When you use DELTA or XDELTA, there are no prompts, few symbols, and one error message. You move through program code by referring directly to address locations. This chapter provides directions for the following actions:

• Referencing addresses
• Referencing registers, the PSL or PS, and the stack
• Interpreting the error message
• Debugging kernel mode code under certain conditions
• Debugging an installed, protected, shareable image
• Using XDELTA on multiprocessor computers
• Debugging code when single-stepping fails (Alpha only)
• Debugging code that does not match the compiler listings (I64 and Alpha only)

For examples of DELTA debugging sessions on various OpenVMS platforms, see Appendix A for I64, Appendix B for Alpha, and Appendix C for VAX.

3.1 Referencing Addresses

When using DELTA or XDELTA to debug programs, you move through the code by referring to addresses. To help you identify address locations within your program, use a list file and a map file. The list file (.LIS) lists each instruction and its offset value from the base address of the program section. The full map file (.MAP) lists the base addresses for each section of your program. To determine the base address of a device driver program, see the OpenVMS VAX Device Support Manual¹.

Once you have the base addresses of the program sections, locate the instruction in the list file where you want to start the debugging work. Add the offset from the list program to the base address from the map file. Remember that all calculations of address locations are done in hexadecimal. You can use DELTA/XDELTA to do the calculations for you with the = command.

To make address referencing easier, you can use offsets to a base address. Then you do not have to calculate all address locations. First, place the base address into a base register. Then move to a location using the offset to the base address stored in the register.

Whenever DELTA/XDELTA displays an address, it will display a relative address if the offset falls within the permitted range (see the ;X command in Chapter 4).

3.1.1 Referencing Addresses (I64 and Alpha Only)

#include <stdio.h> main() { printf("Hello world\n"); }

The following procedure generates information to assist you with the address referencing:

1. Use the /LIST and /MACHINE_CODE qualifiers to compile the program and generate the list file containing the Alpha machine instructions.
   To generate the list file for the previous example, use the following command:

   $ cc/list/machine_code hello

   
   The compiler will generate the following Alpha code in the machine code portion of the listing file:

   .PSECT $CODE, OCTA, PIC, CON, REL, LCL, SHR,- EXE, NORD, NOWRT 0000 main:: ; 000335 0000 LDA SP, -32(SP) ; SP, -32(SP) 0004 LDA R16, 48(R27) ; R16, 48(R27) ; 000337 0008 STQ R27, (SP) ; R27, (SP) ; 000335 000C MOV 1, R25 ; 1, R25 ; 000337 0010 STQ R26, 8(SP) ; R26, 8(SP) ; 000335 0014 STQ FP, 16(SP) ; FP, 16(SP) 0018 LDQ R26, 32(R27) ; R26, 32(R27) ; 000337 001C MOV SP, FP ; SP, FP ; 000335 0020 LDQ R27, 40(R27) ; R27, 40(R27) ; 000337 0024 JSR R26, DECC$GPRINTF ; R26, R26 0028 MOV FP, SP ; FP, SP ; 000338 002C LDQ R28, 8(FP) ; R28, 8(FP) 0030 LDQ FP, 16(FP) ; FP, 16(FP) 0034 MOV 1, R0 ; 1, R0 0038 LDA SP, 32(SP) ; SP, 32(SP) 003C RET R28 ; R28

   
   Notice the statement numbers on the far right of some of the lines. These numbers correspond to the source line statement numbers from the listing file as shown next:

   335 main() 336 { 337 printf("Hello world\n"); 338 }

2. Use the /MAP qualifier with the link command to generate the full map file (.MAP file). To produce a debuggable image, make sure that either /DEBUG or /TRACEBACK (the default) is also specified with the link command.
   To generate the map file for the example program, use the following command:

   $ LINK/MAP/FULL HELLO

3. See the Program Section Synopsis of the map file. Locate the code section that you want to debug and its base address.
   For the example program, the map file is HELLO.MAP. A portion of the Program Section Synopsis is shown below. The $CODE section of the program has a base address of 20000.

