@394 | 245 | -----------------------
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paul@394 | 246 |
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paul@401 | 247 | In Python 2.x, at least with new-style classes, instances have __class__
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paul@401 | 248 | attributes which indicate the class from which they have been instantiated,
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paul@401 | 249 | whereas classes have __class__ attributes which reference the type class.
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paul@401 | 250 | With the object table, it is not possible to provide absolute addresses which
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paul@401 | 251 | can be used for both classes and instances, since this would result in classes
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paul@401 | 252 | and instances having the same class, and thus the class of a class would be
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paul@401 | 253 | the class itself.
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paul@397 | 254 |
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paul@401 | 255 | One solution is to use object-relative values in the table so that referencing
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paul@401 | 256 | the __class__ attribute of an instance produces a value which can be combined
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paul@401 | 257 | with an instance's address to yield the address of the attribute, which itself
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paul@401 | 258 | refers to the instance's class, whereas referencing the __class__ attribute of
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paul@401 | 259 | a class produces a similar object-relative value that is combined with the
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paul@401 | 260 | class's address to yield the address of the attribute, which itself refers to
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paul@401 | 261 | the special type class.
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paul@401 | 262 |
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paul@401 | 263 | Obviously, the above solution requires both classes and instances to retain an
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paul@401 | 264 | attribute location specifically to hold the value appropriate for each object
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paul@401 | 265 | type, whereas a scheme which omits the __class__ attribute on classes would be
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paul@401 | 266 | able to employ an absolute address in the table and maintain only a single
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paul@401 | 267 | address to refer to the class for all instances. The only problem with not
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paul@401 | 268 | providing a sensible __class__ attribute entry for classes would be the need
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paul@401 | 269 | for special treatment of __class__ to prevent inappropriate consultation of
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paul@401 | 270 | the table for classes.
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paul@394 | 271 |
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paul@247 | 272 | Comparing Tables as Matrices with Displacement Lists
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paul@247 | 273 | ----------------------------------------------------
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paul@247 | 274 |
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paul@247 | 275 | Although displacement lists can provide reasonable levels of compaction for
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paul@247 | 276 | attribute data, the element size is larger than that required for a simple
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paul@247 | 277 | matrix: the attribute code (attrcode) need not be stored since each element
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paul@247 | 278 | unambiguously refers to the availability of an attribute for a particular
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paul@247 | 279 | class or instance of that class, and so the data at a given element need not
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paul@247 | 280 | be tested for relevance to a given attribute access operation.
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paul@247 | 281 |
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paul@247 | 282 | Given a program with 20 object types and 100 attribute types, a matrix would
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paul@247 | 283 | occupy the following amount of space:
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paul@247 | 284 |
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paul@247 | 285 | number of object types * number of attribute types * element size
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paul@247 | 286 | = 20 * 100 * 1 (assuming that a single location is sufficient for an element)
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paul@247 | 287 | = 2000
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paul@247 | 288 |
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paul@247 | 289 | In contrast, given a compaction to 40% of the matrix size (without considering
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paul@247 | 290 | element size) in a displacement list, the amount of space would be as follows:
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paul@247 | 291 |
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paul@247 | 292 | number of elements * element size
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paul@247 | 293 | = 40% * (20 * 100) * 2 (assuming that one additional location is required)
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paul@247 | 294 | = 1600
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paul@247 | 295 |
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paul@247 | 296 | Consequently, the principal overhead of using a displacement list is likely to
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paul@247 | 297 | be in the need to check element relevance when retrieving values from such a
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paul@247 | 298 | list.
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paul@247 | 299 |
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paul@199 | 300 | Objects and Structures
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paul@199 | 301 | ======================
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paul@199 | 302 |
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paul@199 | 303 | As well as references, micropython needs to have actual objects to refer to.
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paul@199 | 304 | Since classes, functions and instances are all objects, it is desirable that
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paul@199 | 305 | certain common features and operations are supported in the same way for all
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paul@199 | 306 | of these things. To permit this, a common data structure format is used.
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paul@199 | 307 |
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paul@215 | 308 | Header.................................................... Attributes.................
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paul@200 | 309 |
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paul@401 | 310 | Identifier Identifier Address Identifier Size Object ...
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paul@199 | 311 |
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paul@401 | 312 | 0 1 2 3 4 5 6
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paul@401 | 313 | classcode attrcode/ invocation funccode size attribute ...
