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Arithmetic instructions

#1
03-08-2024, 05:18 PM
You see arithmetic instructions do the core math work inside the processor core. I always notice how they pull numbers straight from registers without much fuss. But you get to see the ALU crunch those values fast when an add operation fires off. Perhaps the carry bit flips on during overflow cases and that changes everything downstream. Also you might spot how subtraction borrows bits in a similar flow to keep results accurate. Then multiplication builds on repeated addition steps that eat more cycles than simple adds ever do. I think you already grasp why division stands out as the slowest one since it loops through shifts and subtracts repeatedly. Or maybe the flags register updates after each step to flag zero results or negative signs for later branches. Now you can imagine coding a loop that relies on these flags to decide repeats without extra checks.
I recall how signed numbers twist the same instructions using two's complement tricks that avoid separate hardware paths. You end up watching overflow flags trip when positive results turn negative unexpectedly. But the processor keeps pushing data through pipelines even if one arithmetic step stalls the next fetch. Perhaps you notice compiler tweaks reorder these instructions to hide latency better during heavy calculations. Also I find that floating point variants extend the basic integer ones with extra exponent handling that eats more silicon space. Then you see how immediate values embed right into the opcode to skip memory loads for small constants. Or the whole sequence might chain multiple arithmetic steps before writing back to memory to cut down on traffic. I wonder if you ever traced an add instruction through the execution stages to count the exact clock ticks involved.
You know the basic add can finish in one cycle on modern cores while multiply often needs four or more depending on the bit width. But you watch for data hazards when one result feeds straight into the next arithmetic call without forwarding paths. Perhaps the architecture hides some of that by buffering results in temporary spots until safe to commit. Also I see how vector extensions bundle several arithmetic ops into one shot for better throughput on arrays. Then division might raise exceptions on zero attempts that force the handler to kick in and save the day. Or maybe you tweak the code to use bit shifts instead of full multiplies when dealing with powers of two. I think you catch on quick that these instructions form the backbone for everything from simple counters to complex simulations running on the machine.
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ron74
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Joined: Feb 2019
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Arithmetic instructions

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