DELAY: MOV R2,#200 1
AGAIN: MOV R3,#250 1
HERE: NOP 1
NOP 1
DJNZ R3,HERE 2
DJNZ R2,AGAIN 2
RET 2
For HERE loop, we have (4x250)x1.085μs=1085μs. For AGAIN loop repeats HERE loop 200 times, so we have 200x1085μs=217000μs. But “MOV R3,#250” and “DJNZ R2,AGAIN” at the start and end of the AGAIN loop add (3x200x1.805)=651μs. As a result we have 217000+651=217651μs.
just keep in mind Clocks per Machine Cycle
DS89C420/30/40/50 Dallas Semi 1
DS5000 Dallas Semi 4
P89C54X2 Philips 6
AT89C51 Atmel 12