3
My initial idea for the disassembler was to do everything in one pass, but
4
I've decided to change to a two-pass approach. One pass would require lots
5
of extra memory to store everything I want to output.
7
Keep track of flags for every address. Possible values are EMPTY, CODE,
8
DATA, and LABELLED. Everything starts out as EMPTY. Everything loaded from
9
the hex file is DATA by default.
11
Pass 1: Start at entry point 0000. Follow code through all branches. Note
12
any change to interrupt vectors. Decode interrupt vector entry points as
15
As I go, mark all decoded addresses as CODE. Mark all jump destinations as
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LABELLED. Give them a label number.
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Pass 2: Start at address 0000. Output "CSEG AT 0000h". Increment through
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"memory" to address 65535. Everything marked as LABELLED gets a label
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looked up from a label table. Everything marked as CODE gets decoded and
21
output. Everything marked as DATA gets a DB command. If EMPTY is
22
encountered, the next non-EMPTY code or data is preceded by at CSEG AT
25
Output "END" at the end.
29
The pass1/pass2 thing worked well. The one "issue" the program has is with
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indirect jumps (JMP @A+DPTR). There is no way to "follow" the code with an
31
indirect jump. It also seems that at least one 8051 C compiler (Keil
32
uVision 1) generates a number of these indirect jumps. My program will take
33
multiple entry points on the command line, so someone could examine the
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assembly output by hand, and make an educated guess about where the jump
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table begins, and then re-disassemble the program with the additional entry
38
I decode SFR addresses, both in byte- and bit-addressed memory, into their
39
symbolic names. This helps make the assembly output a bit more readable. I
40
think the only feature to add is one that examines writes to the interrupt
41
registers and makes a guess about which interrupt vectors are being used.
added
removed
NOTES