   +--------------------------+ ! Program Section Synopsis ! +--------------------------+ Psect Name Module Name Base End Length ---------- ----------- ---- --- ------ $LINKAGE 00010000 0001007F 00000080 ( 128.) HELLO 00010000 0001007F 00000080 ( 128.) $CODE 00020000 000200BB 000000BC ( 188.) HELLO 00020000 000200BB 000000BC ( 188.)

4. See the list file for the location where you want to start debugging. First find the source line statement number. Next find that statement number in the machine code listing portion of the list file. This is the specific instruction where you want to start debugging.
   For the example program, source statement 337 is the following:

   printf("Hello world\n");

   
   Search the machine code listing for statement 337. The first occurrence is the instruction at offset 4 from the start of "main::" and the base of the $CODE PSECT.

5. Enable DELTA using the following commands:

   $ DEFINE LIB$DEBUG SYS$LIBRARY:DELTA $ RUN/DEBUG HELLO

6. If you want to store the base address in a base register, use the ;X command to load the base register.
   For the example program, use the following DELTA/XDELTA command to store the base address of 20000 in base register 0.

   20000,0;X

7. Now you can move to specific address locations.
   For example, if you want to place a breakpoint at offset 4, you would calculate the address as 20000 (base address) plus 4 (offset), or 20004, and specify the ;B command as follows:

   20004;B

   
   Alternatively, if you stored the base address in the base register, you could use the address expression X0+4 (or "X0 4", where the + sign is implied) to set the breakpoint as follows:

   X0+4;B

Reverse this technique to find an instruction displayed by DELTA/XDELTA in the .LIS file, as follows:

1. Note the address of the instruction you want to locate in the .LIS file.
   For example, DELTA/XDELTA displays the following instruction at address 20020:

   20020! LDQ R27,#X0028(R27)

   
   The following steps allow you to find this instruction in the .LIS file.

2. See the .MAP file, and identify the psect and module where the address of the instruction is located. Check the base address value and the end address value of each psect and module. When the instruction address is between the base and end address values, record the psect and module names.
   In the example, the instruction address is located in the HELLO module ($CODE PSECT). The address, 20020, is between the base address 20000 and the end address 200BB.
3. Subtract the base address from the instruction address. Remember that all calculations are in hexadecimal and that you can use the DELTA/XDELTA = command to do the calculations. The result is the offset.
   For example, subtract the base address of 20000 from the instruction address 20020. The offset is 20.
4. See the .LIS file. Look up the module and then find the correct psect. Look for the offset value you calculated in the previous step.
   In the example, there are two psects and one module but only one $CODE psect. Look up the instruction at offset 20, and you will find the following in the .LIS file:

   0020 LDQ R27, 40(R27) ; R27, 40(R27) ; 000337

On VAX, to reference addresses during a DELTA debug session, use the following example as a guide. The example uses a simple VAX MACRO program (EXAMPLE.MAR). You can also use the same commands in an XDELTA debugging session.

0000 1 .title example 0000 2 0000 3 .entry start ^M<r3,r4> 0002 4 clrl r3 0004 5 movl #5,r4 0007 6 10$: addl r4,r3 000A 7 sobgtr r4,10$ 000D 8 ret 000E 9 000E 10 .end start

The following procedure generates information to assist you with address referencing:

1. Use the /LIST qualifier to assemble the program and generate the list file.
   To generate the list file for the previous example, use the following command:

   $ MACRO/LIST EXAMPLE

2. Use the /MAP qualifier with the link command to generate the full map file (.MAP file). Make sure that the default /DEBUG or /TRACEBACK qualifier is active for your link command. If not, specify /DEBUG or /TRACEBACK along with the /MAP qualifier.
   To generate the map file for the example program, use the following command:

   $ LINK/MAP EXAMPLE

3. See the Program Section Synopsis of the map file, locate the section that you want to debug, and look up the base address.
   For the example program, the map file is EXAMPLE.MAP. A portion of the Program Section Synopsis is shown below. The first section of the program has a base address of 200.

