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Text File | 1986-08-30 | 19.3 KB | 551 lines

dis86 - Interactive 8086 Disassembler James R. Van Zandt SYNOPSIS Dis86 is a full-screen, interactive disassembler of object code for the 8086, 8087, 8088, 80186, 80286, and 80386 (products of Intel), and the V20 and V30 (products of NEC). The 80386 disassemblies include 32 bit operands and addresses. Dis86 implements the concept of a "current location" and allows use of the cursor keys to change it. Code can come from a .EXE file (in which case the header is properly interpreted), any other file (assumed to have no header), or anywhere in main memory (0000:0000 - F000:FFFF). Dis86 can install changes, even in an .EXE file, making it a convenient way to install patches. Versions are available for the IBM PC (and clones) and Z-100. STARTING THE DISASSEMBLER To disassemble a file, give the file name (optionally preceded by a path name) on the command line: A>dis86 foo.exe To disassemble from RAM, use an empty command line: A>dis86 There are no command line switches. FILE HEADER INFORMATION For a .EXE file, the information in the file header will be displayed when the program is first run and in response to the H command (see below). DISPLAY SCREEN During disassembly, the screen will resemble the following: 0000:0100 e9 01 90 jmp 9104 0000:0103 55 push bp 0000:0104 8b ec mov bp,sp 0000:0106 83 ec 0e sub sp,0e ... 0000:012C 50 push ax 0000:012D b8 69 00 mov ax,0069 0000:0130 50 push ax 0000:0131 e8 e9 5c call 5e1d dis86 1.00 - A SHAREWARE software product (c) 1986, James R. Van Zandt > ... 0000:0100 0000:0100 0000:0100 Lines 1 through 21 are the disassembled code. Each line starts with the current address, followed by the actual bytes being disassembled. The rest of the line is the assembly language equivalent, if any, of the code. The display for A (ASCII), B (byte), and D (data) formats is similar. All numbers are shown in hexadecimal. Line 22 is a message and prompt line showing, for example, the arguments needed for some commands. Line 23 has the prompt. Typed characters are echoed on the rest of this line. Line 24 has three addresses, which are the first three entries in the stack (see the 'cursor right' and 'cursor left' commands below). CURSOR KEYS The "current location" is the address displayed on the first line of disassembly. The cursor keys are used to adjust the current location. The up and down cursor keys (8 and 2 on the numeric pad) are used to move the current location a small amount (note that they are not inverses): <up> moves up by one byte (lower address) <down> moves down by one line (higher address) The <pg up> and <pg dn> keys (9 and 3 on the numeric pad) move the current location by larger amounts. (These will not move the cursor out of the disassembly buffer. Otherwise, they are inverses.): <pg up> moves up by 32 bytes (lower address) <pg dn> moves down by 32 bytes (higher address) The above keys change only the current location. Other commands change the current location by potentially large amounts, but first save it in a stack. The first three addresses in the stack are shown on the command line at the bottom of the screen. If the instruction at the current location is a jump, call, or a reference to a data location, the cursor right key (6 on the numeric pad) will push the current location on the stack and go to the referenced location. For a data reference, the disassembly format is changed to D (hex and ASCII). <right> follows a jump, call, or data reference The cursor left or left arrow key (4 on the numeric pad) will pop the last address off the stack. Note that right arrow followed by left arrow will return you to the same address, whereas left arrow (returning, let us say, to address X) followed by right arrow will only return you to the same address if there is an appropriate jump or call at X. <left> pops address stack Aûfter using the right arrow or one of the commands A, B, C, D, or G (in next section) to go to a new address, and using the left arrow key to pop the stack, you will sometimes want to return to the previous address. The stack no longer holds the address. However, the left arrow key saves the current location in a special "previous state" before popping the stack. To return to the address stored in the "previous state", type shift right arrow on a Z-100, or control right arrow on an IBM PC. <shift><right> returns to "previous state" (Z-100) <cntrl><right> returns to "previous state" (IBM) In summary, the unshifted keys on the numeric pad are: <home> top of file ^ up 1 byte <pg up> up 32 bytes | <-- pop addr stack --> follow jump/call | <end> end of file v down 1 line <pg dn> down 32 bytes <ins> setup options On the Z-100, the four keys with arrows on them may be used in addition to the 2, 4, 6, and 8 on the numeric pad. LETTER COMMANDS FOR MOVING THE CURSOR There are five letter commands to change the display format and/or disassembly address: A ASCII data B byte data (hex) D data (both hex bytes and ASCII) C code G goto These commands may be in upper or lower case. Each may be followed by: <ret> Only the display format changes. A <expression> <ret> The current location changes to the specified address. S <expression> <expression> <expression> <ret> The disassembler searches from the current address to the end of the buffer for the specified sequence of hex bytes. If an expression has a segment specified using the ':' operator (below), the segment is ignored. S T [string] <ret> The disassembler searches from the current address to the end of the buffer for the specified ASCII string. Cases are not distinct, and the high order bit is ignored. The string can also be introduced by a double quote. S R <expression> <ret> The disassembler searches from the current address to the end of the buffer for a reference (jump or call) to the specified address. An <expression> can involve any of these items: hex numbers (either upper