HP OpenVMS Systems Documentation

OpenVMS Alpha System Analysis Tools Manual

To start SCD on the host side, enter the following command:

$ DEBUG/KEEP

SCD displays the DBG> prompt. With the DBGHK$IMAGE_PATH logical name defined, you can invoke the CONNECT command and the optional qualifiers /PASSWORD and /IMAGE_PATH.

To use the CONNECT command and the optional qualifiers (/PASSWORD and /IMAGE_PATH) to connect to the node with name <node-name>, enter the following command:

DBG> CONNECT %NODE_NAME node-name /PASSWORD="password"

If a password has been set up on the target system, you must use the /PASSWORD qualifier. If a password is not specified, a zero length string is passed to the target system as the password.

The /IMAGE_PATH qualifier is also optional. If you do not use this qualifier, SCD uses the DBGHK$IMAGE_PATH logical name as the default. The /IMAGE_PATH qualifier is a quick way to change the logical name. However, when you use it, you cannot specify a search list. You can use only a logical name or a device and directory, although the logical name can be a search list.

Usually, SCD obtains the source file name from the object file. This is put there by the compiler when the source is compiled with the /DEBUG qualifier. The SET SOURCE command can take a list of paths as a parameter. It treats them as a search list.

9.6 Summary of System Code Debugger Commands

In general, any OpenVMS debugger command can be used in SCD. For a complete list, refer to the OpenVMS Debugger Manual. The following are a few examples:

• Commands to manipulate the source display, such as TYPE and SCROLL.
• Commands used in OpenVMS debugger command programs, such as DO and IF.
• Commands that affect output formats, such as SET RADIX.
• Commands that manipulate symbols and scope, such as EVALUATE, SET LANGUAGE, and CANCEL SCOPE. Note that the debugger SHOW IMAGE command is equivalent to the XDELTA ;L command, and the debugger DEFINE command is equivalent to the XDELTA ;X command.
• Commands that cause code to be executed, such as STEP and GO. Note that the debugger STEP command is equivalent to the XDELTA S and O commands, and the debugger GO command is equivalent to the XDELTA ;P and ;G commands.
• Commands that manipulate breakpoints, such as SET BREAK and CANCEL BREAK. These commands are equivalent to the XDELTA ;B command. However, unlike XDELTA, there is no limit on the number of breakpoints in SCD.
• Commands that affect memory, such as DEPOSIT and EXAMINE. These commands are equivalent to the XDELTA /,!,[,",' commands.

You can also use the OpenVMS debugger command SDA to examine the target system with System Dump Analyzer semantics. This command, which is not available when debugging user programs, is described in the next section.

9.7 Using System Dump Analyzer Commands

Once a connection has been established to the target system, you can use the commands listed in the previous section to examine the target system. You can also use some System Dump Analyzer (SDA) commands, such as SHOW SUMMARY and SHOW DEVICE. This feature allows the system programmer to take advantage of the strengths of both the OpenVMS Debugger and SDA to examine the state of the target system and to debug system programs such as device drivers.

To obtain access to SDA commands, you simply type "SDA" at the OpenVMS Debugger prompt ("DBG>") at any time after a connection has been established to the target system. SDA initializes itself and then outputs the "SDA>" prompt. Enter SDA commands as required. (See Chapter 4 for more information.) To return to the OpenVMS Debugger, you enter "EXIT" at the "SDA>" prompt. Optionally, you may invoke SDA to perform a single command and then return immediately to the OpenVMS Debugger, as in the following example:

DBG>SDA SHOW SUMMARY

You may reenter SDA at any time, with or without the optional SDA command. Once SDA has been initialized, the SDA> prompt is output more quickly on subsequent occasions.

Note that there are some limitations on the use of SDA from within SCD.

• You cannot switch between processes, whether requested explicitly (SET PROCESS <name>) or implicitly (SHOW PROCESS <name>). The exception to this is that access to the system process is possible.
• You cannot switch between CPUs.
• SDA has no knowledge of the OpenVMS debugger's Motif or Windows interfaces. Therefore, all SDA input and output occurs at the terminal or window where the OpenVMS debugger was originally invoked. Also, while using SDA, the OpenVMS debugger window is not refreshed; you must exit SDA to allow the OpenVMS debugger window to be refreshed.
• When you invoke SDA from SCD with an immediate command, and that command produces a full screen of output, SDA displays the message "Press RETURN for more." followed by the "SDA>" prompt before continuing. If you enter another SDA command at this prompt, SDA does not automatically return to SCD upon completion. To do this, you must enter an EXIT command.