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paul@401 | 314 | instance reference reference
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paul@215 | 315 | status
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paul@199 | 316 |
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paul@206 | 317 | Classcode
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paul@206 | 318 | ---------
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paul@206 | 319 |
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paul@206 | 320 | Used in attribute lookup.
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paul@206 | 321 |
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paul@199 | 322 | Here, the classcode refers to the attribute lookup table for the object (as
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paul@200 | 323 | described above). Classes and instances share the same classcode, and their
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paul@200 | 324 | structures reflect this. Functions all belong to the same type and thus employ
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paul@200 | 325 | the classcode for the function built-in type, whereas modules have distinct
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paul@200 | 326 | types since they must support different sets of attributes.
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paul@199 | 327 |
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paul@206 | 328 | Attrcode
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paul@206 | 329 | --------
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paul@206 | 330 |
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paul@206 | 331 | Used to test instances for membership of classes (or descendants of classes).
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paul@206 | 332 |
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paul@242 | 333 | Since, in traditional Python, classes are only ever instances of some generic
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paul@242 | 334 | built-in type, support for testing such a relationship directly has been
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paul@207 | 335 | removed and the attrcode is not specified for classes: the presence of an
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paul@242 | 336 | attrcode indicates that a given object is an instance. In addition, support
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paul@242 | 337 | has also been removed for testing modules in the same way, meaning that the
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paul@242 | 338 | attrcode is also not specified for modules.
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paul@206 | 339 |
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paul@215 | 340 | See the "Testing Instance Compatibility with Classes (Attrcode)" section below
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paul@215 | 341 | for details of attrcodes.
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paul@214 | 342 |
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paul@213 | 343 | Invocation Reference
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paul@213 | 344 | --------------------
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paul@213 | 345 |
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paul@213 | 346 | Used when an object is called.
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paul@213 | 347 |
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paul@213 | 348 | This is the address of the code to be executed when an invocation is performed
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paul@213 | 349 | on the object.
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paul@213 | 350 |
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paul@215 | 351 | Funccode
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paul@215 | 352 | --------
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paul@213 | 353 |
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paul@215 | 354 | Used to look up argument positions by name.
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paul@213 | 355 |
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paul@215 | 356 | The strategy with keyword arguments in micropython is to attempt to position
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paul@215 | 357 | such arguments in the invocation frame as it is being constructed.
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paul@215 | 358 |
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paul@215 | 359 | See the "Parameters and Lookups" section for more information.
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paul@215 | 360 |
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paul@215 | 361 | Size
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paul@215 | 362 | ----
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paul@215 | 363 |
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paul@219 | 364 | Used to indicate the size of an object including attributes.
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paul@213 | 365 |
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paul@209 | 366 | Attributes
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paul@209 | 367 | ----------
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paul@209 | 368 |
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paul@209 | 369 | For classes, modules and instances, the attributes in the structure correspond
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paul@209 | 370 | to the attributes of each kind of object. For functions, however, the
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paul@209 | 371 | attributes in the structure correspond to the default arguments for each
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paul@209 | 372 | function, if any.
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paul@209 | 373 |
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paul@206 | 374 | Structure Types
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paul@206 | 375 | ---------------
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paul@206 | 376 |
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paul@199 | 377 | Class C:
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paul@199 | 378 |
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paul@401 | 379 | 0 1 2 3 4 5 6
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paul@401 | 380 | classcode (unused) __new__ funccode size attribute ...
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paul@401 | 381 | for C reference for reference
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paul@215 | 382 | instantiator
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paul@199 | 383 |
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paul@199 | 384 | Instance of C:
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paul@199 | 385 |
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paul@401 | 386 | 0 1 2 3 4 5 6
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paul@401 | 387 | classcode attrcode C.__call__ funccode size attribute ...
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paul@401 | 388 | for C for C reference for reference
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paul@215 | 389 | (if exists) C.__call__
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paul@199 | 390 |
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paul@200 | 391 | Function f:
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paul@199 | 392 |
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paul@401 | 393 | 0 1 2 3 4 5 6
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paul@401 | 394 | classcode attrcode code funccode size attribute ...
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paul@401 | 395 | for for reference (default)
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paul@401 | 396 | function function reference
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paul@200 | 397 |
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paul@200 | 398 | Module m:
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paul@200 | 399 |
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paul@401 | 400 | 0 1 2 3 4 5 6
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paul@401 | 401 | classcode attrcode (unused) (unused) (unused) attribute ...
