1. About Debug/X

    Debug/X is a package of debuggers. The most important ones are:
    
    - Debug: is a clone of the MS-DOS DEBUG command - although, "clone"
      isn't the correct name, because it has many features not available in
      the original MS DEBUG. For details see below. 

    - DebugX: is an extended version of Debug that allows to debug both
      real-mode and protected-mode (DPMI) DOS programs.

    - additional variants like DebugXv, DebugXg, DebugB or DebugR that are
      useful for special cases. For details see below.


 2. Added/Missing Features

  a. Debug extends MS-Debug in the following ways:

   - The assembler and disassembler support all publicly documented
     instructions for Intel chips through the Pentium Pro (P6), except for
     the MMX instructions (as for DebugX, MMX is partially supported since
     v1.18). The assembler and disassembler inform you if any instruction is 
     inappropriate for the current processor.

   - FPU Opcodes are supported.

   - The current processor (for the purposes of assembler and disassembler
     warnings, above) can be declared to be something else via the following
     commands:

        m [x] set current processor.
              x=0 current processor is 8088.
              x=1..6 current processor is 80x86. 80586 is a Pentium and
              80686 is a Pentium Pro.
              no argument = print current CPU/FPU types.
        mc [2|N] set math coprocessor. 
             2 = math coprocessor is a 287 (only valid if current
                 processor is a 386).
             N = math coprocessor is absent
             no argument = math coprocessor is present

   - 'r register [value]' accepts 32-bit register names (for cpu 80386+).

   - You can do `r cx 1234' instead of having to put the `1234' on a
     separate line.

   - 'rn' displays FPU register status.
      Just the raw hex values of the registers are displayed, though.

   - 'rx' switches among 8086 and 80386 register display (for cpu 80386+).

   - When doing `debug < file', debug will not hang if it reaches an end of
     file before encountering a `q' command.

   - This debugger saves and restores the program's Control-C and critical
     error interrupts, providing for better isolation between the debugger
     and the program being debugged.

   - 'tm 0|1' sets trace command mode, 0 = process INTs (default),
     1 = single-step INTs (the MS-DOS Debug compatible mode).

   - 'xr' allows to reallocate an EMS handle and 'xa' allows to allocate
     an "empty" EMS handle (with zero pages) - but only if an EMM v4.0
     is present.  

   - Besides 'i' and 'o' exist the variations 'iw', 'id', 'ow' and 'od'
     to read/write a WORD or DWORD value from/to a port [DWORD values
     require a 80386+ cpu].

   - 'h' can handle dword values.

   - register names can be used anywhere where a number is expected as
     input. That is, things like "u cs:ip" do work.

   - 'dm' displays the DOS memory control block (MCB) chain and the
     current PSP.

   - if DOS has set its InDOS flag, Debug will avoid to use int 21h
     internally, which allows to single-step through DOS code.

   - 'd', 't' and 'u' are automatically repeated if a blank line is
     entered.

  b. DebugX has the following additional features:

   - besides real-mode applications it can debug both 16-bit and 32-bit
     DPMI clients.
     [To debug DPMI clients a DPMI host has to be installed. Tested with
      cwsdpmi, hdpmi, WinXP NTVDM, 32rtm. In DosEmu and Windows 95/98/ME
      DOS boxes it is required to single-step through the initial switch
      to protected-mode, else DebugX will loose control of the debuggee.]

   - 'dl': display LDT descriptor(s) (in protected-mode only)

   - 'di': display interrupt vector(s)

   - 'dx': display extended memory (read via Int 15h, ah=87h). Requires a
     80386 cpu. 'dx' is automatically repeated if a blank line is entered.
     Be aware that Int 15h, ah=87h most likely will disable "unreal"-mode
     if it was enabled. This may be an unwanted side effect during debugging.

   - 'g','p','t': the 80386+ debug registers may be used if breakpoints are
     to be set. This allows setting breakpoints in code that is read-only.
     The feature may be switched off by cmdline option /s. It's never active
     if DebugX runs on NTVDM or DosEmu, since the debug registers aren't
     accessible there.

   - 'rn' displays FPU registers in "readable" format.

   - when running in a Windows XP DOS bos, DebugX will try to load and use
     DEBXXVDD.DLL. This DLL will allow DebugX to successfully execute its
     low-level disk access commands L and W in this OS - if the user has
     administrator rights. DEBXXVDD.DLL is public domain, the source can
     be found at http://github.com/Baron-von-Riedesel/debxxf. Please
     be aware that you can do severe damage to your disk data if you use
     those low-level functions without knowing what you are doing.

   - 'rm' displays MMX registers.

