Intel 80287, 80286 manual Clocks, Description, Flags Modified

THE 80286 INSTRUCTION SET

xb: a simple byte memory variable without a base or index register. MOY instructions between AL

and memory have this optimized form if no indexing is required. ..

xw: a simple word memory variable without a base or index register. MOY instructions between AX and memory have this optimized form if no indexing is required.

Clocks

This column gives the number of clock cycles that this form of the instruction .takes to execute. The amount of time for each clock cycle is computed by dividing one microsecond by the number of MHz at which the 80286 is running. For example, a lO-MHz 80286 (with the eLK pin connected to a 20-MHz crystal) takes 100 nanoseconds for each clock cycle.

Add one clock to instructioris that use the base plus index plus displacement form of addressing. Add two clocks for each 16-bit memory based operand reference located on an odd physical address. Add one clock for each wait state added to each memory read. Wait states inserted in memory writes or instruction fetches do not necessarily increase execution time.

The clock counts establish the maximum execution rate of the 80286. With no delays in bus cycles, the actual clock count of an 80286 program will average 5-10% more than the calculated cl()ck count due to instruction sequences that execute faster than they can be fetched from memory.

Some instruction forms give two clock counts, one unlabelled and one labelled. These counts indicate that the instruction has two different clock times for two different circumstances. Following are the circumstances for each possible label:

mem: The instruction has an operand that can either be a register or a memory variable. The unlabelled time is for the register; the mem time is for the memory variable. Also, one additional clock cycle is taken for indexed memory variables for which all three possible indices (base register, index register, and displacement) must be added.

noj: The instruction involves a conditional jump or interrupt. The unlabelled time holds when the jump is made; the noj time holds when the jump is not made.

pm: If the instruction takes more time to execute when the 80286 is in Protected Mode. The unlabelled time is for Real Address Mode; the pm time is for Protected Mode.

Description

This is a concise description of the. operation performeulur ili;5 fviill Gf the :~~tr!!ct!0!!. MOT" rletails are given in the "Operation" section that appears later in this chapter.

Flags Modified

This is a list of the flags that are set to a meaningful value by the instruction. If a flag is always set to

the same value by the instruction, the value is given ("=0" or "= I") after the flag name. .

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