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paul@401 | 402 | for m for m (global)
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paul@401 | 403 | reference
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paul@200 | 404 |
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paul@200 | 405 | The __class__ Attribute
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paul@200 | 406 | -----------------------
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paul@200 | 407 |
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paul@401 | 408 | All objects should support the __class__ attribute, and in most cases this is
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paul@401 | 409 | done using the object table, yielding a common address for all instances of a
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paul@401 | 410 | given class.
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paul@401 | 411 |
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paul@401 | 412 | Function: refers to the function class
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paul@401 | 413 | Instance: refers to the class instantiated to make the object
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paul@401 | 414 |
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paul@401 | 415 | The object table cannot support two definitions simultaneously for both
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paul@401 | 416 | instances and their classes. Consequently, __class__ access on classes must be
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paul@401 | 417 | tested for and a special result returned.
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paul@200 | 418 |
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paul@200 | 419 | Class: refers to the type class (type.__class__ also refers to the type class)
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paul@401 | 420 |
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paul@401 | 421 | For convenience, the first attribute of a class will be the common __class__
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paul@401 | 422 | attribute for all its instances. As noted above, direct access to this
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paul@401 | 423 | attribute will not be possible for classes, and a constant result will be
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paul@401 | 424 | returned instead.
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paul@200 | 425 |
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paul@203 | 426 | Lists and Tuples
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paul@203 | 427 | ----------------
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paul@203 | 428 |
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paul@203 | 429 | The built-in list and tuple sequences employ variable length structures using
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paul@203 | 430 | the attribute locations to store their elements, where each element is a
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paul@203 | 431 | reference to a separately stored object.
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paul@203 | 432 |
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paul@214 | 433 | Testing Instance Compatibility with Classes (Attrcode)
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paul@200 | 434 | ------------------------------------------------------
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paul@200 | 435 |
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paul@200 | 436 | Although it would be possible to have a data structure mapping classes to
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paul@200 | 437 | compatible classes, such as a matrix indicating the subclasses (or
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paul@200 | 438 | superclasses) of each class, the need to retain the key to such a data
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paul@200 | 439 | structure for each class might introduce a noticeable overhead.
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paul@200 | 440 |
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paul@200 | 441 | Instead of having a separate structure, descendant classes of each class are
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paul@200 | 442 | inserted as special attributes into the object table. This requires an extra
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paul@200 | 443 | key to be retained, since each class must provide its own attribute code such
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paul@200 | 444 | that upon an instance/class compatibility test, the code may be obtained and
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paul@200 | 445 | used in the object table.
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paul@200 | 446 |
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paul@200 | 447 | Invocation and Code References
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paul@200 | 448 | ------------------------------
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paul@200 | 449 |
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paul@200 | 450 | Modules: there is no meaningful invocation reference since modules cannot be
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paul@200 | 451 | explicitly called.
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paul@200 | 452 |
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paul@200 | 453 | Functions: a simple code reference is employed pointing to code implementing
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paul@200 | 454 | the function. Note that the function locals are completely distinct from this
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paul@200 | 455 | structure and are not comparable to attributes. Instead, attributes are
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paul@200 | 456 | reserved for default parameter values, although they do not appear in the
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paul@200 | 457 | object table described above, appearing instead in a separate parameter table
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paul@200 | 458 | described below.
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paul@200 | 459 |
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paul@200 | 460 | Classes: given that classes must be invoked in order to create instances, a
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paul@200 | 461 | reference must be provided in class structures. However, this reference does
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paul@200 | 462 | not point directly at the __init__ method of the class. Instead, the
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paul@200 | 463 | referenced code belongs to a special initialiser function, __new__, consisting
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paul@200 | 464 | of the following instructions:
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paul@200 | 465 |
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paul@200 | 466 | create instance for C
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paul@200 | 467 | call C.__init__(instance, ...)
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paul@200 | 468 | return instance
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paul@200 | 469 |
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paul@200 | 470 | Instances: each instance employs a reference to any __call__ method defined in
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paul@200 | 471 | the class hierarchy for the instance, thus maintaining its callable nature.
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paul@200 | 472 |
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paul@200 | 473 | Both classes and modules may contain code in their definitions - the former in
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paul@200 | 474 | the "body" of the class, potentially defining attributes, and the latter as
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paul@200 | 475 | the "top-level" code in the module, potentially defining attributes/globals -
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paul@200 | 476 | but this code is not associated with any invocation target. It is thus
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paul@200 | 477 | generated in order of appearance and is not referenced externally.
