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When I started to learn Machine Code,
I desperately needed a DISASSEMBLER to investigate and study the
Machine Code Programs written by the Advanced ATARI users. The only
Disassembler program I could find, wasn't a very helpful and
detailing one, but I had to manage with it till I reached a certain
knowledge level. I then felt I had to design a better and more
detailing program, and now I believe that the DISASSEMBLER program
below is a very good one for Machine Code beginners and advanced
programmers. It is fast and can detail Addressing Modes, and can
address the backward and forward branches.
Before running the Program please
save it to Disk as "D:DIS.ASM" or to cassette and then RUN
it.
Structure of Disassembler
Program
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A)
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Strings and
Variables
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| M$
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DIMensioned (1024)
originating from 255 possibilities in a 6502 Machine Code
System Byte. (255*4+4=1024) Adding 4 to multiplication 255*4,
is the last 4 locations of 255th place. |
| LCT$ |
DIMensioned (256)
based on same reason explained above. Since 0th place can not
be called in a String, for 255 possibilities LCT$ is
DIMensioned to 256 places to be used as LCT$(1,256) instead of
LCT$(0,255). |
| TR$ |
Transfer String to
be used in specific location transfer from M$. |
| LOC1 |
Content of Main
ADDRESS, 1st location. |
| LOC2 |
Content of 2nd
location. |
| LOC3 |
Content of 3rd
location. |
| LOC4 |
Used in Branches.
Backward branching. |
| LOC5 |
Used in Branches.
Forward branching. |
| ADRS |
Main Address with
MNEmonic. |
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B)
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Contents of
Strings
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M$
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Consisted of 255
main MNEmonics, each 4 locations long. First 3 locations hold
the Mnemonic. 4th location classifies the Mnemonic.
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"#"
Immediate
" " (Void) Absolute or Zero Page
"a"
Absolute or Zero Page
"b" Absolute,Y
"c" (indirect,X)
"d" (Indirect),Y
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LCT$
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Total of 256
codes that classifies each Addressing mode in detail,
separates Absolute and Zero Page, branching Mnemonics, Implied
and Relative addresses.
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"1" 1 byte long,
Implied or Relative
"2" 2 bytes long, Zero Page
"3" 3 bytes long, Absolute.
"4" Branchers, 1 byte long, Relative.
"5" No MNEmonic, 1 byte long.
"6" JMP (Arg), Implied, 1 byte long, location 108
only. |
Combination of M$ and LCT$ specifies
Addressing Mode very clearly. For example: Void " " in 4th
place of M$ with 2 code in LCT$, specifies a ZERO PAGE, 2 BYTE LONG
ADDRESSING MODE.
How The Program
Flows
| Up to 130 |
DIMensioning,
Opening a Channel and text Graphic plus screen and border
colour. |
| GOSUB 800 |
This section holds
M$ and LCT$. I could keep these values in DATA and use
READ-DATA-POKE combination but this routine is boring when you
hurry for a quick start |
| 130-200 |
Head line, start
Address, End Address and check address. |
| 200-210 |
Start of 20 line per
screen Disassembling. |
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220-280
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Content of ADRS. Locates exact place in M$. Get the 4 place
long string. Get the classifying Code from LCT$ string, exactly the
same location. Print address and content of Address.
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290
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According to the value
of TR, GOTO to the matching section.
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400-450
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1 byte long Mode section. Determines the exact Addressing
Mode, Implied or BRK-Implied
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450-500
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2 byte long Mode section. Determines Zero Page or Absolute.
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500-550
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3 byte long Mode Section. Determines Absolute or
Absolute,X.
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550-600
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Branching Modes only. Determines forward branch or Backward
branch.
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600-650
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1 byte long but No
MNEmonics, abbreviated as "? "
in M$ string.
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650-700
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JMP(Arg), Implied Mode only, which is a special jump.
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700-760
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Final check and request to continue. If END of address
reached, stops the execution.
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1000
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Gosub section for Determining very quickly the Addressing
Modes.
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