Intel 80C186XL, 80C188XL user manual Invalid Opcode Type

OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE

Breakpoint Interrupt — Type 3

The Breakpoint Interrupt is a single-byte version of the INT instruction. It is commonly used by software debuggers to set breakpoints in RAM. Because the instruction is only one byte long, it can substitute for any instruction.

Interrupt on Overflow — Type 4

The Interrupt on Overflow trap occurs if the Overflow Flag (OF) bit is set in the Processor Status Word and the INT0 instruction is executed. Interrupt on Overflow is a common method for han- dling arithmetic overflows conditionally.

Array Bounds Check — Type 5

An Array Bounds trap occurs when the array index is outside the array bounds during execution of the BOUND instruction (see Appendix A, “80C186 Instruction Set Additions and Exten- sions”).

Invalid Opcode — Type 6

Execution of an undefined opcode causes an Invalid Opcode trap.

Escape Opcode — Type 7

The Escape Opcode fault is used for floating point emulation. With 80C186 Modular Core family members, this fault is enabled by setting the Escape Trap (ET) bit in the Relocation Register (see Chapter 4, “Peripheral Control Block”). When a floating point instruction is executed with the Escape Trap bit set, the Escape Opcode fault occurs, and the Escape Opcode service routine em- ulates the floating point instruction. If the Escape Trap bit is cleared, the CPU sends the floating point instruction to an external 80C187.

80C188 Modular Core Family members do not support the 80C187 interface and always generate the Escape Opcode Fault. The 80C186XL will generate the Escape Opcode Fault regardless of the state of the Escape Trap bit unless it is in Numerics Mode.

Numerics Coprocessor Fault — Type 16

The Numerics Coprocessor fault is caused by an external 80C187 numerics coprocessor. The 80C187 reports the exception by asserting the ERROR pin. The 80C186 Modular Core checks the ERROR pin only when executing a numerics instruction. A Numerics Coprocessor Fault in- dicates that the previous numerics instruction caused the exception. The 80C187 saves the ad- dress of the floating point instruction that caused the exception. The return address pushed onto the stack during the interrupt processing points to the numerics instruction that detected the ex- ception. This way, the last numerics instruction can be restarted.

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