Intel 80286, 80287 manual Extended Instruction SET

EXTENDED INSTRUCTION SET

BOUND (Detect Value Out of Range) verifies that the signed value contained in the specified register lies within specified limits. An interrupt (INT 5) occurs if the value contained in the register is less than the lower bound or greater than the upper bound.

The BOUND instruction includes two operands. The first operand specifies the register being tested. The second operand contains the effective relative address of the two signed BOUND limit values. The BOUND instruction assumes that it can obtain the upper limit from the memory word that immedi- ately follows the lower limit. These limit values cannot be register operands; if they are, an invalid opcode exception occurs.

BOUND is useful for checking array bounds before using a new index value to access an element within the array. BOUND provides a simple way to check the value of an index register before the program overwrites information in a location beyond the limit of the array.

The two-word block of memory that specifies the lower and upper limits of an array might typically reside just before the array itself. This makes the array bounds accessible at a constant offset of -4 from the beginning of the array. Because the address of the array will already be present in a register, this practice avoids extra calculations to obtain the effective address of the array bounds.

Example: BOUND BX,ARRAY-4. Compares the value in BX with the lower limit at address ARRAY-4 and the upper limit at address ARRAY-2. If the signed value in BX is less than the lower bound or greater than the upper bound, the interrupt for this instruction (INT 5) occurs. Otherwise, this instruction has no effect.

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