Intel 80C186XL Clocking the 80C187, Processor Bus Cycles Accessing the 80C187, Math Coprocessing

MATH COPROCESSING

11.4.1 Clocking the 80C187

The microprocessor and math coprocessor operate asynchronously, and their clock rates may dif- fer. The 80C187 has a CKM pin that determines whether it uses the input clock directly or divided by two. Direct clocking works up to 12.5 MHz, which makes it convenient to feed the clock input from the microprocessor’s CLKOUT pin. Beyond 12.5 MHz, the 80C187 must use a multiply- by-two clock input up to a maximum of 32 MHz. The microprocessor and the math coprocessor have correct timing relationships, even with operation at different frequencies.

11.4.2 Processor Bus Cycles Accessing the 80C187

Data transfers between the microprocessor and the 80C187 occur through the dedicated, 16-bit I/O ports shown in Table 11-7. When the processor encounters a numerics opcode, it first writes the opcode to the 80C187. The 80C187 decodes the instruction and passes elementary instruction information (Opcode Status Word) back to the processor. Since the 80C187 is a slave processor, the Modular Core processor performs all loads and stores to memory. Including the overhead in the microprocessor’s microcode, each data transfer between memory and the 80C187 (via the mi- croprocessor) takes at least 17 processor clocks.

Table 11-7. 80C187 I/O Port Assignments

I/O Address	Read Definition	Write Definition
00F8H	Status/Control	Opcode
00FAH	Data	Data
00FCH	Reserved	CS:IP, DS:EA
00FEH	Opcode Status	Reserved

The microprocessor cannot process any numerics (ESC) opcodes alone. If the CPU encounters a numerics opcode when the Escape Trap (ET) bit in the Relocation Register is a zero and the 80C187 is not present, its operation is indeterminate. Even the FINIT/FNINIT initialization in- struction (used in the past to test the presence of a coprocessor) fails without the 80C187. If an application offers the 80C187 as an option, problems can be prevented in one of three ways:

•Remove all numerics (ESC) instructions, including code that checks for the presence of the 80C187.

•Use a jumper or switch setting to indicate the presence of the 80C187. The program can interrogate the jumper or switch setting and branch away from numerics instructions when the 80C187 socket is empty.

•Trick the microprocessor into predictable operation when the 80C187 socket is empty. The fix is placing pull-up or pull-down resistors on certain data and handshaking lines so the CPU reads a recognizable Opcode Status Word. This solution requires a detailed knowledge of the interface.

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