   +--------------------------+ ! Program Section Synopsis ! +--------------------------+ Psect Name Module Name Base End Length ---------- ----------- ---- --- ------ . BLANK . 00000200 0000020D 0000000E ( 14.) EXAMPLE 00000200 0000020D 0000000E ( 14.)

4. See the list file for the location of the specific instruction where you want to start debugging.
   For the example program, start with the second instruction (MOVL #5,R4) with an offset of 4.
5. Enable DELTA using the following commands:

   $ DEFINE LIB$DEBUG SYS$LIBRARY:DELTA $ RUN/DEBUG EXAMPLE

6. If you want to store the base address in a base register, use the ;X command to load the base register.
   For the example program, use the following DELTA/XDELTA command to store the base address 200 in base register 0.

   200,0;X [Return]

7. Now you can move to specific address locations.
   For example, if you want to place a breakpoint at the second instruction (MOVL #5,R4), you would calculate the address as 200 (base address) plus 4 (offset), or 204, and specify the ;B command as follows:

   204;B [Return]

   
   Alternatively, if you stored the base address in the base register, you could use the address expression X0+4 (or "X0 4", where the + sign is implied), as follows:

   X0+4;B [Return]

Reverse this technique to find an instruction displayed by DELTA/XDELTA in the .LIS file, as follows:

1. Note the address of the instruction you want to locate in the .LIS file.
   For example, DELTA/XDELTA displays the following instruction at address 020A:

   20A! sobgtr r4,00000207

   
   The following steps allow you to find the instruction at location 207:

2. See the .MAP file and identify the PSECT and MODULE where the address of the instruction is located. Check the base address value and the end address value of each PSECT and MODULE. When the instruction address is between the base and end address values, record the PSECT and MODULE names.
   In the example, the instruction address is located in the EXAMPLE module (.BLANK. psect). The address instruction, 207, is between the base address 200 and the end address 20D.
3. Subtract the base address from the instruction address. Remember that all calculations are in hexadecimal and that you can use the DELTA/XDELTA = command to do the calculations. The result is the offset.
   For the example, subtract the base address 200 from the instruction address 207. The offset is 7.
4. See the .LIS file. Look up the MODULE and then find the correct PSECT. Look for the offset value you calculated in the previous step.
   In the example, there is only one PSECT and MODULE. Look up the instruction at offset 7. The program is branching to the following instruction:

   10$: addl r4,r3

3.2 Referencing Registers

When using DELTA or XDELTA to debug programs, you can view the contents of registers. The following sections describe the types of registers that are referenced by each OpenVMS platform.

3.2.1 Referencing Registers (I64 Only)

On I64, you can reference the following kinds of registers: integer, floating, application, branch, control, special purpose, and software equivilents of special OpenVMS symbolic locations.

Table 3-1 lists the Intel Itanium registers and symbols by which they are identified.

Table 3-1 Intel Itanium Registers and their Associated Symbols

Register	Symbol
General	R0 through R127
Floating	FP0 through FP127
Branch	BR0 through BR7
Predicate	P0 through P63
Application	AR16 (RSC), AR17 (BSP), AR18 (BSPSTORE), AR19 (RNAT), AR25 (CSD), AR26 (SSD), AR32 (CCV), AR36 (UNAT), AR64 (PFS), AR65 (LC), AR66 (EC)
Control	CR0 (DCR), CR1 (ITM), CR2 (IVA), CR8 (PTA), CR16 (IPSR), CR17 (ISR), CR19 (IIP), CR20 (IFA), CR21 (ITIR), CR22 (IIPA), CR23 (IFS), CR24 (IIM), CR25 (IHA),CR65 (IVR)

In addition, there is a program counter (PC) register, which is obtained from the hardware IP register and the ri field of the PSR register.