The SCD host and the target kernel use a private Ethernet protocol to communicate. For the two systems to see each other, they have to be on the same Ethernet segment.

The network portion of the target system finds the first Ethernet device and communicates through it. The network portion of the host system also finds the first Ethernet device and communicates through it. However, if for some reason, SCD picks the wrong device, you can override this by defining the logical DBGHK$ADAPTOR to the template device name for the appropriate adaptor.

9.9 Troubleshooting Checklist

If you have trouble starting a connection, perform the following tasks to correct the problem:

• Check SCSNODE on the target system.
  It must match the name you are using in the host CONNECT command.
• Make sure that both the Ethernet and boot device are on the boot command.
• Make sure that the host system is using the correct Ethernet device, and that the host and target systems are connected to the same Ethernet segment.
• Check the version of the operating system and make sure that both the host and target systems are running the same version of the OpenVMS Alpha operating system.

There are three possible network errors:

• NETRETRY
  Indicates the system code debugger connection is lost
• SENDRETRY
  Indicates a message send failure
• NETFAIL
  Results from the two previous errors

The netfail error message has a status code that can be one of the following values:

Accessing OpenVMS executive images' symbols is not always straightforward with SCD. Only a subset of the symbols may be accessible at one time and in some cases, the symbol value the debugger currently has may be stale. To understand these problems and their solutions, you must understand how the debugger maintains its symbol tables and what symbols exist in the OpenVMS executive images. The following sections briefly summarize these topics.

9.11.1 Overview of How the OpenVMS Debugger Maintains Symbols

The debugger can access symbols from any image in the OpenVMS loaded system image list by reading in either the .DSF or .EXE file for that particular image. The .EXE file contains information only about symbols that are part of the symbol vector for that image. The current image symbols for any set module are defined. (You can tell if you have the .DSF or .EXE file by doing a SHOW MODULE. If there are no modules, you have the .EXE file.) This includes any symbols in the SYS$BASE_IMAGE.EXE symbol vector for which the code or data resides in the current image. However, you cannot access a symbol that is part of the SYS$BASE_IMAGE.EXE symbol vector that resides in another image.

In general, at any one point in time, the debugger can access only the symbols from one image. It does this to reduce the time it takes to search for a symbol in a table. To load the symbols for a particular image, use the SET IMAGE command. When you set an image, the debugger loads all the symbols from the new image and makes that image the current image. The symbols from the previous image are in memory, but the debugger will not look through them to translate symbols. To remove symbols from memory for an image, use the CANCEL IMAGE command (which does not work on the main image, SYS$BASE_IMAGE).

There is a set of modules for each image the debugger accesses. The symbol tables in the image that are part of these modules are not loaded with the SET IMAGE command. Instead they can be loaded with the SET MODULE <module-name> or SET MODULE/ALL commands. As they are loaded, a new symbol table is created in memory under the symbol table for the image. Figure 9-1 shows what this looks like.

Figure 9-1 Maintaining Symbols

When the debugger needs to look up a symbol name, it first looks at the current image to find the information. If it does not find it there, it then looks into the appropriate module. It determines which module is appropriate by looking at the module range symbols which are part of the image symbol table.

To see the symbols that are currently loaded, use the debugger's SHOW SYMBOL command. This command has a few options to obtain more than just the symbol name and value. (See the OpenVMS Debugger Manual for more details.)

9.11.2 Overview of OpenVMS Executive Image Symbols

Depending on whether the debugger has access to the .DSF or .EXE file, different kinds of symbols could be loaded. Most users will have the .EXE file for the OpenVMS executive images and a .DSF file for their private images---that is, the images they are debugging.

The OpenVMS executive consists of two base images, SYS$BASE_IMAGE.EXE and SYS$PUBLIC_VECTORS.EXE, and a number of separately loadable executive images.

The two base images contain symbol vectors. For SYS$BASE_IMAGE.EXE, the symbol vector is used to define symbols accessible by all the separately loadable images. This allows these images to communicate with each other through cross-image routine calls and memory references. For SYS$PUBLIC_VECTORS.EXE, the symbol vector is used to define the OpenVMS system services. Because these symbol vectors are in the .EXE and the .DSF files, the debugger can load these symbols no matter which one you have.

