Intel 80286, 80287 manual Software-Generated Interrupts

BASIC INSTRUCTION SET

LOOPE (Loop While Equal) and LOOPZ (Loop While Zero) are physically the same instruction. These instructions auto-decrement the ex register before testing ex and ZF for the branch conditions. If ex is non-zero and ZF= 1, the program branches to the target label specified in the instruction. If LOOPE or LOOPZ finds that ex=o or ZF=O, control transfers to the instruction immediately succeeding the LOOPE or LOOPZ instruction.

Example: LOOPE START_LOOP (or LOOPZ START_LOOP). Each time the program encounters this instruction, it decrements ex and tests ex and ZF. If the value in ex is non-zero and the value of ZF is 1, the program branches to the instruction labeled START_LOOP. If ex=o or ZF=O, the program continues with the instruction that follows the LOOPE (or LOOPZ) instruction.

LOOPNE (Loop While Not Equal) and LOOPNZ (Loop While Not Zero) are physically the same instruction. These instructions auto-decrement the ex register before testing ex and ZF for the branch conditions. If ex is non-zero and ZF=O, the program branches to the target label specified in the instruction. If LOOPNE or LOOPNZ finds that ex=o or ZF= 1, control transfers to the instruction immediately succeeding the LOOPNE or LOOPNZ instruction.

Example: LOOPNE START_LOOP (or LOOPNZ START_LOOP). Each time the program encounters this instruction, it decrements ex and tests ex and ZF. If the value of ex is non-zero and the value of ZF is 0, the program branches to the instruction labeled START_LOOP. If ex=o or ZF= 1, the program continues with the instruction that follows the LOOPNE (or LOOPNZ) instruction.

3.6.2.3 EXECUTING A LOOP OR REPEAT ZERO TIMES

JCXZ (Jump if CX Zero) branches to the label specified in the, instruction if it finds a value of zero in ex. Sometimes, it is desirable to design a loop that executes zero times if the count variable in ex is initialized to zero. Because the LOOP instructions (and repeat prefixes) decrement ex before they test it, a loop will execute 65,536 times if the program enters the loop with a zero value in ex. A programmer may conveniently overcome this problem with JeXZ, which enables the program to branch around the code within the loop if ex is zero when JeXZ executes.

Example: JeXZ TARGETLABEL. Causes the program to branch to the instruction labeled TARGETLABEL if ex=o when the instruction executes.

3.6.3 Software-Generated Interrupts

The INT n and INTO instructions allow the programmer to specify a transfer to an interrupt service routine from within a program. Interrupts 0-31 are reserved by Intel.

3.6.3.1 SOFTWARE INTERRUPT INSTRUCTION

INT n (Software Interrupt) activates the interrupt service routine that corresponds to the number coded within the instruction. Interrupt type 3 is reserved for internal software-generated interrupts. However, the INT instruction may specify any interrupt type to allow multiple types of internal inter- rupts or to test the operation of a service routine. The interrupt service routine terminates with an IRET instruction that returns control to the instruction that follows INT.

Example: INT 3. Transfers control to the interrupt service routine specified by a type 3 interrupt.

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