| 1 | dis88
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| 2 | Beta Release
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| 3 | 87/09/01
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| 4 | ---
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| 5 | G. M. HARDING
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| 6 | POB 4142
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| 7 | Santa Clara CA 95054-0142
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| 8 |
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| 9 |
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| 10 | "Dis88" is a symbolic disassembler for the Intel 8088 CPU,
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| 11 | designed to run under the PC/IX operating system on an IBM XT
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| 12 | or fully-compatible clone. Its output is in the format of, and
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| 13 | is completely compatible with, the PC/IX assembler, "as". The
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| 14 | program is copyrighted by its author, but may be copied and re-
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| 15 | distributed freely provided that complete source code, with all
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| 16 | copyright notices, accompanies any distribution. This provision
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| 17 | also applies to any modifications you may make. You are urged
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| 18 | to comment such changes, giving, as a miminum, your name and
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| 19 | complete address.
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| 20 |
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| 21 | This release of the program is a beta release, which means
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| 22 | that it has been extensively, but not exhaustively, tested.
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| 23 | User comments, recommendations, and bug fixes are welcome. The
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| 24 | principal features of the current release are:
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| 25 |
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| 26 | (a) The ability to disassemble any file in PC/IX object
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| 27 | format, making full use of symbol and relocation information if
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| 28 | it is present, regardless of whether the file is executable or
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| 29 | linkable, and regardless of whether it has continuous or split
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| 30 | I/D space;
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| 31 |
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| 32 | (b) Automatic generation of synthetic labels when no sym-
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| 33 | bol table is available; and
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| 34 |
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| 35 | (c) Optional output of address and object-code informa-
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| 36 | tion as assembler comment text.
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| 37 |
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| 38 | Limitations of the current release are:
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| 39 |
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| 40 | (a) Numeric co-processor (i.e., 8087) mnemonics are not
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| 41 | supported. Instructions for the co-processor are disassembled
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| 42 | as CPU escape sequences, or as interrupts, depending on how
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| 43 | they were assembled in the first place. This limitation will be
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| 44 | addressed in a future release.
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| 45 |
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| 46 | (b) Symbolic references within the object file's data
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| 47 | segment are not supported. Thus, for example, if a data segment
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| 48 | location is initialized to point to a text segment address, no
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| 49 | reference to a text segment symbol will be detected. This limi-
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| 50 | tation is likely to remain in future releases, because object
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| 51 | code does not, in most cases, contain sufficient information to
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| 52 | allow meaningful interpretation of pure data. (Note, however,
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| 53 | that symbolic references to the data segment from within the
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| 54 | text segment are always supported.)
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| 55 |
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| 56 | As a final caveat, be aware that the PC/IX assembler does
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| 57 | not recognize the "esc" mnemonic, even though it refers to a
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| 58 | completely valid CPU operation which is documented in all the
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| 59 | Intel literature. Thus, the corresponding opcodes (0xd8 through
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| 60 | 0xdf) are disassembled as .byte directives. For reference, how-
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| 61 | ever, the syntactically-correct "esc" instruction is output as
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| 62 | a comment.
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| 63 |
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| 64 | To build the disassembler program, transfer all the source
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| 65 | files, together with the Makefile, to a suitable (preferably
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| 66 | empty) PC/IX directory. Then, simply type "make".
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| 67 |
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| 68 | To use dis88, place it in a directory which appears in
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| 69 | your $PATH list. It may then be invoked by name from whatever
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| 70 | directory you happen to be in. As a minimum, the program must
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| 71 | be invoked with one command-line argument: the name of the ob-
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| 72 | ject file to be disassembled. (Dis88 will complain if the file
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| 73 | specified is not an object file.) Optionally, you may specify
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| 74 | an output file; stdout is the default. One command-line switch
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| 75 | is available: "-o", which makes the program display addresses
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| 76 | and object code along with its mnemonic disassembly.
