CP/M/CPM_CDROM.iso/simtel/sigm/vols000/vol023/disasmb.doc

Text File | 1984-04-29 | 10.3 KB | 270 lines

DISASMB -- A TABLE DRIVEN DISASSEMBLER WRITTEN IN MICROSOFT BASIC

This package consists of two main programs and a set of support programs
and files.

Among its features:

-- By loading assembler mnenomics tables at run-time, the program can
   disassemble into any assembly language for which tables are available.

-- Output can be sent to the console, printer and/or a disk file.

-- A cross-reference file can be created as part of disassembly,
   sorted, and listed on the printer and/or console.

-- Source code can be either a disk file or located in memory.

-- All 16-bit values are converted to labels, which permit relocation.

-- Every byte of code is disassembled. The program treats secondary code
   as a comment.

-- In addition to its mnenomic value, the hex value and ASCII equivalent
   of each byte is listed. This facilitates location of data and text
   blocks.

-- The program runs reasonably fast under the Basic interpreter and can
   presumably be compiled with the Microsoft Basic compiler, although
   this does require a few modifications of the program.

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RUNNING DISASMB.BAS

To begin with, you have to have Microsoft Basic-80 ver 5.1 or later,
since this program makes use of the long variable names facilitated
as part of ver 5.

The disassembler first requests the name of the assembly language to
be used. A .LST file, which contains the opcode and operand mnenomics,
and a .TAB file, which contains a table to define the names of each
byte, must be available under that name. Thus, if INTEL is specified,
the files INTEL.LST and INTEL.TAB must be found. Otherwise, an error
message will be printed and a new name will be requested.

Once the tables have been loaded, a menu is displayed and the user is
allowed to specify actions to be taken. The options available are:

-- END
   Self explanatory.

-- CONSOLE toggle
   PRINTER toggle
   Switching these will determine where generated listings will be
   displayed. If the printer is enabled, hitting "P" disables it,
   hitting "P" again re-enables it.

-- TABLE LOAD
   If you want to switch assembly languges you can do so. If the
   .LST file is not found an error message is issued and the old files
   remain loaded.

-- LIST OPCODES
   This generates a table showing the mnenomic equivalent for each
   opcode under the currently loaded assembly language tables. Note
   that since Zilog index codes are disassembled by testing for
   reference to the HL register, these codes are not listed in this
   table.

-- WRITE LISTING TO DISK
   The dissembled code is written to disk. The standard source file
   extension is provided as part of the .LST file, and this is used
   as a default value. If a cross-reference file has been specifed,
   this file name is used as a default as well. A new name and/or
   extension can, of course, be provided.

-- X-REF FILE GENERATION
   A cross-reference file is generated, listing each 16-bit value loaded,
   the current address and whether it is appears to be a primary or
   secondary [ie: not necessarily valid] value.

-- MEMORY DISASSEMBLE
   The first value requested is the starting address in memory for the
   program to be disassembled. A second program starting address is then
   required. The memory starting address is assumed, but if a different
   value is entered the diassembler assumes the program has been
   relocated in memory and acts accordingly. The final value to be
   entered is the program end address, relative to the program starting
   address, NOT the memory start.

-- DISK DISASSEMBLE
   This disassembles from a disk file. If a write or x-ref file has been
   specified that file name is assumed, as is the extension .COM. The
   program start is assumed to be at 100H, but a different value can
   be entered.

Miscellaneous notes:

To begin with, the disassembler has no real way of knowing what is
machine instruction and what is data. Similarly, it can not know
if a 16-bit value loaded into a register is an absolute value or a
memory reference. Thus, for a file to be re-assembled it will first
have to be edited. Some of this is easy -- blocks of text will stick
out in the ASCII column -- but determining whether a 16-bit value
should be be converted from a label to an absolute value, by removing the
the prefix "x", can prove a bit tricky, although the x-ref listings
should help.

It is also important to keep in mind that every byte of source code
generates 30-40 bytes of disassembled disk file. Which means that
even rather small source files can generate rather large output files.

Commands issued from the menu are entered via the INPUT$(1) command,
which means that simply hitting a key serves to enter the command.
By the time you hit a return it's too late.

When an assumed value is given, hitting a return means that it will
be used.

The menu is updated to show the status of each function, including the
names of any designated output disk files.

Output disk files are disabled at the end of any disassembly cycle
but console and printer status are not changed.

The program includes a convert-to-caps routine, so that data can be entered
as lower case if desired.

