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BOUND -- Check Array Index Against Bounds




Opcode    Instruction          Clocks    Description

62 /r     BOUND r16,m16&16     10        Check if r16 is within bounds
                                         (passes test)
62 /r     BOUND r32,m32&32     10        Check if r32 is within bounds
                                         (passes test)

Operation




IF (LeftSRC < [RightSRC] OR LeftSRC > [RightSRC + OperandSize/8])
   (* Under lower bound or over upper bound *)
THEN Interrupt 5;
FI;

Description

BOUND ensures that a signed array index is within the limits specified by a block of memory consisting of an upper and a lower bound. Each bound uses one word for an operand-size attribute of 16 bits and a doubleword for an operand-size attribute of 32 bits. The first operand (a register) must be greater than or equal to the first bound in memory (lower bound), and less than or equal to the second bound in memory (upper bound). If the register is not within bounds, an Interrupt 5 occurs; the return EIP points to the BOUND instruction.

The bounds limit data structure is usually placed just before the array itself, making the limits addressable via a constant offset from the beginning of the array.

Flags Affected

None

Protected Mode Exceptions

Interrupt 5 if the bounds test fails, as described above; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

The second operand must be a memory operand, not a register. If BOUND is executed with a ModRM byte representing a register as the second operand, #UD occurs.

Real Address Mode Exceptions

Interrupt 5 if the bounds test fails; Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH; Interrupt 6 if the second operand is a register

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

up: Chapter 17 -- 80386 Instruction Set
prev: ARPL Adjust RPL Field of Selector
next: BSF Bit Scan Forward