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| 77 |
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| 78 | The "-o" option is useful primarily for verifying the cor-
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| 79 | rectness of the program's output. In particular, it may be used
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| 80 | to check the accuracy of local relative jump opcodes. These
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| 81 | jumps often target local labels, which are lost at assembly
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| 82 | time; thus, the disassembly may contain cryptic instructions
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| 83 | like "jnz .+39". As a user convenience, all relative jump and
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| 84 | call opcodes are output with a comment which identifies the
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| 85 | physical target address.
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| 86 |
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| 87 | By convention, the release level of the program as a whole
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| 88 | is the SID of the file disrel.c, and this SID string appears in
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| 89 | each disassembly. Release 2.1 of the program is the first beta
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| 90 | release to be distributed on Usenet.
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| 91 |
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| 92 |
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| 93 | .TH dis88 1 LOCAL
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| 94 | .SH "NAME"
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| 95 | dis88 \- 8088 symbolic disassembler
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| 96 | .SH "SYNOPSIS"
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| 97 | \fBdis88\fP [ -o ] ifile [ ofile ]
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| 98 | .SH "DESCRIPTION"
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| 99 | Dis88 reads ifile, which must be in PC/IX a.out format.
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| 100 | It interprets the binary opcodes and data locations, and
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| 101 | writes corresponding assembler source code to stdout, or
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| 102 | to ofile if specified. The program's output is in the
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| 103 | format of, and fully compatible with, the PC/IX assembler,
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| 104 | as(1). If a symbol table is present in ifile, labels and
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| 105 | references will be symbolic in the output. If the input
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| 106 | file lacks a symbol table, the fact will be noted, and the
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| 107 | disassembly will proceed, with the disassembler generating
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| 108 | synthetic labels as needed. If the input file has split
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| 109 | I/D space, or if it is executable, the disassembler will
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| 110 | make all necessary adjustments in address-reference calculations.
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| 111 | .PP
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| 112 | If the "-o" option appears, object code will be included
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| 113 | in comments during disassembly of the text segment. This
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| 114 | feature is used primarily for debugging the disassembler
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| 115 | itself, but may provide information of passing interest
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| 116 | to users.
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| 117 | .PP
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| 118 | The program always outputs the current machine address
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| 119 | before disassembling an opcode. If a symbol table is
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| 120 | present, this address is output as an assembler comment;
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| 121 | otherwise, it is incorporated into the synthetic label
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| 122 | which is generated internally. Since relative jumps,
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| 123 | especially short ones, may target unlabelled locations,
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| 124 | the program always outputs the physical target address
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| 125 | as a comment, to assist the user in following the code.
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| 126 | .PP
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| 127 | The text segment of an object file is always padded to
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| 128 | an even machine address. In addition, if the file has
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| 129 | split I/D space, the text segment will be padded to a
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| 130 | paragraph boundary (i.e., an address divisible by 16).
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| 131 | As a result of this padding, the disassembler may produce
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| 132 | a few spurious, but harmless, instructions at the
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| 133 | end of the text segment.
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| 134 | .PP
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| 135 | Disassembly of the data segment is a difficult matter.
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| 136 | The information to which initialized data refers cannot
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| 137 | be inferred from context, except in the special case
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| 138 | of an external data or address reference, which will be
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| 139 | reflected in the relocation table. Internal data and
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| 140 | address references will already be resolved in the object file,
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| 141 | and cannot be recreated. Therefore, the data
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| 142 | segment is disassembled as a byte stream, with long
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| 143 | stretches of null data represented by an appropriate
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| 144 | ".zerow" pseudo-op. This limitation notwithstanding,
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| 145 | labels (as opposed to symbolic references) are always
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| 146 | output at appropriate points within the data segment.
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| 147 | .PP
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| 148 | If disassembly of the data segment is difficult, disassembly of the
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| 149 | bss segment is quite easy, because uninitialized data is all
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| 150 | zero by definition. No data
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| 151 | is output in the bss segment, but symbolic labels are
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| 152 | output as appropriate.
