![]() ![]() I relish such minor details as this in the engineering of next generation hardware and wanted to share them with readers who might find a similar appreciation. A generic Snapdragon ARM SoC, say, would deliver notably less performance in this specific scenario - getting non-native apps running solid and fast during this transition period - that is critically important to Mac users. ![]() Apple’s move here demonstrates the benefits of their position in controlling all aspects of the systems. While few modern applications likely read these AF and PF bits, Apple - who make their own CPUs and can control the entire process - put in place a hardware step to increase performance when running translated 圆4 code on their own processors. Rosetta 2 chooses the second option, and this mostly works out fine, because they have an “unused flags” optimisation that avoids the computation a lot of the time. Or you can compute them the long way shown above. Either, you can skip computing the flags, because they’re mostly useless and most software won’t care. In a VM, it isn’t able to configured the host CPU, so it can’t use this functionality. Rosetta 2 can also run in Linux VMs on Apple Silicon. With that set Rosetta 2 does:Īnd both flags are computed for it by the hardware. When running Rosetta 2, the CPU is configured to enable this functionality (since it’d break the ARM specification to have it enabled all the time). So, since Apple design their own CPUs, they decided to also build this logic. That’s a lot of work for one subtraction (5x as many instructions), and that’s not all of it – we didn’t compute AF.īut it’s not really a lot of work for Intel – in every x86 CPU, they just build some logic that computes both AF and PF at the same time as doing the subtraction. For example, to compute the parity flag on ARM, a subtraction turns into something like: This was used for binary-coded decimal – that flag would indicate a carry from one 4-bit digit to the next.īoth these flags are computed on every ADD or SUB instruction (extremely often), and 64-bit ARM has no such functionality. The adjust flag is set if there’s a “carry out” from the low four-bits of the addition (and otherwise cleared). ![]() The parity flag is set if, in the last eight bits of the result, the number of set-bits is odd. So, both the “adjust flag” (AF) and the “parity flag” (PF) come from the 8080-family CPUs from the 1970s. As he explained in his multi-part Mastodon response: Intrigued by mention of this “secret extension,” I reached out to the author and asked if he could expand on what Apple has done here. The Apple M1 has an undocumented extension that, when enabled, ensures instructions like ADDS, SUBS and CMP compute PF and AF and store them as bits 26 and 27 of NZCV respectively, providing accurate emulation with no performance penalty. Most software won’t notice if you get these wrong, but some software will. On ARM these can optionally set the four-bit NZVC register, whereas on x86 these always set six flag bits: CF, ZF, SF and OF (which correspond well-enough to NZVC), as well as PF (the parity flag) and AF (the adjust flag).Įmulating the last two in software is possible (and seems to be supported by Rosetta 2 for Linux), but can be rather expensive. There are only a handful of different instructions that account for 90% of all operations executed, and, near the top of that list are addition and subtraction. But one detail of the post really grabbed my attention. It’s a fascinating read for a tech nerd like me that has a particular interest in OS technology. Dougall delves into various aspects of Rosetta 2 in an effort to explain why it is so performant in many instances the translated binary runs faster on Apple Silicon than on the fastest Intel machines that Apple has ever released. ![]() It translates the entire binary - once - at launch time, making best-guess choices along the way. Rosetta 2 is the ahead-of-time compile translator that’s part of macOS Big Sur (and later) that, upon launch of an 圆4 Intel binary, translates it to 64-bit ARM code for execution on ARM-based Apple Silicon processors before execution. ![]()
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