or lower case letters) cs, ds, es, ss, fs, gs currently assumed segment register values $ current location @ offset of top address on the stack ...and any of these operators: + - * / add, subtract, multiply, divide : separate segment and offset Note that G with no address is a noop. OPTIONS The 'O' command or <ins> (0 on the numeric pad) brings up menus for changing setup options and allows the user to reset the disassembly window. Use <space> or <esc> to move to the next screen. The first menu allows the user to select the processor which is supposed to execute the code. There is some conflict in op codes between the V20 and V30 on one hand and the 80286 and 80386 on the other. That is, the two families use the same op codes for different instructions. Dis86 selects the instruction appropriate for the chip shown in this menu. In addition, instructions not implemented by the indicated chip will be flagged. The other item on the first menu lets the user specify 16 or 32 bit mode for the 80386. In the 16 bit mode the 80386 is similar to the 8086. In the 32 bit mode arithmetic is performed in 32 bit registers and all address offsets are 32 bits. (The 80386 itself selects the mode based on a bit in the segment table entry for the code segment.) The second menu allows the user to indicate the byte value which matches any byte in a byte or character search (the "wild card" byte) and select the number of bytes displayed on each line for the A, B, or D formats. The latter value can also be set using the W command. The last options display is a small map of the code being disassembled which will resemble the following: ds= -10 cs=0000 | ss=0960 es= -10 | | cursor=0000:0453 | CCCCCCCCCCCCCCcccccccccccccc ^0000:0000 ^0000:6144 The Cs represent the code being disassembled. The capital Cs are the portion of code in the disassembly window (see discussion below). The assumed values for the segment registers, the current location (labeled "cursor"), and the beginning and end addresses of the disassembly window are also shown. The window can be adjusted using the right and left cursor keys. By entering the options menu with the <ins> key and stepping from one menu to the next with <ret>, you can leave your right hand on the numeric pad. MISCELLANEOUS COMMANDS The 'P' command is used to print a disassembly listing to a file. The first time this command is used, it prompts for a file name. The default file name is "printout". To actually send the listing to a printer, specify the filename "prn". If the file already exists the new information will be appended. The file is automatically closed before the disassembler exits. The command also prompts for the beginning and end addresses of the code to be printed. The default addresses print the current screen. When the printing is finished, the current address is advanced to the first byte not printed. Thus, you can repeat the sequence P <ret> <ret> to print a large section. Enter 'R' to display and/or change the assumed segment register values. Entries may be full expressions. For example, to copy the value from SS into DS, use the cursor keys to select the DS register and type "ss". The 'S' command selects a new segment register value for displaying addresses. The new register is shown on the message line. The actual address being disassembled is not changed (see "segmentation" below). The 'W' command is used to set the number of bytes displayed on each line for the A, B, and D formats. This is useful for displaying tables. For example, when dis86 is executed without a file, it displays bytes starting at address 0000:0000 and the width is set to four so each interrupt vector is shown on a separate line. Type '?' to get a series of help screens. Type <esc> to return to the disassembly, or any other key to advance to the next screen The 'E' command allows the user to modify the program being disassembled. Changes are initially made only in the disassembly buffer. Before the buffer is overwritten or the disassembler terminates, the user is asked whether the changes are to be written to the file or RAM area being disassembled. Enter 'Q' to stop the disassembler. TYPING REQUESTED DATA Many commands supply default entries for requested data. If you decide to accept the default, just enter <ret>. For editing entries, you can position the cursor using the left and right cursor keys to move by one character, <home> (7 on the numeric pad) to move to the left end of the string, or <end> (1 on the numeric pad) to move to the right end. Use the <del> or <backspace> keys to delete incorrect characters, or just type characters to be inserted. (There is no "replace" typing mode.) In every case but one, you can also edit the default entry by making <right>, <end>, or <del> your first keystroke. The exception is the default for the byte search function. DISASSEMBLY WINDOW The disassembler uses a buffer to hold the code being disassembled. For most purposes, this disassembly window is transparent to the user. If the user requests an address within the file but outside the disassembly window, the appropriate code is automatically read in. The existence of the window is apparent in only three cases: 1. If the disassembler is started near the end of the window and reaches the end before it fills the screen, the rest of the screen will be left blank. 2. The searches are done only from the current location to the end of the buffer. 