All images in the OpenVMS executive also contain global and local symbols. However, none of these symbols ever gets into the .EXE file for the image. These symbols are put in the specific module's section of the .DSF file if that module was compiled using /DEBUG and the image was linked using /DSF.

9.11.3 Possible Problems You May Encounter

• Access to All Executive Image Symbols
  When the current image is not SYS$BASE_IMAGE, but one of the separately loaded images, the debugger does not have access to any of the symbols in the SYS$BASE_IMAGE symbol vector. This means you cannot access (set breakpoints, and so on) any of the cross-image routines or data cells. The only symbols you have access to are the ones defined by the current image.
  If the debugger has access only to the .EXE file, then only symbols that have vectors in the base image are accessible. For .DSF files, the current image symbols for any set module are defined. (You can tell if you have the .DSF or .EXE by using the SHOW MODULE command---if there are no modules you have the .EXE). This includes any symbols in the SYS$BASE_IMAGE.EXE symbol vector for which the code or data resides in the current image. However, the user cannot access a symbol that is part of the SYS$BASE_IMAGE.EXE symbol vector that resides in another image. For example, if you are in one image and you want to set a breakpoint in a cross-image routine from another image, you do not have access to the symbol. Of course, if you know in which image it is defined, you can do a SET IMAGE, SET MODULE/ALL, and then a SET BREAK.
  There is a debugger workaround for this problem. The debugger and SCD let you use the SET MODULE command on an image by prefixing the image name with SHARE$ (SHARE$SYS$BASE_IMAGE, for example). This treats that image as a module which is part of the current image. In the previous figure, think of it as another module in the module list for an image. Note, however, that only the symbols for the symbol vector are loaded. None of the symbols for the modules of the SHARE$xxx image are loaded. Therefore, this command is only useful for base images.
  So, in other words, by doing SET MODULE SHARE$SYS$BASE_IMAGE, the debugger gives you access to all cross-image symbols for the OpenVMS executive.
• Stale Data from the Symbol Vector
  When an OpenVMS executive based image is loaded, the values in the symbol vectors are only correct for information that resides in that based image. For all symbols that are defined in the separately loaded images, the based image contains a pointer to a placeholder location. For routine symbols this is a routine that just returns "an image not loaded" failure code. A symbol vector entry is fixed to contain the real symbol address when the image in which the data resides is loaded.
  Therefore, if you do a SET IMAGE command to a base image before all the symbol entries are corrected, the SET IMAGE obtains the placeholder value for those symbols. Then, once the image containing the real data is loaded, the debugger will still have the placeholder value. This means that you are looking at stale data. One solution to this is to make sure to do a SET IMAGE command on the base image in order to get the most up-to-date symbol vector loaded into memory.
  The CANCEL IMAGE/SET IMAGE combination does not currently work for SYS$BASE_IMAGE because it is the main image and DEBUG does not allow you to CANCEL the main image. Therefore, if you connect to the target system early in the boot process, you will have stale data as part of the SYS$BASE_IMAGE symbol table. However, the SET MODULE SHARE$xxx command always reloads the information from the symbol vector. So, to solve this problem you could SET IMAGE to an image other than SYS$BASE_IMAGE and then use the CANCEL MODULE SHARE$SYS$BASE_IMAGE and SET MODULE SHARE$SYS$BASE_IMAGE commands to do the same thing. The only other solution is to always connect to the target system once all images are loaded that define the real data for values in the symbol vectors. You could also enter the following commands, and you would obtain the latest values from the symbol vector:

  SET IMAGE EXEC_INIT SET MODULE/ALL SET MODULE SHARE$SYS$BASE_IMAGE

• Problems with SYS$BASE_IMAGE.DSF
  For those who have access to the SYS$BASE_IMAGE.DSF file, there may be another complication with accessing symbols from the symbol vector. The problem is that the module SYSTEM_ROUTINES contains the placeholder values for each symbol in the symbol vector. So, if SYSTEM_ROUTINES is the currently set module (which is the case if you are sitting at the INI$BRK breakpoint) then the debugger will have the placeholder value of the symbol as well as the value in the symbol vector. You can see what values are loaded with the SHOW SYMBOL/ADDRESS command. The symbol vector version should be marked with (global); the local one is not.
  To set a breakpoint at the correct code address for a routine when in this state, use the SHOW SYMBOL/ADDRESS command on the routine symbol name. If the global and local values for the code address are the same, then the image with the routine has not yet been loaded. If not, set a breakpoint at the code address for the global symbol.

