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11.1 Coprocessing

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

11.1.1 Coprocessor Identification

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.

11.1.2 ESC and WAIT Instructions

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:

Tests the emulation mode (EM) flag to determine whether coprocessor functions are being emulated by software.
Tests the TS flag to determine whether there has been a context change since the last ESC instruction.
For some ESC instructions, tests the ERROR# pin to determine whether the coprocessor detected an error in the previous ESC instruction.

The WAIT instruction is not an ESC instruction, but WAIT causes the CPU to perform some of the same tests that it performs upon encountering an ESC instruction. The processor performs the following actions for a WAIT instruction:

Waits until the coprocessor no longer asserts the BUSY# pin.
Tests the ERROR# pin (after BUSY# goes inactive). If ERROR# is active, the 80386 signals exception 16, which indicates that the coprocessor encountered an error in the previous ESC instruction.
WAIT can therefore be used to cause exception 16 if an error is pending from a previous ESC instruction. Note that, if no coprocessor is present, the ERROR# and BUSY# pins should be tied inactive to prevent WAIT from waiting forever or causing spurious exceptions.

11.1.3 EM and MP Flags

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.

11.1.4 The Task-Switched Flag

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.

11.1.5 Coprocessor Exceptions

Three exceptions aid in interfacing to a coprocessor: interrupt 7 (coprocessor not available), interrupt 9 (coprocessor segment overrun), and interrupt 16 (coprocessor error).

11.1.5.1 Interrupt 7 -- Coprocessor Not Available

This exception occurs in either of two conditions:

The CPU encounters an ESC instruction and EM is set. In this case, the exception handler should emulate the instruction that caused the exception. TS may also be set.
The CPU encounters either the WAIT instruction or an ESC instruction when both MP and TS are set. In this case, the exception handler should update the state of the coprocessor, if necessary.

11.1.5.2 Interrupt 9 -- Coprocessor Segment Overrun

An operand of a coprocessor instruction wraps around an addressing limit (0FFFFH for small segments, 0FFFFFFFFH for big segments, zero for expand-down segments). An operand may wrap around an addressing limit when the segment limit is near an addressing limit and the operand is near the largest valid address in the segment. Because of the wrap-around, the beginning and ending addresses of such an operand will be near opposite ends of the segment.
Both the first byte and the last byte of the operand (considering wrap-around) are at addresses located in the segment and in present and accessible pages.
The operand spans inaccessible addresses. There are two ways that such an operand may also span inaccessible addresses:
1. The segment limit is not equal to the addressing limit (e.g., addressing limit is FFFFH and segment limit is FFFDH); therefore, the operand will span addresses that are not within the segment (e.g., an 8-byte operand that starts at valid offset FFFC will span addresses FFFC-FFFF and 0000-0003; however, addresses FFFE and FFFF are not valid, because they exceed the limit);
2. The operand begins and ends in present and accessible pages but intermediate bytes of the operand fall either in a not-present page or in a page to which the current procedure does not have access rights.

The address of the failing numerics instruction and data operand may be lost; an FSTENV does not return reliable addresses. As with the 80286/80287, the segment overrun exception should be handled by executing an FNINIT instruction (i.e., an FINIT without a preceding WAIT). The return address on the stack does not necessarily point to the failing instruction nor to the following instruction. The failing numerics instruction is not restartable.

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.

11.1.5.3 Interrupt 16 -- Coprocessor Error

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.

up: Chapter 11 -- Coprocessing and Multiprocessing
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