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In the ARM Architecture pipelining stages, we know that the instructions pass from fetch to decode and so on? But, from where does the fetch unit get the instructions?

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The instructions come from some part of memory, pointed to by the Program Counter (PC) aka Instruction Pointer (IP).

In modern computers, instructions tend to share the same RAM used for everything else (with protections in place to keep things sane), but especially in older or special-purpose machines it's not uncommon for instructions to be baked into ROM instead.

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    $\begingroup$ The first instructions that a computer will perform just after starting will come from ROM, on any modern computer. There must be enough code in the ROM to make the computer read more instructions from the hard drive. On a modern Mac, the ROM has enough code to actually download the operating sytem through the Internet, in case you replaced your hard drive with a new, empty one. $\endgroup$ – gnasher729 Feb 22 '19 at 8:06

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