Developer Manual/Crashdump

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The wiki is being retired!

Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to Contributions welcome!


Let begin by example and dissect it. An example Coreboot crash dump is shown:

  PCI: 00:18.7: enabled 1
Mainboard NF81-T56N-LF Enable.
Unexpected Exception: 6 @ 10:00246faa - Halting
Code: 0 eflags: 00010402
eax: fdb01c7c ebx: 0027c4a4 ecx: 000c34b1 edx: 000045fd
edi: 00000002 esi: 00000078 ebp: 00287ff0 esp: 00287f6d

00246f68:       0d 26 00 c7 04 24 06 00
00246f70:       00 00 e8 a9 c5 00 00 c6
00246f78:       05 00 0e d8 fe ff c6 05
00246f80:       01 0e d8 fe 00 fd 05 02
00246f88:       0e d8 fe 00 c6 00 03 0e
00246f90:       d8 fe 00 c6 05 83 0e d8
00246f98:       fe 00 c7 44 24 66 80 00
00246fa0:       00 00 c7 04 24 fd 00 00
00246fa8:       00 e8 fe d8 ff 00 c7 44
00246fb0:       24 04 61 00 00 83 c7 04
00246fb8:       24 28 00 00 00 66 ea d8
00246fc0:       ff ff 83 c4 1c c3 c3 c3
00246fc8:       57 bf 00 0c 00 00 56 53
00246fd0:       83 ec 30 8b 44 24 40 c7
00246fd8:       44 24 04 00 00 00 00 89
00246fe0:       04 24 e8 a3 fb 00 00 8d

Here the line, Unexpected Exception: 6 @ 10:00246faa - Halting, is key to understanding the dump. In this example an Exception: 6 signifies an unknown Opcode was attempt to execute and 10:00246faa has two components, the segment selector (0x10) and address (e000_0001).

x86 Interlude

To understand the next chunk:

Code: 0 eflags: 00010402
eax: fdb01c7c ebx: 0027c4a4 ecx: 000c34b1 edx: 000045fd
edi: 00000002 esi: 00000078 ebp: 00287ff0 esp: 00287f6d

we remind ourselves of some x86 asmebler.

Register Summary Flags
General registers Segment registers Index and pointers Indicator

General registers

The following are 32 bits wide registers : EAX EBX ECX EDX

EAX : Called the Accumulator register. 
      It is used for I/O port access, arithmetic, interrupt calls, etc...
EBX : Called the Base register
      It is used as a base pointer for memory access
      Gets some interrupt return values
ECX : Called the Counter register
      It is used as a loop counter and for shifts
      Gets some interrupt values
EDX : Called the Data register
      It is used for I/O port access, arithmetic, some interrupt calls.

Segment registers

Segment registers hold the segment address of various items. They are only available in 16 values. They can only be set by a general register or special instructions.

CS         : Holds the Code segment in which your program runs.
             Changing its value might make the computer hang.
DS         : Holds the Data segment that your program accesses.
             Changing its value might give erronous data.
ES,FS,GS   : These are extra segment registers available for
             far pointer addressing like video memory and such.
SS         : Holds the Stack segment your program uses.
             Sometimes has the same value as DS.
             Changing its value can give unpredictable results,
             mostly data related.

Indexes and Pointers

Indexes and pointer and the offset part of and address. They have various uses but each register has a specific function. They some time used with a segment register to point to far address (in a 1Mb range). The register with an "E" prefix can only be used in protected mode.

ES:EDI EDI DI : Destination index register
                Used for string, memory array copying and setting and
                for far pointer addressing with ES
DS:ESI EDI SI : Source index register
                Used for string and memory array copying
SS:EBP EBP BP : Stack Base pointer register
                Holds the base address of the stack
SS:ESP ESP SP : Stack pointer register
                Holds the top address of the stack
CS:EIP EIP IP : Index Pointer
                Holds the offset of the next instruction
                It can only be read

The EFLAGS Register

The EFLAGS register hold the state of the processor. It is modified by many intructions and is used for comparing some parameters, conditional loops and conditionnal jumps. Each bit holds the state of specific parameter of the last instruction.

