User's Manual

THE 80286 INSTRUCTION SET
BOUND-Check Array Index Against Bounds
Opcode Instruction Clocks
Description
62. /r
BOUND rw,md noj=13
INT 5 if rw not within bounds
FLAGS MODIFIED
None
FLAGS UNDEFINED
None
OPERATION
BOUND is used to ensure that a signed array index is within the limits defined by a two-word block
of memory. The
first operand (a register) tnustbe greater than or equal to the first word in memory,
and
·less than or equal to the second word in memory. If the register is not within the bounds, an
INTERRUPT 5 occurs. .
The two-word block might typically be found just before the array itself and therefore would be acces-
sible at a constant offset of -4 from the array, simplifying the addressing.
PROTECTED MODE EXCEPTIONS.
INTERRUPT 5 if the bounds test fails, as described above. #GP(O) for an illegal memory operand
effective address in the C8, D8, or
E8 segments; #88(0) for an illegal address in the 88 segment.
The second operand must be a memory operand, not a register.
If the BOUND instruction is executed
with a ModRM
byte representing a register second operand, then fault #UD will occUr.
REAL ADDRESS MODE EXCEPTIONS
INTERRUPT 5 if the bounds test fails, as described above. Interrupt 13 for a second operand at offset
OFFFDH or higher. Interrupt 6 if the second operand
is a register,as described in the paragraph above.
B-22