Exceptions: Difference between revisions

'''Exceptions''', as described in this article, are a type of [[interrupt]] generated by the CPU when an 'error' occurs. Some exceptions are not really errors in most cases, such as [[#Page Fault|page faults]]. Exceptions are a type of [[interrupt]].

Some exceptions will push a 32-bit "error code" on to the top of the stack, which provides additional information about the error. This value must be pulled from the stack before returning control back to the currently running program. (i.e. before calling IRET). In [[Long Mode]], the error code is padded with zeros to form a 64-bit push, so that it can be popped like any other value.

! [[#Divide-by-zeroDivision Error|Divide-by-zeroDivision Error]]

| 2122-2927 (0x150x16-0x1D0x1B)

| -Fault

==== Divide-by-zeroDivision Error ====

The '''Divide-by-zeroDivision Error''' occurs when dividing any number by 0 using the DIV or IDIV instruction., Manyor OSwhen developersthe usedivision thisresult exceptionis totoo testlarge whetherto theirbe exceptionrepresented handlingin codethe worksdestination. ThisSince exceptiona mayfaulting alsoDIV occuror whenIDIV the resultinstruction is toovery largeeasy to beinsert representedanywhere in the destinationcode, many OS developers use this exception to test whether their exception handling code works.

The Invalid Opcode exception occurs when the processor tries to execute an invalid or undefined opcode, or an instruction with invalid prefixes. It also occurs whenin another instruction exceeds 15 bytescases, but this only occurs with redundantsuch prefixes.as:

An Invalid TSS exception occurs when an invalid segment selector is referenced as part of a task whichswitch, or as a result of a control transfer through a gate descriptor, which results in an invalid stack-segment reference using an SS selector in the TSS.

The Segment Not Present exception occurs when trying to load a segment or gate which has it'sits `Present-` bit set to 0.

'''Error code:''' The Stack-Segment Fault sets an error code, which is the stack [[#Selector Error Code|segment selector index]] when a non-present segment descriptor was referenced or a limit check failed during a hardware task switch. Otherwise (for present segments and already in use), the error code is 0.

* Writing a 1 in a reserved register field or writing invalid value combinations (e.g. CR0 with PE=0 and PG=1).

31 15 4 0

+---+-- --+---+-----+---+-- --+---+----+----+---+---+---+---+---+

| Reserved | SGX | Reserved | SS | PK | I | R | U | W | P |

+---+-- --+---+-----+---+-- --+---+----+----+---+---+---+---+---+

| When set, the page fault was caused by a page write access. When not set, it was caused by a page read access.

| When set, the page fault was caused by an instruction fetch. This only applies when the No-Execute bit is supported and enabled.

An Overflow exception is raised when the INTO instruction is executed while the overflow bit in RFLAGS is set to 1, or when the result of div/idiv insructions is bigger than 64 bit /32 bit / 16 bit /8bit depending on the instruction operand size .( only if bigger than the operand size.)

The saved instruction pointer points to the instruction after the INTO instruction, or when an div/idiv is the cuase of the exeption , the insruction pointer that pushed points to the faulty insruction.

In the old days, the floating point unit was a dedicated chip that could be attached to the processor. It lacked direct wiring of FPU errors to the processor, so instead it used [[PIC|IRQ 13]], allowing the CPU to deal with errors at it'sits own leasure. When the 486 was developed and multiprocessor support was added, the [[FPU]] was embedded on die and a global interrupt for FPUs became undesirable, instead getting an option for direct error handling. By default, this method is not enabled at boot for backwards compatibility, but an OS should update the settings accordingly.