V - Exception/Trap/Fault Vector Commands

This group of commands manipulates IDT entries 0 through e to point to Kernel Debugger supplied interrupt handlers. By this means the Kernel Debugger may selectively be made to intercept each system exception before the system is allowed to process the exception. When a system exception is intercepted the Kernel Debugger gives control to the user. The original IDT entries are retained so that they may be re-instated, or given control following an exception which the Kernel Debugger has been intercepted. See the GT and TT commands for information in returning control to the system exception handlers.

Syntax:

────── V ─┬──  L  ──┬──┬───────┬───┬────────────────────┬─────
            ├──  S  ──┤  ├── R ──┤   ├──── interrupt ──┬──┤
            ├──  T  ──┤  ├── V ──┤   │                │  │
            └──  C  ──┘  ├── P ──┤   │  └───── , ──────┘  │
                         ├── F ──┤   └──────── * ─────────┘
                         ├── N ──┤
                         └── U ──┘

Parameters:

L

The List subcommand list active Kernel Debugger trap and interrupt vectors.

Only a category specification (R, V, P, F or N) may be optionally specified with the List subcommand.

S
The Set subcommand activates a Kernel Debugger exception vector according to criteria specified in the remaining parameters. Vectors set using this option cause the Kernel Debugger to receive control only when the corresponding exceptions are generated in ring 2 and 3 code.
T
The Trap subcommand activates a Kernel Debugger exception vector according to criteria specified in the remaining parameters. Vectors set using this option cause the Kernel Debugger to receive control whenever the corresponding exceptions are generated regardless of the current privileged level.
C
The Clear subcommand re-instates one or more system exception handlers according to the criteria specified in the remaining parameters.
R
This option refines the exception criteria to Real-mode exceptions only.

If no refining category is specified then the vector subcommand being executed applies to the R, V, P and F options simultaneously.

V
This option refines the exception criteria to V86-mode exceptions only.
P
This option refines the exception criteria to Protect-mode exceptions only.
F
This option refines the exception criteria to those exceptions that would be 'fatal' to a process or the system. If a Local (system) Exception handler is registered then the exception is not intercepted. User Exception Handlers do not affect the operation of the Vector command. Local Exception Handlers protect the system from recoverable errors, in particular bad parameters passed in API calls. If a parameter causes the system to trap, the Local Exception Handler is given control and the application is terminated. VSF will not intercept such traps. For further information on exception handling and how to intercept exceptions in general, see Trap and Exception Processing.
U
This option allows exceptions, fatal to a process, to be intercepted before the process is terminated. Interception occurs if the exception is not recovered by any user exception handler. VSU will intercept 'user-fatal' exceptions whether they originate from the system or the user. In particular if a local (system) exception handler has intercepted a kernel trap due to a bad API parameter then this will be intercepted if no user exception handler recovers from the error.

Note: This option was introduced with OS/2 Warp V3.0 fix pack 26 and OS/2 Warp V4.0 fix pack 1.

N
This option causes the Kernel Debugger exception handler to 'beep' continuously instead of giving control to the user. The user may then break into the Kernel Debugger by entering Ctrl-C at the debugging console.

The N option works in conjunction with the four refining categories, that is, it does not by itself cause an interrupt to be trapped but instead specifies an action when that event occurs.

The N option must be explicitly specified for all four subcommands (L S, T and C) when required.

interrupt
This allows one or more interrupt vectors, separated by commas, to be specified with the vector command as a refining criterion.

It is not valid with the List subcommand.

The abbreviation * may be specified as an alternative to the following interrupts, in each of the refining categories:

Real-mode:

0,1,2,3,4,5,6
V86-mode:
0,1,3,4,5,6,7,9,a,b,c,d,e,(f,10)
Protect-mode:
0,1,3,4,5,6,7,9,a,b,c,d,e,(f,10)
Fatal option:
0,1,3,4,5,6,7,9,a,b,c,d,e,(f,10)
User-fatal option:
0,1,3,4,5,6,7,9,a,b,c,d,e,(f,10)
Noise option:
0,1,3,4,5,6,7,9,a,b,c,d,e,(f,10)
Note:

Vectors f and 10 are only allowed with OS/2 Warp SMP

Results & Notes:

Only the List subcommand gives immediate output, which is of the form in the following example:

---------------------------------------------------##VL
R 0 1 2 3 4 5 6
V
P d
U e
F e d
---------------------------------------------------

As can bee seen from this example each category is shown with its one-letter abbreviation followed by a list of interrupt vectors currently being intercepted by the Kernel Debugger

Notes:

The N option must be specified explicitly to be listed.

The U option is only from OS/2 Warp V3.0 fix pack 26 and OS/2 Warp V4.0 fix pack 1.

All other subcommands only cause output to appear when an interrupt is intercepted. When this happens the following events occur:

• The N option is checked, if enabled the Kernel Debugger emits a continuous beep until the user breaks in through the debugging console.

• A trap message is issued if the default command is set to the R command.

• The default command is executed.

The following figure shows the format of the trap messages issued by the Kernel Debugger exception handlers:

Trap 0 - Divide Error Exception
Trap 1 - Unexpected trace interrupt
Trap 2 - NMI Interrupt
Trap 4 - INTO Detected Overflow Exception
Trap 5 - BOUND Range Exceeded Exception
Trap 6 - Invalid Opcode Exception
Trap 7 - Processor Extension Not Available Exception
Trap 8 - Double Exception Detected nnnn
Trap 9 - Processor Extension Segment Overrun
Trap 10 (0AH) - Invalid TSS nnnn, mmmmmmmm
Trap 11 (0BH) - Segment Not Present nnnn, mmmmmmmm
Trap 12 (0CH) - Stack Segment Overrun or Not Present nnnn, mmmmmmmm
Trap 13 (0DH) - General Protection Fault nnnn, mmmmmmmm
Trap 14 (0EH) - Page Fault nnnn, mmmmmmm

In the messages above nnnn is substituted with the Intel exception code and mmmmmmmm is substituted with an interpretation of the Intel error code flags. For Trap 10, Trap 11, Trap 12 and Trap 13 the error code flags are interpreted as:

External

External event
IDT Gate
IDT gate selector error
GDT
GDT selector error
LDT
LDT Selector error

For Trap 14 the error code flags are interpreted as a combination of:

Not Present

Page not present
Read Access
Read Access failure
Write Access
Write Access Failure
User mode
Fault occurred when executing in User mode
Supervisor
Fault occurred when executing in Supervisor mode

If a trap occurs in the debugger component of the Kernel Debugger the trap message will be appended with:

- In Debugger

If this happens then the only hope of recovering the system is to set the registers, using the R command, to a known consistent set of values.

See the INTEL x86 Programmer's Reference or the INTEL Pentium User's Guide for further information on exceptions and error codes.

Notes

Trap 1 normally occurs as part of the operation of the Kernel Debugger. Only unexpected Trap 1 exception are therefore reported.

When a Trap 1 is generated through use of the Debug Registers then the Kernel Debugger signals this with the message Debug register hit.

Trap 3 occurs through use of the INT 3 instruction. This is used both by the Kernel Debugger and user programs in implementing break-points. User programs may use the INT 3 instruction as a program controlled technique for breaking into the debugger. In these cases a trap message is not displayed.

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