3. If the contents of the buffer has been changed (see 'E' command) they are optionally written out before being overwritten. LOAD ADDRESS Code from a .COM file is displayed as though its Program Segment Prefix were at 0000:0000 and its load address were 0000:0100. Code from a .EXE file is displayed as though its load address were 0000:0000. This puts its Program Segment Prefix is 10 paragraphs or 100 (hex) bytes lower. This is somewhat awkward, because the DS and ES registers are initialized to point to the PSP. The disassembler displays this segment value as -10. The advantage of a load address of 0000:0000 is that no relocation is necessary. The bytes displayed are exactly the same as those in the file. This also means that the code can be modified (see below for the 'E' command) and written back to the file without being "unrelocated". SEGMENTATION Addresses are displayed in segment:offset form, using the current assumed value of the current segment register. The current segment register can be selected using the 'S' command to step among the available registers (CS, SS, DS, ES, FS, and GS - the last two only with 80386 code). Changing segment registers or their values does not move the disassembler cursor. Only the displayed segment and offset values will change to reflect the new assumptions. Legal offsets will be displayed as a four digit hex number (0000 to FFFF). Other offsets (negative or greater than 64K) will also be calculated and displayed correctly, although they are illegal on the 8086. Illegal offsets will have more than four digits. The segment register values are initialized as indicated in the file header (for .EXE files) or to zero (for other files or RAM). The disassembler has no way of determining the values which may be set during execution. For example, the initialization code for DeSmet C programs resets DS to the same value as the initial SS before executing main(). The assumed segment register values can be altered in two ways. Any segment register can be changed using the register menu reached by the 'R' command. In addition, when the right arrow key is used to follow a far call or jump, the new code segment value is loaded into the CS register. When the user specifies a new segment value on an A, B, C, D, or G command, that value is used for subsequent displays but none of the assumed segment register values is changed. The segmentation models of the protected modes of the 80286 and 80386 are not supported. ALIGNMENT Dis86 will correctly disassemble code if started on the first byte of an instruction. If started in the middle of an instruction, it will disassemble that instruction and perhaps several more incorrectly. In this case the disassembler is said to be out of alignment with the object code. The disassembler will tend to correct its alignment if it continues long enough. 8086 instructions tend to be longer than, for example, those for the 8080, so the disassembler will tend to stay out of alignment for more instructions. Generally speaking, the alignment will be correct after the first half dozen lines. SUMMARY Here are all the letter commands: A nnnn ASCII data B nnnn byte data (hex) C nnnn code (disassembly) D nnnn data (hex and ASCII) E enter new data (follow with a hex expression for each new byte) G nnnn goto address nnnn H display file header information (.EXE files only) O change setup options P print disassembly listing to file Q quit to DOS R change segment register values S select new segment register W set bytes of data per line for A, B, and D formats X exchange current address (at top of screen) with top of stack ? display help screens EXAMPLE 1 In the examples, <left>, <right>, <up>, and <down> refer to the four cursor keys (4, 6, 8, and 2 on the numeric pad, plus the four arrow keys on the Z-100 keyboard). <pg up> and <pg dn> refer to 9 and 3 on the numeric pad. To investigate the bootstrap code, type A>dis86 <ret> and press <space> to advance to the disassembly display (which will be the interrupt vectors). Next type c a ffff:0000 <ret> (for Code format at the Address ffff:0000). On an IBM, the ROM release date and machine ID appear in the last 16 bytes of the ROM. To see them, type D <ret> The release data is at addresses ffff:0005 - ffff:000c in ASCII. The machine ID is at ffff:000e. Some of the possible values are: ff IBM PC fe IBM XT and Portable IBM PC fd IBM PCjr fc IBM AT 2d Compaq 9a Compaq-Plus Return to code format by typing C <ret> One of the instructions displayed will almost certainly be a jump. If so, press <down> enough times to bring the jump to the top line, then <right> to follow the jump. Note that the previous addresses were pushed onto the stack, as shown on the bottom line. To return to the most recent address, press <left> To leave the disassembler, press Q EXAMPLE 2 For a second example, let us disassemble the disassembler itself. Begin by typing A>dis86 dis86.exe <ret> Note the header information, including the entry point of 0000:0000 and the initial stack location of approximately 09e0:9eb8. Proceed to the disassembly screen by typing <space> The disassembler starts in C (code) format at the entry point, which is a jump to the initialization code. To follow the jump, type <right> One of the early instructions in the initialization code refers to the first location in the stack segment. Bring this location to the top of the screen by typing <pg dn> <down> <down> and follow the reference by typing <right> Since it was a data reference, the disassembler automatically switched to D (data) format. Note that the two previous addresses have been pushed onto the stack, as shown at the bottom of the screen. Return to the most recent one by typing <left> The initialization code gets rather involved, but one of its functions is to initialize DS to the same value as SS. To reflect this, use the R command: R DS is the first register in the list, so you need only enter the appropriate value: ss <ret> <space> The code for the main program immediately followed the jump at 0000:0000. To return there, type <left> Send a copy of this screen to the file "printout" by typing P <ret> <ret> <ret> To inspect the data segment, type A ds:0 <ret> To display more characters on each line, use the W command: W 60 <ret> Use the search command to find one of the messages: G S T hime <ret> This string won't be found. To correct the spelling to "home" and try again, type G S T <right> o <del> <ret> Once again, leave the disassembler by pressing Q