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Instruction Timing

Instruction Clock Cycle Calculation

Some instructions require additional clock cycles due to a “Next Instruction Component” identified by a “+m” in the instruction clock cycle listings. This is due to the prefetch queue being purge on a control transfers. Below is the general rule for calculating “m”:

88/86 not applicable
286 “m” is the number of bytes in the next instruction
386 “m” is the number of components in the next instruction (the instruction coding (each byte), plus the data and the displacement are all considered components)

8088/8086 Effective Address (EA) Calculation

Description Clock Cycles
Displacement 6
Base or Index (BX,BP,SI,DI) 5
Displacement+(Base or Index) 9
Base+Index (BP+DI,BX+SI) 7
Base+Index (BP+SI,BX+DI) 8
Base+Index+Displacement (BP+DI,BX+SI) 11
Base+Index+Displacement (BP+SI+disp,BX+DI+disp) 12
  • add 4 cycles for word operands at odd addresses
  • add 2 cycles for segment override
  • 80188/80186 timings differ from those of the 8088/8086/80286

Task State Calculation

“TS” is defined as switching from VM/486 or 80286 TSS to one of the following:

New Task
Old Task 486 TSS
486 TSS
386 TSS
386 TSS
286 TSS
386 TSS (VM=0) 309 226 282
386 TSS (VM=1) 314 231 287
386 CPU/286 TSS 307 224 280
486 CPU/286 TSS 199 177 180


- all timings are for best case and do not take into account wait states, instruction alignment, the state of the prefetch queue, DMA refresh cycles, cache hits/misses or exception processing. - to convert clocks to nanoseconds divide one microsecond by the processor speed in MegaHertz:

(1000~MHz) / (n~MHz) = X~nanoseconds

back2root/ibm-pc-ms-dos/hardware/instructions-timing.txt · Dernière modification : 2023/02/24 14:16 de frater