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paul@200 | 478 |
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paul@200 | 479 | Invocation Operation
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paul@200 | 480 | --------------------
|
paul@200 | 481 |
|
paul@200 | 482 | Consequently, regardless of the object an invocation is always done as
|
paul@200 | 483 | follows:
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paul@200 | 484 |
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paul@200 | 485 | get invocation reference from the header
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paul@200 | 486 | jump to reference
|
paul@200 | 487 |
|
paul@200 | 488 | Additional preparation is necessary before the above code: positional
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paul@200 | 489 | arguments must be saved in the invocation frame, and keyword arguments must be
|
paul@200 | 490 | resolved and saved to the appropriate position in the invocation frame.
|
paul@200 | 491 |
|
paul@200 | 492 | See invocation.txt for details.
|
paul@200 | 493 |
|
paul@200 | 494 | Parameters and Lookups
|
paul@200 | 495 | ======================
|
paul@200 | 496 |
|
paul@200 | 497 | Since Python supports keyword arguments when making invocations, it becomes
|
paul@200 | 498 | necessary to record the parameter names associated with each function or
|
paul@200 | 499 | method. Just as object tables record attributes positions on classes and
|
paul@200 | 500 | instances, parameter tables record parameter positions in function or method
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paul@200 | 501 | parameter lists.
|
paul@200 | 502 |
|
paul@200 | 503 | For a given program, a parameter table can be considered as being like a
|
paul@200 | 504 | matrix mapping functions/methods to parameter names. For example:
|
paul@200 | 505 |
|
paul@200 | 506 | def f(x, y, z):
|
paul@200 | 507 | pass
|
paul@200 | 508 |
|
paul@200 | 509 | def g(a, b, c):
|
paul@200 | 510 | pass
|
paul@200 | 511 |
|
paul@200 | 512 | def h(a, x):
|
paul@200 | 513 | pass
|
paul@200 | 514 |
|
paul@200 | 515 | This would provide the following table, referred to as a parameter table in
|
paul@200 | 516 | the context of functions and methods:
|
paul@200 | 517 |
|
paul@200 | 518 | Function/param a b c x y z
|
paul@200 | 519 |
|
paul@200 | 520 | f 1 2 3
|
paul@200 | 521 | g 1 2 3
|
paul@200 | 522 | h 1 2
|
paul@200 | 523 |
|
paul@233 | 524 | Confusion can occur when functions are adopted as methods, since the context
|
paul@233 | 525 | then occupies the first slot in the invocation frame:
|
paul@233 | 526 |
|
paul@233 | 527 | def f(x, y, z):
|
paul@233 | 528 | pass
|
paul@233 | 529 |
|
paul@233 | 530 | f(x=1, y=2, z=3) -> f(<context>, 1, 2, 3)
|
paul@233 | 531 | -> f(1, 2, 3)
|
paul@233 | 532 |
|
paul@233 | 533 | class C:
|
paul@233 | 534 | f = f
|
paul@233 | 535 |
|
paul@233 | 536 | def g(x, y, z):
|
paul@233 | 537 | pass
|
paul@233 | 538 |
|
paul@233 | 539 | c = C()
|
paul@233 | 540 |
|
paul@233 | 541 | c.f(y=2, z=3) -> f(<context>, 2, 3)
|
paul@233 | 542 | c.g(y=2, z=3) -> C.g(<context>, 2, 3)
|
paul@233 | 543 |
|
paul@200 | 544 | Just as with parameter tables, a displacement list can be prepared from a
|
paul@200 | 545 | parameter table:
|
paul@200 | 546 |
|
paul@200 | 547 | Functions with parameter (attribute) offsets
|
paul@200 | 548 |
|
paul@200 | 549 | funccode f
|
paul@200 | 550 | attrcode a b c x y z
|
paul@200 | 551 |
|
paul@200 | 552 | g
|
paul@200 | 553 | a b c x y z
|
paul@200 | 554 |
|
paul@200 | 555 | h
|
paul@200 | 556 | a b c x y z
|
paul@200 | 557 |
|
paul@200 | 558 | List . . . 1 2 3 1 2 3 1 . . 2 . .