9.12 Sample System Code Debugging Session

To reproduce this sample session, the host system needs access to the SYSTEM_DEBUG.DSF matching the SYSTEM_DEBUG.EXE file on your target system, and to the source file C_TEST_ROUTINES.C, which is available in SYS$EXAMPLES. The target system is booted with the boot flags 0, 8004, so it stops at an initial breakpoint, and the devices DKB200,ESA0.

>>> b -fl 0,8004 dkb200,esa0
INIT-S-CPU...
INIT-S-RESET_TC...
INIT-S-ASIC...
INIT-S-MEM...
INIT-S-NVR...
INIT-S-SCC...
INIT-S-NI...
INIT-S-SCSI...
INIT-S-ISDN...
INIT-S-TC0...
AUDIT_BOOT_STARTS ...
AUDIT_CHECKSUM_GOOD
AUDIT_LOAD_BEGINS
AUDIT_LOAD_DONE

%SYSBOOT-I-GCTFIL, Using a configuration file to boot as a Galaxy instance.


    OpenVMS (TM) Alpha Operating System, Version V7.2


DBGTK: Initialization succeeded.  Remote system debugging is now possible.

DBGTK: Waiting at breakpoint for connection from remote host.

The example continues by invoking the system code debugger's character cell interface on the host system.

$ define dbg$decw$display " "
$ debug/keep

         OpenVMS Alpha Debug64 Version V7.2-019


DBG>

Use the CONNECT command to connect to the target system. In this example, the target system's default password is the null string, and the logical name DBGHK$IMAGE_PATH is used for the image path; so the command qualifiers /PASSWORD and /IMAGE_PATH are not being used. You may need to use them.

When you have connected to the target system, the DBG> prompt is displayed. Enter the SHOW IMAGE command to see what has been loaded. Because you are reaching a breakpoint early in the boot process, there are very few images. See Example 9-3. Notice that SYS$BASE_IMAGE has an asterisk next to it. This is the currently set image, and all symbols currently loaded in the debugger come from that image.

DBG> connect %node_name TSTSYS
%DEBUG-I-INIBRK, target system interrupted
%DEBUG-I-DYNMODSET, setting module SYSTEM_ROUTINES
DBG> show image
 image name                      set    base address           end address

 ERRORLOG                        no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80084000       FFFFFFFF80086FFF
    NPRW1                               FFFFFFFF80CA3600       FFFFFFFF80CA3BFF
 EXEC_INIT                       no     FFFFFFFF8306E000       FFFFFFFF830A2000
*SYS$BASE_IMAGE                  yes    0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80002000       FFFFFFFF8000EDFF
    NPRW1                               FFFFFFFF80C05C00       FFFFFFFF80C2AFFF
 SYS$CNBTDRIVER                  no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF8001A000       FFFFFFFF8001AFFF
    NPRW1                               FFFFFFFF80C2D600       FFFFFFFF80C2D9FF
 SYS$CPU_ROUTINES_0402           no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80010000       FFFFFFFF800191FF
    NPRW1                               FFFFFFFF80C2B000       FFFFFFFF80C2D5FF
 SYS$ESBTDRIVER                  no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF8002C000       FFFFFFFF8002E1FF
    NPRW1                               FFFFFFFF80C30C00       FFFFFFFF80C30FFF
 SYS$NISCA_BTDRIVER              no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF8001C000       FFFFFFFF8002ADFF
    NPRW1                               FFFFFFFF80C2DA00       FFFFFFFF80C30BFF
 SYS$OPDRIVER                    no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80030000       FFFFFFFF800337FF
    NPRW1                               FFFFFFFF80C31000       FFFFFFFF80C319FF
 SYS$PUBLIC_VECTORS              no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80000000       FFFFFFFF80001FFF
    NPRW1                               FFFFFFFF80C00000       FFFFFFFF80C05BFF
 SYSTEM_DEBUG                    no     FFFFFFFF82FFE000       FFFFFFFF83056000
 SYSTEM_PRIMITIVES_MIN           no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80034000       FFFFFFFF800775FF
    NPRW1                               FFFFFFFF80C31A00       FFFFFFFF80CA11FF
 SYSTEM_SYNCHRONIZATION_UNI      no     0000000000000000       FFFFFFFFFFFFFFFF
    NPRO0                               FFFFFFFF80078000       FFFFFFFF800835FF
    NPRW1                               FFFFFFFF80CA1200       FFFFFFFF80CA35FF

 total images: 12                bytes allocated: 1517736

Example 9-4 shows the target system's console display during the connect sequence. Note that for security reasons, the name of the host system, the user's name, and process ID are displayed.

