The components of the coprocessor interface include:
- ET bit of control register zero (CR0) - The EM, and MP bits of CR0 - The ESC instructions - The WAIT instruction - The TS bit of CR0 - Exceptions
The 80386 is designed to operate with either an 80287 or 80387 math coprocessor. The ET bit of CR0 indicates which type of coprocessor is present. ET is set automatically by the 80386 after RESET according to the level detected on the ERROR# input. If desired, ET may also be set or reset by loading CR0 with a MOV instruction. If ET is set, the 80386 uses the 32-bit protocol of the 80387; if reset, the 80386 uses the 16-bit protocol of the 80287.
The 80386 interprets the pattern 11011B in the first five bits of an instruction as an opcode intended for a coprocessor. Instructions thus marked are called ESCAPE or ESC instructions. The CPU performs the following functions upon encountering an ESC instruction before sending the instruction to the coprocessor:
The EM and MP flags of CR0 control how the processor reacts to coprocessor instructions.
The EM bit indicates whether coprocessor functions are to be emulated. If the processor finds EM set when executing an ESC instruction, it signals exception 7, giving the exception handler an opportunity to emulate the ESC instruction.
The MP (monitor coprocessor) bit indicates whether a coprocessor is actually attached. The MP flag controls the function of the WAIT instruction. If, when executing a WAIT instruction, the CPU finds MP set, then it tests the TS flag; it does not otherwise test TS during a WAIT instruction. If it finds TS set under these conditions, the CPU signals exception 7.
The EM and MP flags can be changed with the aid of a MOV instruction using CR0 as the destination operand and read with the aid of a MOV instruction with CR0 as the source operand. These forms of the MOV instruction can be executed only at privilege level zero.
The TS bit of CR0 helps to determine when the context of the coprocessor does not match that of the task being executed by the 80386 CPU. The 80386 sets TS each time it performs a task switch (whether triggered by software or by hardware interrupt). If, when interpreting one of the ESC instructions, the CPU finds TS already set, it causes exception 7. The WAIT instruction also causes exception 7 if both TS and MP are set. Operating systems can use this exception to switch the context of the coprocessor to correspond to the current task. Refer to the 80386 System Software Writer's Guide for an example.
The CLTS instruction (legal only at privilege level zero) resets the TS flag.
Three exceptions aid in interfacing to a coprocessor: interrupt 7 (coprocessor not available), interrupt 9 (coprocessor segment overrun), and interrupt 16 (coprocessor error).
This exception occurs in either of two conditions:
This exception occurs in protected mode under the following conditions:
Case 2 can be avoided by either aligning all segments on page boundaries or by not starting them within 108 bytes of the start or end of a page. (The maximum size of a coprocessor operand is 108 bytes.) Case 1 can be avoided by making sure that the gap between the last valid offset and the first valid offset of a segment is either no less than 108 bytes or is zero (i.e., the segment is of full size). If neither software system design constraint is acceptable, the exception handler should execute FNINIT and should probably terminate the task.
The numerics coprocessors can detect six different exception conditions during instruction execution. If the detected exception is not masked by a bit in the control word, the coprocessor communicates the fact that an error occurred to the CPU by a signal at the ERROR# pin. The CPU causes interrupt 16 the next time it checks the ERROR# pin, which is only at the beginning of a subsequent WAIT or certain ESC instructions. If the exception is masked, the numerics coprocessor handles the exception according to on-board logic; it does not assert the ERROR# pin in this case.