Bit   Label    Desciption
0      CF      Carry flag
2      PF      Parity flag
4      AF      Auxiliary carry flag
6      ZF      Zero flag
7      SF      Sign flag
8      TF      Trap flag
9      IF      Interrupt enable flag
10     DF      Direction flag
11     OF      Overflow flag
12-13  IOPL    I/O Priviledge level
14     NT      Nested task flag
16     RF      Resume flag
17     VM      Virtual 8086 mode flag
18     AC      Alignment check flag (486+)
19     VIF     Virutal interrupt flag
20     VIP     Virtual interrupt pending flag
21     ID      ID flag

*Protected-mode Programming

Protected mode segmentation registers are GDTR (Global Descriptor Table Register), IDTR (Interrupt Descriptor Table Register), LDTR (Local DTR), and TR.

x86 Exception Summary

The following chart lists the exceptions that can be generated by the Intel 80286, 80386, 80486, and Pentium processors:

Exception | Description
(dec/hex) |
  0  00h  | Divide error:
          | Occurs during a DIV or an IDIV instruction when the
          | divisor is zero or a quotient overflow occurs.

  1  01h  | Single-step/debug exception:
          | Occurs for any of a number of conditions:
          | - Instruction address breakpoint fault
          | - Data address breakpoint trap
          | - General detect fault
          | - Single-step trap
          | - Task-switch breakpoint trap

  2  02h  | Nonmaskable interrupt:
          | Occurs because of a nonmaskable hardware interrupt.

  3  03h  | Breakpoint:
          | Occurs when the processor encounters an INT 3 instruction.

  4  04h  | Overflow:
          | Occurs when the processor encounters an INTO instruction
          | and the OF (overflow) flag is set.

  5  05h  | Bounds check (BOUND instruction):
          | Occurs when the processor, while executing a BOUND
          | instruction, finds that the operand exceeds the specified
          | limit.

  6  06h  | Invalid opcode:
          | Occurs when an invalid opcode is detected.

  7  07h  | Coprocessor not available:
          | Occurs for one of two conditions:
          | - The processor encounters an ESC (escape) instruction
          |   and the EM (emulate) bit of CR0 (control register zero)
          |   is set.
          | - The processor encounters either the WAIT instruction or
          |   an ESC instruction and both the MP (monitor
          |   coprocessor) and TS (task switched) bits of CR0 are set.

  8  08h  | Double fault:
          | Occurs when the processor detects an exception while
          | trying to invoke the handler for a prior exception.

  9  09h  | Coprocessor segment overrun:
          | Occurs when a page or segment violation is detected while
          | transferring the middle portion of a coprocessor operand
          | to the NPX.

 10  0Ah  | Invalid TSS:
          | Occurs if, during a task switch, the new TSS is invalid.

 11  0Bh  | Segment not present:
          | Occurs when the processor detects that the present bit of
          | a descriptor is zero.

 12  0Ch  | Stack exception:
          | Occurs for one of two conditions:
          | - As a result of a limit violation in any operation that
          |   refers to SS (stack segment register)
          | - When attempting to load SS with a descriptor that is
          |   marked as not-present but is otherwise valid

 13  0Dh  | General protection violation:
          | Each protection violation that does not cause another
          | exception causes a general protection exception.
          | - Exceeding segment limit when using CS, DS, ES, FS, or GS
          | - Exceeding segment limit when referencing a descriptor
          |   table
          | - Transferring control to a segment that is not executable
          | - Writing to a read-only data segment or a code segment
          | - Reading from an execute-only segment
          | - Loading SS with a read-only descriptor
          | - Loading SS, DS, ES, FS, or GS with a descriptor of a system
          |   segment
          | - Loading DS, ES, FS, or GS with the descriptor of an
          |   executable segment that is also not readable
          | - Loading SS with the descriptor of an executable segment
          | - Accessing memory through DS, ES, FS, or GS when the segment
          |   register contains a NULL selector
          | - Switching to a busy task
          | - Violating privilege rules
          | - Loading CR0 with PG=1 and PE=0
          | - Interrupt or exception through trap or interrupt gate from
          |   V86 mode to a privilege level other than 0

 14  0Eh  | Page fault:
          | Occurs when paging is enabled (PG=1) and the processor
          | detects one of the following conditions while translating
          | a linear address to a physical address:
          | - The page-directory or page-table entry needed for the
          |   address translation has 0 in its present bit.
          | - The current procedure does not have sufficient privilege
          |   to access the indicated page.

 15  0Fh  | (Reserved)

 16  10h  | Coprocessor error:
          | Occurs when the processor detects a signal from the
          | coprocessor on the ERROR# input pin.

 17  11h  | (Reserved)
 through  |
 31  1Fh  |

See Intel 80386 Programmer's Reference Manual for further detail.