|
paul@200 | 559 |
|
paul@200 | 560 | Here, the funccode refers to the offset in the list at which a function's
|
paul@200 | 561 | parameters are defined, whereas the attrcode defines the offset within a
|
paul@200 | 562 | region of attributes corresponding to a single parameter of a given name.
|
paul@200 | 563 |
|
paul@200 | 564 | Instantiation
|
paul@200 | 565 | =============
|
paul@200 | 566 |
|
paul@200 | 567 | When instantiating classes, memory must be reserved for the header of the
|
paul@200 | 568 | resulting instance, along with locations for the attributes of the instance.
|
paul@200 | 569 | Since the instance header contains data common to all instances of a class, a
|
paul@200 | 570 | template header is copied to the start of the newly reserved memory region.
|
paul@222 | 571 |
|
paul@222 | 572 | Register Usage
|
paul@222 | 573 | ==============
|
paul@222 | 574 |
|
paul@222 | 575 | During code generation, much of the evaluation produces results which are
|
paul@450 | 576 | implicitly recorded in the "active value" or "working" register, and various
|
paul@450 | 577 | instructions will consume this active value. In addition, some instructions
|
paul@450 | 578 | will consume a separate "active source value" from a register, typically those
|
paul@450 | 579 | which are assigning the result of an expression to an assignment target.
|
paul@222 | 580 |
|
paul@222 | 581 | Since values often need to be retained for later use, a set of temporary
|
paul@222 | 582 | storage locations are typically employed. However, optimisations may reduce
|
paul@222 | 583 | the need to use such temporary storage where instructions which provide the
|
paul@450 | 584 | "active value" can be re-executed and will produce the same result. Whether
|
paul@450 | 585 | re-use of instructions is possible or not, values still need to be loaded into
|
paul@450 | 586 | a "source" register to be accessed by an assignment instruction.
|
paul@450 | 587 |
|
paul@450 | 588 | RSVP instructions generally have the notion of working, target and source
|
paul@450 | 589 | registers (see registers.txt). These register "roles" are independent from the
|
paul@450 | 590 | actual registers defined for the RSVP machine itself, even though the naming
|
paul@450 | 591 | is similar. Generally, instructions do regard the RSVP "working" registers as
|
paul@450 | 592 | the class of register in the "working" role, although some occurrences of
|
paul@450 | 593 | instruction usage may employ other registers (such as the "result" registers)
|
paul@450 | 594 | and thus take advantage of the generality of the RSVP implementation of such
|
paul@450 | 595 | instructions.
|
paul@245 | 596 |
|
paul@245 | 597 | List and Tuple Representations
|
paul@245 | 598 | ==============================
|
paul@245 | 599 |
|
paul@245 | 600 | Since tuples have a fixed size, the representation of a tuple instance is
|
paul@245 | 601 | merely a header describing the size of the entire object, together with a
|
paul@245 | 602 | sequence of references to the object "stored" at each position in the
|
paul@245 | 603 | structure. Such references consist of the usual context and reference pair.
|
paul@245 | 604 |
|
paul@245 | 605 | Lists, however, have a variable size and must be accessible via an unchanging
|
paul@245 | 606 | location even as more memory is allocated elsewhere to accommodate the
|
paul@245 | 607 | contents of the list. Consequently, the representation must resemble the
|
paul@245 | 608 | following:
|
paul@245 | 609 |
|
paul@245 | 610 | Structure header for list (size == header plus special attribute)
|
paul@245 | 611 | Special attribute referencing the underlying sequence
|
paul@245 | 612 |
|
paul@245 | 613 | The underlying sequence has a fixed size, like a tuple, but may contain fewer
|
paul@245 | 614 | elements than the size of the sequence permits:
|
paul@245 | 615 |
|
paul@245 | 616 | Special header indicating the current size and allocated size
|
paul@245 | 617 | Element
|
paul@245 | 618 | ... <-- current size
|
paul@245 | 619 | (Unused space)
|
paul@245 | 620 | ... <-- allocated size
|
paul@245 | 621 |
|
paul@245 | 622 | This representation permits the allocation of a new sequence when space is
|
paul@245 | 623 | exhausted in an existing sequence, with the new sequence address stored in the
|
paul@245 | 624 | main list structure. Since access to the contents of the list must go through
|
paul@245 | 625 | the main list structure, underlying allocation activities may take place
|
paul@245 | 626 | without the users of a list having to be aware of such activities.
|