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1 LWASM Internals
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2 ===============
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3
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4 LWASM is a table-driven assembler that notionally uses two passes. However,
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5 it implements its assembly in several passes as follows.
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6
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7 Pass 1 - Preprocessing & Parsing
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8 --------------------------------
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9
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10 This pass reads the source file and all included source files. It handles
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11 macro definition and expansion.
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12
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13 As it reads the various lines, it also identifies any symbol associated with
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14 the line, the operation code, and, based on the operation code, the operand,
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15 if any. Upon examination of the operand, any expressions are stored in an
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16 internal postfix notation for later evaluation. During this pass,
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17 preliminary values are assigned to all symbols using the largest possible
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18 instruction size. A table of lines that reference every symbol is generated
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19 to be used in the following pass. Note that any symbols for which the value
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20 is known with no uncertainty factor will be generated with the smallest
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21 possible instruction.
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22
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23 At this stage, simple optimizations are performed on expressions. This
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24 includes coalescing constants (1+2+x => 3+x). It also includes some basic
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25 algebra (x+x => 2*x, 2*x+4*x => 6*x, x-x => 0).
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26
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27 Pass 2 - Optimization
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28 ---------------------
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29
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30 This pass sweeps the code looking for operations which could use a shorter
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31 instruction. If it finds one, it must then re-define all symbols defined
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32 subsequently and all symbols defined in terms of one of those symbols in a
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33 cascade. This process is repeated until no more possible reductions are
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34 discovered.
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35
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36 If, in the process of implementing an instruction reduction, a phasing error
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37 or other conflict is encountered, the reduction is backed out and marked as
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38 forced.
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39
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40 The following may be candidates for reduction, depending on assembler
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41 options:
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42
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43 - extended addressing -> direct addressing (not in obj target)
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44 - 16 bit offset -> 8 bit offset (indirect indexed)
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45 - 16 bit offset -> 8 bit or 5 bit offset (direct indexed)
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46 - 16 bit offset -> no offset (indexed)
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47 - 16 bit relative -> 8 bit relative (depending on configuration)
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48
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49
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