DBGTK: Connection attempt from host HSTSYS  user GUEST       process 2E801C2F
DBGTK: Connection attempt succeeded

To set a breakpoint at the first routine in the C_TEST_ROUTINES module of the SYSTEM_DEBUG.EXE execlet, do the following:

1. Load the symbols for the SYSTEM_DEBUG image with the DEBUG SET IMAGE command.
2. Use the SET MODULE command to obtain the symbols for the module.
3. Set the language to be C and set a breakpoint at the routine test_c_code.
   The language must be set because C is case sensitive and test_c_code needs to be specified in lowercase. The language is normally set to the language of the main image, in this example SYS$BASE_IMAGE.EXE. Currently that is not C.

DBG> set image system_debug
DBG> show module
module name                     symbols   language   size

AUX_TARGET                      no        C         15928
BUFSRV_TARGET                   no        C         11288
BUGCHECK_CODES                  no        BLISS     26064
CRTLPRINTF                      no        C         29920
C_TEST_ROUTINES                 no        C          3808
FATAL_EXC                       no        C          1592
HIGH_ADDRESS                    no        C           372
LIB$CALLING_STANDARD_AUX        no        MACRO64    1680
LINMGR_TARGET                   no        C         13320
LOW_ADDRESS                     no        C           368
OBJMGR                          no        C          5040
PLUMGR                          no        C         19796
POOL                            no        C           116
PROTOMGR_TARGET                 no        C         17868
SOCMGR                          no        C          3324
SYS$DOINIT                      no        AMACRO    81740
TARGET_KERNEL                   no        C        207244
TMRMGR_TARGET                   no        C          3516
XDELTA                          no        BLISS    189940
XDELTA_ISRS                     no        MACRO64    2428

total modules: 20.              bytes allocated: 1585168.
DBG> set module c_test_routines
DBG> show module c_test_routines
module name                     symbols    size

C_TEST_ROUTINES                 yes        3808

total C modules: 1.             bytes allocated: 1592264.
DBG> set language c
DBG> show symbol test_c_code*
routine C_TEST_ROUTINES\test_c_code5
routine C_TEST_ROUTINES\test_c_code4
routine C_TEST_ROUTINES\test_c_code3
routine C_TEST_ROUTINES\test_c_code2
routine C_TEST_ROUTINES\test_c_code
DBG> set break test_c_code

Now that the breakpoint is set, you can proceed and activate the breakpoint. When that occurs, the debugger tries to open the source code for that location in the same place as where the module was compiled. Because that is not the same place as on your system, you need to tell the debugger where to find the source code. This is done with the debugger's SET SOURCE command, which takes a search list as a parameter so you can make it point to many places.

DBG> set source/latest sys$examples,sys$library
DBG> go
break at routine C_TEST_ROUTINES\test_c_code
   166:     x = xdt$fregsav[0];

Now that the debugger has access to the source, you can put the debugger into screen mode to see exactly where you are and the code surrounding it.

DBG> Set Mode Screen; Set Step Nosource

- SRC: module C_TEST_ROUTINES -scroll-source------------------------------------
   151:     xdt$fregsav[5] = in64;
   152:     xdt$fregsav[6] = in32;
   153:     if (xdt$fregsav[9] > 0)
   154:         *pVar =  (*pVar + xdt$fregsav[17])%xdt$fregsav[9];
   155:     else
   156:         *pVar = (*pVar + xdt$fregsav[17]);
   157:     xdt$fregsav[7] = test_c_code3(10);
   158:     xdt$fregsav[3] = test;
   159:     return xdt$fregsav[23];
   160: }
   161: void test_c_code(void)
   162: {
   163:     int x,y;
   164:     int64 x64,y64;
   165:
-> 166:     x = xdt$fregsav[0];
   167:     y = xdt$fregsav[1];
   168:     x64 = xdt$fregsav[2];
   169:     y64 = xdt$fregsav[3];
   170:     xdt$fregsav[14] = test_c_code2(x64+y64,x+y,x64+x,&y64);
   171:     test_c_code4();
   172:     return;
   173: }
- OUT -output-------------------------------------------------------------------







- PROMPT -error-program-prompt--------------------------------------------------





DBG>