MS-DOS DEBUG Basics for Fixed-Format Files

The Debug Dump/Display Format

Sample DEBUG Dump
1373:0100  46 FE 06 47 DB EB EF E8-DB F9 75 04 FE 06 A3 D8  F..G......u.....
1373:0110  3C 3F 75 05 80 0E A7 D8-02 3C 2A 75 05 80 0E A7  <?u......<*u....
1373:0120  D8 02 3A 06 94 D2 75 C9-4E 32 C0 86 04 46 3C 0D  ..:...u.N2...F<.
1373:0130  75 02 88 04 89 36 6B D7-89 0E 69 D7 C3 BE 48 DB  u....6k...i...H.
1373:0140  8B 4C 05 8B 74 09 E8 08-00 03 F1 E8 03 00 3C 0D  .L..t.........<.

• Address Every byte in memory has its own unique address. The pair of 4-hexadecimal digit values at the left side of each line of the dump indicates the address of the first data byte displayed on that line. For now we will ignore the initial 4-hexadecimal digit value (1373 in this case) and consider only the second 4 digits to be relevant. For reasons to be discussed later in this course, most files are loaded in memory by DEBUG starting at address 0100 hex. (.EXE files are the only exception). 16 (10 hex) data bytes are displayed on each line, so an address of 0120 (for example) at the beginning of a dump line indicates that the addresses for the bytes on that line start at 0120 and go up to 12F inclusive; the byte at address 012C above has the value 04, as an example.
• Hexadecimal Data Values As has already been mentioned the middle portion of each dump line shows (in hexadecimal) the internal bit pattern of 16 (10 hex) bytes of data in memory. The "dash" character displayed in the middle of each dump line has no significance for the values stored in memory; it is simple an indicator to help us count addresses (the byte immediately following the dash is at an address 8 more than the address at the beginning of the line).
• ASCII codes At the far right of each line is a display of the character each byte of the line could be interpreted as representing (assuming that each byte was being used to hold an ASCII encoded character... which of course will not always be the case). Non-displayable ASCII codes and codes beyond the standard ASCII range are displayed in this area as periods.

Running Debug &: Basic Commands

• Starting the DEBUG program - From the MS-DOS command prompt, type DEBUG and hit the [Enter] key. If your system responds with a message indicating that it could not find this program, check that DEBUG.EXE is in fact installed on your computer and that there is a PATH to the directory containing DEBUG.EXE. The DEBUG program should respond with a fresh line starting with a dash, the DEBUG prompt.
• Quitting DEBUG When you are finished using DEBUG, type the letter Q (for Quit) at the DEBUG prompt (and hit [Enter]).
• Naming a file to load and Loading it A file can be specified on the command line when you start DEBUG. A more general method is to identify the name of the file (or the complete file path) of the file to be loaded, by typing the letter N followed immediately by the name (or path) of the file to be used; you must then (on the next line) type the letter L (for load) to tell DEBUG to load this file from disk to memory.
• Dumping/Displaying the data values - By typing the letter D at the DEBUG prompt, you can ask DEBUG to display (or "dump") several lines of memory. The first time you do this, the display will start from where-ever your file was loaded (address 0100, usually); subsequent D requests will display memory starting from where the last display ended. You can get around this default by specifying an address (in hex) immediately after the letter D before hitting the [Enter] key; the address you specify becomes the starting address for the display. You can also specify two addresses, separated by a space, between the D and the [Enter] key; a start address followed by an ending address.

A File Dump Example

• A Simple File Format for demonstration purposes, suppose we have a file called DEMOFILE in the current directory; this file contains 3 records; each record is composed of a 6-byte ASCII character field followed by a 2-byte 2's Complement field. Note: for files created on an IBM PC (or most microcomputers), numeric fields are stored with their bytes in the reverse order to the way we normally read.
  
  
• The DEBUG run

C:\CSA>DEBUG
-Ndemofile
-L
-D
1373:0100  46 49 52 53 54 20 FE FF-53 45 43 4F 4E 44 32 00  FIRST ..SECOND2.
1373:0110  54 48 49 52 44 20 46 00-BF FC 46 36 91 00 62 13  THIRD F...F6..b.
1373:0120  D6 C6 06 47 DB 00 E3 31-49 AC E8 D9 F6 74 08 49  ...G...1I....t.I
1373:0130  46 FE 06 47 DB EB EF E8-DB F9 75 04 FE 06 A3 D8  F..G......u.....
1373:0140  3C 3F 75 05 80 0E A7 D8-02 3C 2A 75 05 80 0E A7  <?u......<*u....
1373:0150  D8 02 3A 06 94 D2 75 C9-4E 32 C0 86 04 46 3C 0D  ..:...u.N2...F<.
1373:0160  75 02 88 04 89 36 6B D7-89 0E 69 D7 C3 BE 48 DB  u....6k...i...H.
1373:0170  8B 4C 05 8B 74 09 E8 08-00 03 F1 E8 03 00 3C 0D  .L..t.........<.
-Q
C:\CSA>




  

• The first record
• 46 49 52 53 54 20 FE FF
• The 6-byte character field: "FIRST " (obtained either by translating each byte using an ASCII table, or by reading from the ASCII area on the right)
• The 2's Complement value: -2 (FE FF reversed becomes FFFE; remaining translation is by standard decoding)

• The second record
• 53 45 43 4F 4E 44 32 00
• The 6-byte character field: "SECOND"
• The 2's Complement value: +50 (note this is not the ASCII code decoded value, "2")



• The third record
• 54 48 49 52 44 20 46 00
• The 6-byte character field: "THIRD "
• The 2's Complement value: +70 (again note that this is not the ASCII decoded value, "F")



• ...the remaining values in the dump/display are "garbage", whatever pattern of bits happened to be in memory and are not related to the contents of the file which was loaded.