Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

5-22 Vol. 3A

INTERRUPT AND EXCEPTION HANDLING

5.13 ERROR CODE

When an exception condition is related to a specific segment, the processor pushes

an error code onto the stack of the exception handler (whether it is a procedure or

task). The error code has the format shown in Figure 5-6. The error code resembles

a segment selector; however, instead of a TI flag and RPL field, the error code

contains 3 flags:

EXT External event (bit 0) — When set, indicates that an event external

to the program, such as a hardware interrupt, caused the exception.

IDT Descriptor location (bit 1) — When set, indicates that the index

portion of the error code refers to a gate descriptor in the IDT; when

clear, indicates that the index refers to a descriptor in the GDT or the

current LDT.

TI GDT/LDT (bit 2) — Only used when the IDT flag is clear. When set,

the TI flag indicates that the index portion of the error code refers to

a segment or gate descriptor in the LDT; when clear, it indicates that

the index refers to a descriptor in the current GDT.

The segment selector index field provides an index into the IDT, GDT, or current LDT

to the segment or gate selector being referenced by the error code. In some cases

the error code is null (that is, all bits in the lower word are clear). A null error code

indicates that the error was not caused by a reference to a specific segment or that a

null segment descriptor was referenced in an operation.

The format of the error code is different for page-fault exceptions (#PF). See the

“Interrupt 14—Page-Fault Exception (#PF)” section in this chapter.

The error code is pushed on the stack as a doubleword or word (depending on the

default interrupt, trap, or task gate size). To keep the stack aligned for doubleword

pushes, the upper half of the error code is reserved. Note that the error code is not

popped when the IRET instruction is executed to return from an exception handler, so

the handler must remove the error code before executing a return.

Error codes are not pushed on the stack for exceptions that are generated externally

(with the INTR or LINT[1:0] pins) or the INT n instruction, even if an error code is

normally produced for those exceptions.

Figure 5-6. Error Code

Reserved

123

Segment Selector Index