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USING XREF.BAS

Since this program is not table-dependent, the menu is displayed
immediately. The options available are:

-- CONSOLE TOGGLE
   PRINTER TOGGLE
   These work the same as with DISASMB.BAS.

-- SORT FILE
   This sorts an .XRF file generated by DISASMB and then prints it.

-- LIST FILE
   This takes a sorted file and simply lists it.

The sort program is a heap sort but under the interpreter this still
takes time. So a submit file, XREF.SUB, is provided to permit MicroPro's
SuperSort to do the deed with great dispatch.

The first column of the listing is the destination address, the second
column contains primary source addresses, the third column contains
secondary source addresses.

The listing is paginated and normally displays two columns to a page.
Both of these values can be changed by modifying indicated constants at
the start of the program.

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CREATING .LST AND .TAB FILES

This package contains four sets of files:

-- INTEL -- standard Intel mnenomics for the 8080 and CP/M assembler.

-- 8085 -- adds the SIM and RIM instructions to INTEL.

-- ZILOG -- standard Zilog mnenomics with extension .MAC for Macro-80.

-- TDL -- TDL's Intel-extended-to-Zilog mnenomics with extension .SRC
   for the InterSystems assembler.

I have included the TAB<name>.BAS and LST<name>.BAS programs which
generated these files in order to facilitate changes.

The .LST file contains the mnenomic names and miscellaneous values
for the disassemblers. The order is:
    ALEN -- The number of columns in the .TAB file. Standard
        Intel has 3, Zilog has 9.
    ZCOM -- The comment character/string, generally ";".
    ZLAB -- The label terminator string. This is optional but
        permissible to most assemblers, but Macro-80 apparently
        requires it.
    ZBYTE -- The byte pseudo-op.
    ZEXT -- The extension assumed by the assembler.
    AZIL -- Since the index register codes can be easily
        disassembled by testing for use of the HL registers,
        this approach was used. However, this requires
        language-dependent code, so this value tells the
        disassembler what, if any, procedures to use.
        The provided values are:
            0 - no index-register codes
            1 - standard Zilog mnenomics
            2 - TDL mnenomics
    Number of operands
    null operand
    1..n operands
    Number of opcodes
    null operand
    "N" operand
    "NN" operand
    "(NN)" operand
    relative displacement operand
    5..n non-reserved operands

The reserved operands must be allowed for, whether or not the language
provides for that instruction. Other than that, opcodes and operands
may be listed in any order.

The .TAB file contains the .LST position of the opcode and the first
and second operands for each byte. The values are stored as bytes.
New .LST and .TAB files can be checked using the list opcodes procedure
in DISASMB.

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FINAL BITS OF INFORMATION

[1] MBasic allows strings to be of an arbitrary length. There is a price,
however: When a string is created or changed it is simply tossed on top
of the string stack. From time to time this process exhausts available
memory and the intepreter has to stop to purge the stack of extraneous
strings. Thus, during the generation of a disassembler listing the
process will stall for a number of seconds to permit the purge.

[2] In order to conserve memory, the opcode, operand and table arrays
are dynamically redimensioned in DISASMB. If this program is to be
compiled using BASCOM, these have to be changed to absolute values.
The maximum values for the various tables are:

    name        opcodes        operands    table
    INTEL         80        22        2,255
    8085         80        22        2,255
    ZILOG         67        48        8,255
    TDL        136        22        8,255

[3] There are two ways which MBasic can tell whether it has reached
the end of an input file: (a) It is told by CP/M that it is on the
last sector of the current extent and that there are no further
last sector of the current extent and taht there are no further extents
or (b) it encounters a control-z code at the start of a logical block
(a line in a sequential file or a sector in a random file).

There are some problems with this approach. Since DISASMB reads source
files as a series of sector-long records it has been instructed to
continue reading if both EOF is true and the first byte on the sector
is a 1AH (control-z). This may cause the program to read past the end
of a file, but ultimately it will be forced to initialize a sector
which will result in a proper stop.

For .XRF files the end of file is indicated by two 0FFH bytes followed
by 1AH's. This is the FFZZ end flag for SuperSort.

[4] Disassembled disk files use horizontal tabs to separate columns in
order to compress the length of the files at least a little.

[5] MBasic clears the input buffer for normal data input but not for
INPUT$. With interrupt-based input, however, this means that it is
possible to accidently enter commands in response to the menu's prompt.

[6] XREF.BAS creates and then erases two files named XREFWORK.### and
XREFWORK.$$$. XREF.SUB creates and then erates XREFWORK.###.

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Scott Custin
2410 20th St NW #10
Washington DC 20009