05-11-2020, 01:14 PM
Now direct mode adds one memory fetch. That fetch eats extra time compared to immediate stuff. You watch the clock tick longer as the address gets resolved from memory. I see it snag extra cycles when programs jump around a lot. But register modes cut that down by keeping values close. You avoid memory hits altogether with registers. Processors zip through those faster since nothing leaves the chip. Or indirect modes pile on more fetches. They force two trips to memory which drags execution out. Perhaps indexed modes tweak this with offsets that add a bit of calc time. You mix them and watch the total time stretch or shrink based on access patterns. I think about cache effects too since modes that hit memory often miss caches more.
Then base plus offset modes combine ideas from several approaches. They let you reach arrays without full recalcs each time. You gain some efficiency but still pay for address math. Also relative modes help with position independent code that runs quicker in spots. Processors handle those with fewer stalls if branches stay predictable. I find execution time drops when modes reduce overall memory traffic. But complex modes like auto increment eat cycles on updates. You see this in older designs where every access counted heavy. Processors today hide some costs with pipelines yet modes still influence throughput a ton. Maybe you test this by swapping modes in a benchmark and timing the results yourself.
Or scaled index modes speed array traversals by embedding multipliers. They cut separate shift operations which saves steps. I notice this helps in tight numeric work where data moves constantly. Processors execute those with less overhead when hardware supports the scaling direct. But you pay if the mode needs extra register reads that compete for ports. Execution time varies wildly across architectures due to these choices. Now memory modes always lag behind register ones in raw speed. You trade that for bigger data spaces that programs need. I see indirect chains prolong things further when pointers nest deep. Processors stall waiting on those chained loads. Perhaps modern out of order execution masks some delays yet modes still matter for worst case paths.
You combine several modes in one program and the average time reflects the mix. I watch how frequent memory modes inflate total cycles. Processors favor modes that keep data nearby for better performance overall. But rare modes might force slower paths if unsupported well. Execution time changes with workload too since data heavy tasks suffer more from slow modes. You experiment by rewriting sections and measuring differences. Processors reveal these effects clearly in cycle accurate sims. Also branch modes affect time through prediction accuracy. They tie into how addresses get calculated on the fly. I find this interplay makes tuning tricky but rewarding when time drops.
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