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Updated internals docs
author lost
date Fri, 12 Mar 2010 06:01:47 +0000
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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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7 Pass 1
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8 ------
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10 This pass reads the entire source code and parses each line into an internal
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11 representation. Macros, file inclusions, and conditional assembly
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12 instructions are resolved at this point as well.
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14 Pass 2
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15 ------
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17 This pass assigns instruction sizes to all invariate instructions. Invariate
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18 instructions are any instructions with a fixed size, including those with
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19 forced addressing modes.
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21 Pass 3
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22 ------
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24 This pass resolves all instruction sizes that can be resolved without
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25 setting addresses for instructions. This process is repeated until no
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26 further instructions sizes are resolved.
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28 Pass 4
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29 ------
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31 This pass assigns addresses to all symbols where values are known. It does
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32 the same for instructions. Then a repeat of similar algorithms as in the
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33 previous pass is used to resolve as many operands as possible.
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35 This pass is repeated multiple times until no further instructions or
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36 symbols are resolved.
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38 Pass 5
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39 ------
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41 Finalization of all instruction sizes by forcing them to the maximum
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42 addressing mode. Then all remaining instruction addresses and symbol values
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43 are resolved.
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45 Pass 6
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46 ------
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48 This pass does actual code generation. This is the notional second pass. All
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49 other passes are the notional first pass.
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52 Expression Evaluation
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53 =====================
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55 Each expression carries a certainty flag. Any expression in which any term
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56 is flagged as uncertain is, itself, uncertain. There are a few specific
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57 cases where such uncertainty can cancel out. For instance, X-X where X is
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58 uncertain is guaranteed to be 0 and so there is no uncertainty.
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