Intel 80386 manual Instruction SET, Instruction Encoding

intJ80386

8. INSTRUCTION SET

This section describes the 80386 instruction set. A table lists all instructions along with instruction en- coding diagrams and clock counts. Further details of the instruction encoding are then provided in the fol- lowing sections, which completely describe the en- coding structure and the definition of all fields occur- ring within 80386 instructions.

8.1 80386 INSTRUCTION ENCODING

AND CLOCK COUNT SUMMARY

To calculate elapsed time for an instruction, multiply the instruction clock count, as listed in Table 8-1 below, by the processor clock period (e.g. 62.5 ns for an 80386-16 operating at 16 MHz (32 MHz CLK2 signal)). The actual clock count of an 80386 pro- gram will average 5% more than the calculated clock count due to instruction sequences which exe- cute faster than they can be fetched from memory.

For more detailed information on the encodings of instructions refer to section 8.2 Instruction Encod- ings. Section 8.2 explains the general structure of instruction encodings, and defines exactly the en- codings of all fields contained within the instruction.

Instruction Clock Count Assumptions

1.The instruction has been prefetched, decoded, and is ready for execution.

2.Bus cycles do not require wait states.

3.There are no local bus HOLD requests delaying processor access to the bus.

4.No exceptions are detected during instruction ex- ecution.

5.If an effective address is calculated, it does not use two general register components. One regis- ter, scaling and displacement can be used within the clock counts shown. However, if the effective address calculation uses two general register components, add 1 clock to the clock count shown.

Instruction Clock Count Notation

1.If two clock counts are given, the smaller refers to a register operand and the larger refers to a mem- ory operand.

2.n = number of times repeated.

3.m = number of components in the next instruc- tion executed, where the entire displacement (if any) counts as one component, the entire imme- diate data (if any) counts as one component, and all other bytes of the instruction and prefix(es) each count as one component.

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