Processor Programming (Continued)
Single-step operation is enabled by setting the TF bit (bit
8) in the EFLAGS register. When the TF is set, the CPU
generates a debug exception (vector 1) after the execution
of every instruction. Data breakpoints also generate a
debug exception and are specified by loading the debug
registers (DR0-DR3, see Table 3-12 on page 55) with the
appropriate values.
3.6.3.2 Interrupt Descriptor Table
The interrupt vector number is used by the CPU to locate
an entry in the interrupt descriptor table (IDT). In real
mode, each IDT entry consists of a 4-byte far pointer to
the beginning of the corresponding interrupt service rou-
tine. In protected mode, each IDT entry is an 8-byte
descriptor. The Interrupt Descriptor Table Register (IDTR)
specifies the beginning address and limit of the IDT. Fol-
lowing RESET, the IDTR contains a base address of
00000000h with a limit of 3FFh.
A Fault exception is reported before completion of the
instruction that generated the exception. By reporting the
fault before instruction completion, the CPU is left in a
state that allows the instruction to be restarted and the
effects of the faulting instruction to be nullified. Fault
exceptions include divide-by-zero errors, invalid opcodes,
page faults and coprocessor errors. Debug exceptions
(vector 1) are also handled as faults (except for data
breakpoints and single-step operations). After execution
of the fault service routine, the instruction pointer points to
the instruction that caused the fault.
The IDT can be located anywhere in physical memory as
determined by the IDTR register. The IDT may contain dif-
ferent types of descriptors: interrupt gates, trap gates and
task gates. Interrupt gates are used primarily to enter a
hardware interrupt handler. Trap gates are generally used
to enter an exception handler or software interrupt han-
dler. If an interrupt gate is used, the Interrupt Enable Flag
(IF) in the EFLAGS register is cleared before the interrupt
handler is entered. Task gates are used to make the tran-
sition to a new task.
An Abort exception is a type of fault exception that is
severe enough that the CPU cannot restart the program at
the faulting instruction. The double fault (vector 8) is the
only abort exception that occurs on the CPU.
Table 3-29. Interrupt Vector Assignments
3.6.3 Interrupt Vectors
When the CPU services an interrupt or exception, the cur-
rent program’s instruction pointer and flags are pushed
onto the stack to allow resumption of execution of the
interrupted program. In protected mode, the processor
also saves an error code for some exceptions. Program
control is then transferred to the interrupt handler (also
called the interrupt service routine). Upon execution of an
IRET at the end of the service routine, program execution
resumes at the instruction pointer address saved on the
stack when the interrupt was serviced.
Interrupt
Vector
Exception
Type
Function
Divide error
0
Fault
Trap/Fault*
---
1
Debug exception
NMI interrupt
2
3
Breakpoint
Trap
4
Interrupt on overflow
BOUND range exceeded
Invalid opcode
Trap
5
Fault
Fault
Fault
Abort
---
6
7
Device not available
Double fault
3.6.3.1 Interrupt Vector Assignments
Each interrupt (except SMI#) and exception are assigned
one of 256 interrupt vector numbers as shown in Table 3-
29. The first 32 interrupt vector assignments are defined
or reserved. INT instructions acting as software interrupts
may use any of interrupt vectors, 0 through 255.
8
9
Reserved
10
11
12
13
14
15
16
17
18:31
32:55
0:255
Invalid TSS
Fault
Fault
Fault
Trap/Fault
Fault
---
Segment not present
Stack fault
The non-maskable hardware interrupt (NMI) is assigned
vector 2. Illegal opcodes including faulty FPU instructions
will cause an illegal opcode exception, interrupt vector 6.
NMI interrupts are enabled by setting bit 2 of the CCR7
register (Index EBh[2] = 1, see Table 3-11 on page 52 for
register format).
General protection fault
Page fault
Reserved
FPU error
Fault
Fault
---
Alignment check exception
Reserved
In response to a maskable hardware interrupt (INTR), the
CPU issues interrupt acknowledge bus cycles used to read
the vector number from external hardware. These vectors
should be in the range 32 to 255 as vectors 0 to 31 are pre-
defined.
Maskable hardware interrupts
Programmed interrupt
Trap
Trap
Note: *Data breakpoints and single steps are traps. All other
debug exceptions are faults.
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