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| 153 | .PP
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| 154 | For each opcode which takes an operand, a particular
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| 155 | symbol type (text, data, or bss) is appropriate. This
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| 156 | tidy correspondence is complicated somewhat, however,
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| 157 | by the existence of assembler symbolic constants and
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| 158 | segment override opcodes. Therefore, the disassembler's
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| 159 | symbol lookup routine attempts to apply a certain amount
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| 160 | of intelligence when it is asked to find a symbol. If
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| 161 | it cannot match on a symbol of the preferred type, it
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| 162 | may return a symbol of some other type, depending on
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| 163 | preassigned (and somewhat arbitrary) rankings within
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| 164 | each type. Finally, if all else fails, it returns a
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| 165 | string containing the address sought as a hex constant;
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| 166 | this behavior allows calling routines to use the output
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| 167 | of the lookup function regardless of the success of its
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| 168 | search.
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| 169 | .PP
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| 170 | It is worth noting, at this point, that the symbol lookup
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| 171 | routine operates linearly, and has not been optimized in
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| 172 | any way. Execution time is thus likely to increase
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| 173 | geometrically with input file size. The disassembler is
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| 174 | internally limited to 1500 symbol table entries and 1500
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| 175 | relocation table entries; while these limits are generous
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| 176 | (/unix, itself, has fewer than 800 symbols), they are not
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| 177 | guaranteed to be adequate in all cases. If the symbol
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| 178 | table or the relocation table overflows, the disassembly
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| 179 | aborts.
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| 180 | .PP
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| 181 | Finally, users should be aware of a bug in the assembler,
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| 182 | which causes it not to parse the "esc" mnemonic, even
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| 183 | though "esc" is a completely legitimate opcode which is
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| 184 | documented in all the Intel literature. To accommodate
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| 185 | this deficiency, the disassembler translates opcodes of
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| 186 | the "esc" family to .byte directives, but notes the
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| 187 | correct mnemonic in a comment for reference.
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| 188 | .PP
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| 189 | In all cases, it should be possible to submit the output
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| 190 | of the disassembler program to the assembler, and assemble
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| 191 | it without error. In most cases, the resulting object
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| 192 | code will be identical to the original; in any event, it
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| 193 | will be functionally equivalent.
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| 194 | .SH "SEE ALSO"
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| 195 | adb(1), as(1), cc(1), ld(1).
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| 196 | .br
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| 197 | "Assembler Reference Manual" in the PC/IX Programmer's
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| 198 | Guide.
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| 199 | .SH "DIAGNOSTICS"
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| 200 | "can't access input file" if the input file cannot be
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| 201 | found, opened, or read.
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| 202 | .sp
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| 203 | "can't open output file" if the output file cannot be
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| 204 | created.
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| 205 | .sp
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| 206 | "warning: host/cpu clash" if the program is run on a
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| 207 | machine with a different CPU.
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| 208 | .sp
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| 209 | "input file not in object format" if the magic number
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| 210 | does not correspond to that of a PC/IX object file.
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| 211 | .sp
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| 212 | "not an 8086/8088 object file" if the CPU ID of the
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| 213 | file header is incorrect.
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| 214 | .sp
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| 215 | "reloc table overflow" if there are more than 1500
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| 216 | entries in the relocation table.
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| 217 | .sp
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| 218 | "symbol table overflow" if there are more than 1500
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| 219 | entries in the symbol table.
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| 220 | .sp
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| 221 | "lseek error" if the input file is corrupted (should
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| 222 | never happen).
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| 223 | .sp
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| 224 | "warning: no symbols" if the symbol table is missing.
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| 225 | .sp
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| 226 | "can't reopen input file" if the input file is removed
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| 227 | or altered during program execution (should never happen).
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| 228 | .SH "BUGS"
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| 229 | Numeric co-processor (i.e., 8087) mnemonics are not currently supported.
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| 230 | Instructions for the co-processor are
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| 231 | disassembled as CPU escape sequences, or as interrupts,
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| 232 | depending on how they were assembled in the first place.
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| 233 | .sp
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| 234 | Despite the program's best efforts, a symbol retrieved
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| 235 | from the symbol table may sometimes be different from
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| 236 | the symbol used in the original assembly.
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| 237 | .sp
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| 238 | The disassembler's internal tables are of fixed size,
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| 239 | and the program aborts if they overflow.
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