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Port manual forward with corrections from Jason Law
author lost
date Sun, 28 Feb 2010 05:55:07 +0000
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1 LWASM Internals
2 ===============
3
4 LWASM is a table-driven assembler that notionally uses two passes. However,
5 it implements its assembly in several passes as follows.
6
7 Pass 1 - Preprocessing & Parsing
8 --------------------------------
9
10 This pass reads the source file and all included source files. It handles
11 macro definition and expansion.
12
13 As it reads the various lines, it also identifies any symbol associated with
14 the line, the operation code, and, based on the operation code, the operand,
15 if any. Upon examination of the operand, any expressions are stored in an
16 internal postfix notation for later evaluation. During this pass,
17 preliminary values are assigned to all symbols using the largest possible
18 instruction size. A table of lines that reference every symbol is generated
19 to be used in the following pass. Note that any symbols for which the value
20 is known with no uncertainty factor will be generated with the smallest
21 possible instruction.
22
23 At this stage, simple optimizations are performed on expressions. This
24 includes coalescing constants (1+2+x => 3+x). It also includes some basic
25 algebra (x+x => 2*x, 2*x+4*x => 6*x, x-x => 0).
26
27 Pass 2 - Optimization
28 ---------------------
29
30 This pass sweeps the code looking for operations which could use a shorter
31 instruction. If it finds one, it must then re-define all symbols defined
32 subsequently and all symbols defined in terms of one of those symbols in a
33 cascade. This process is repeated until no more possible reductions are
34 discovered.
35
36 If, in the process of implementing an instruction reduction, a phasing error
37 or other conflict is encountered, the reduction is backed out and marked as
38 forced.
39
40 The following may be candidates for reduction, depending on assembler
41 options:
42
43 - extended addressing -> direct addressing (not in obj target)
44 - 16 bit offset -> 8 bit offset (indirect indexed)
45 - 16 bit offset -> 8 bit or 5 bit offset (direct indexed)
46 - 16 bit offset -> no offset (indexed)
47 - 16 bit relative -> 8 bit relative (depending on configuration)
48
49