3.2.2 Referencing Registers (Alpha Only)

On Alpha, to view the contents of the 32 integer registers, the program counter (PC), the stack pointer (SP), the processor status (PS), the 32 floating point registers, the floating point control register (FPCR), and the internal processor registers (IPRs), use the same DELTA/XDELTA commands that you use to view the contents of any memory location. These commands include /, LINEFEED, and ESC. The symbols for identifying these registers follow:

• Integer registers are referenced by the symbol R and a decimal number from 0 to 31. For example, register 1₁₀ is R1₁₀ and register 10₁₀ is R10₁₀. (Decimal notation differs from the original implementation on VAX which uses hexadecimal notation.)
• PC is referenced symbolically by PC.
• PS is referenced symbolically by PS.
• FP is referenced by R29.
• SP is referenced by R30.
• Floating point registers are referenced by FP and a decimal number from 0 to 31. For example, floating point register 1₁₀ is FP1₁₀ and floating point register 10₁₀ is FP10₁₀.
• FPCR is treated like any other floating point register except, to explicitly open it, you specify FPCR/.
• Internal processor registers (IPRs) are accessed symbolically, for example, P(ASTEN). For IPR names, see the Alpha Architecture Reference Manual.

Floating point registers can be accessed from DELTA and from XDELTA but only if floating point arithmetic is enabled in the current process.

DELTA runs in the context of a process. Access to floating point registers is enabled as soon as the first floating point instruction in the code being examined is executed. Access is disabled as soon as that image completes execution.

When the system enters XDELTA, some process is the current process, and that current process may not be obvious. If that process happens to have floating point enabled at the time (because a floating point instruction had executed and the image containing the floating point instruction was still executing), then you can access the floating point registers. Otherwise, you cannot. XDELTA checks the FEN (floating point enable) IPR (internal processor register) to see if it needs to provide access to floating point registers.

3.2.3 Referencing Registers (VAX Only)

On VAX, to view the contents of the 16 general registers (including the program counter and the stack pointer) and the processor status longword (PSL), use the same DELTA/XDELTA commands as you use to view the contents of any memory location (for example, the /, LINEFEED, and the ESC commands). The symbols used to identify the locations of the registers and PSL are as follows:

• The general registers are referred to by the symbol R and a hexadecimal number from 0₁₆ to F₁₆ representing the number of the register. For example, general register 1₁₀ is R1₁₆ and general register 10₁₀ is RA₁₆. The stack pointer is located in general register 14₁₀, RE₁₆. The program counter is in general register 15₁₀, RF₁₆.
• Upon entry to DELTA or XDELTA, the PSL is stored in the longword directly following the longword representing general register F₁₆. Reference it by using the general register F₁₆ symbol plus a longword (RF+4).

When you make an error entering a command in DELTA or XDELTA, you get the "Eh?" error message. This is the only error message generated by DELTA and XDELTA. It is displayed under the following circumstances:

• You entered characters that DELTA/XDELTA does not recognize
• You entered a command incorrectly
• You exceeded the limits of the command (for example, trying to set another breakpoint when all breakpoints are used)
• You attempted to display a particular memory address and one or more of the following is true:
  • Location is not a valid memory address
  • You have no privilege to read the address
  • The process to which the read applies does not exist (DELTA only)
• You attempted to change a particular memory address (including setting a breakpoint) and one or more of the following is true:
  • The location is not a valid memory address
  • You have no privilege to write to the address
  • The process to which the write applies does not exist (DELTA only)

On I64, the error message is also displayed if you are unable to step over a subroutine call due to no write access to the address of the next instruction.

On Alpha, the error message is also displayed if you are unable to single-step or proceed due to no write access to the address of the next instruction.

3.4 Debugging Kernel Mode Code Under Certain Conditions

Some programs exist which, while running in process space, change mode to kernel and raise IPL. Typically, this code is debugged with both DELTA and XDELTA. DELTA is used to debug the kernel mode code at IPL zero. XDELTA is used to debug the code at elevated IPL. (DELTA does not work at elevated IPL.)

Before you can debug such code with XDELTA, you must complete some setup work.