   - commandline option /2 may direct DebugX's output to a monochrome video
     adapter if one is available. Useful for emulators that support this
     feature ( DosBox-x or 86Box, for example ). Needs at least a VGA
     compatible card, and there's no guarantee that it works as expected.


  c. Optionally, one may create special-purpose variants of Debug.
     Run file makex.bat to create them; you'll need the JWasm assembler
     for a successful run. The many variants may be confusing, basically
     there are 3 branches:

     - debugger without protected-mode support ( Debug, DebugB )
     - debugger with DPMI protected-mode support ( DebugX variants )
     - debugger running in protected-mode ring 0 ( DebugR variants )
  
   - DebugB.BIN: a version that is supposed to be loaded during the
     boot process, before DOS is loaded. Has to be installed in "free"
     disk space ( for example, in the "reserved" sectors of a FAT partition
     or behind the disk's MBR ) and must be initialized before it becomes
     active. Definitely not for beginners. See "Installing and Initializing
     DebugB" in DEBUG.TXT for details.

   - DebugX variants:

     + DebugXd.COM: a version that doesn't hook int 01 and int 03. This
       allows to debug this variant with a "normal" version of Debug(X).

     + DebugXe.COM: a version which hooks interrupts 06, 0Ch and 0Dh in
       real-mode. This is for - rare - cases when such exceptions may
       occur in real-mode applications.

     + DebugXf.COM: a version which hooks interrupt 31h in protected-mode
       and then rejects any attempt to modify exception vectors 01, 03,
       0Dh and 0Eh. This allows to debug DOS-extended applications that
       modify these vectors.

     + DebugXg.EXE: a version that is in DOS device driver format. This
       variant can be used to debug other device drivers. There are a
       few peculiarities:
       - commands N and Q are rejected;
       - commands R and W can't be used to read/write files;
       - interrupt 06 (invalid opcode exception) is hooked;
       - commands BP/BC to manage permanent breakpoints are activated.

     + DebugXu.COM: a version that uses "unreal" mode to access extended
       memory for the DX command. This is useful to debug applications
       that set/use "unreal" mode themselves, because DebugXU will not
       change the current "unreal" state by its DX command.

     + DebugXv.COM: a variant that has a few nice features activated
       compared to the standard DebugX:
       - takes care that screen output of debuggee and debugger are strictly
         separated; the 'v' command may be used to watch the debuggee screen
         while the debugger is active.
       - the SysReq key is detected and will activate the debugger.
       - the BP/BC commands are active, allowing permanent breakpoints.
       DebugXv is intended to help debugging fullscreen text mode programs.

   - DebugR variants. These variants run in ring0 protected-mode. The
     binaries need additional software a) to be loaded and b) to handle the
     i/o requests. No DOS API is used ( actually, not even the BIOS is used )
     and hence these are not really "DOS debuggers" anymore.
     See "DebugR Basics" in DEBUG.TXT for more details.
   
     + DebugR.BIN: variant for legacy mode, no support for V86 mode.
     + DebugRV.BIN: variant for legacy mode with support for V86 mode.
     + DebugRL.BIN: variant for long mode.

  d. The following features of MS Debug are not implemented:

   - Loading of .HEX files.


 3. How to (re)build the Debug.COM/DebugX.COM Binaries

    MAKE.BAT is intended for rebuilding the debugger binaries. The
    recommended tool for this step is to use the JWasm assembler.
    Alternatively, one can use MS Masm, but then a 16-bit OMF linker 
    will be needed as well (Open Watcom's WLink might do the job). Also,
    the DebugRL variant cannot be created with Masm, since it contains
    a 64-bit segment.


 4. Files Overview

    The files in this distribution are:

    debug.com     the executable binary.
    debugx.com    a DPMI aware version of Debug.
    debxxvdd.dll  a Win32 dll (VDD) loaded if debugx runs in a Windows XP 
                  NTVDM "DOS box". Makes debugx's L and W command work with
                  sectors on this platform.
    readme.txt    this file.
    history.txt   list of changes.
    debug.txt     a MS-Debug tutorial originally written by Michael Webster,
                  extended to describe all additional features of Debug(X).
    SAMPLES       contains samples of 16 and 32-bit DPMI clients in Masm
                  syntax.

    The following files, which may be distributed separately, are needed if 
    debug.com or debugx.com is to be rebuilt:

    debug.asm     Assembler source code for both debug.com and debugx.com.
                  Assemble with JWasm or Masm (see Make.bat).
    *.inc         several assembly include files, two of them (asmtbl.inc
                  and distbl.inc) generated by tool mktables.
    instr.*       input data for mktables.
    mktables.c    C program to compile the internal instruction-set
                  tables into files asmtbl.inc/distbl.inc. It's supposed to
                  be translated with Open Watcom's 16bit compiler WCC.
    make.bat      batch file to (re)create debug.com and debugx.com.
    makecm.bat    batch file to (re)create mktables.exe with MS C.
    makecw.bat    batch file to (re)create mktables.exe with OW C.
    makex.bat     batch file to create special versions of Debugx.


 5. Copyright

    This program is copyrighted, but feel free to distribute and use it
    as you wish.  For full copyright conditions, see the file debug.asm.

    Paul Vojta
    vojta@math.berkeley.edu