Key takeaways: - ARM is a little-endian, 32-bit RISC architecture widely used by mobile devices. - The original iPhone's core processor is ARM1176JZF-S (aka A

Key takeaways: - ARM is a little-endian, 32-bit RISC architecture widely used by mobile devices. - The original iPhone's core processor is ARM1176JZF-S (aka ARM11) which implements an instruction set called ARMv6. - iPhone 3GS and later models use the Cortex A8 processor core that implements ARMv7, a newer instruction set. - The so-called A4 processor is in fact a bundle of the Cortex A8 core and several other systems-on-a-chip components including the graphics accelerator and audio/video codecs. - To take advantage of ARMv7 features while supporting ARMv6, your code must be compiled for both targets and put into a fat binary. - When you debug optimized ARM code, remember that most ARM instructions can be conditionally executed -- the processor may go through a code block without actually running it. - Thumb is a compressed subset of ARM instructions. 16-bit thumb code is generally smaller but can not be freely mixed with 32-bit uncompressed ARM code. - Thumb for ARMv6 has a run-time overhead that makes it undesirable for high-performance apps; however, Thumb for ARMv7 has no such drawbacks and should be turned on by default. - iOS supports unaligned memory access, but it's slooooow. Don't use it. - Integer division on ARM is not natively supported. Surprise, surprise! - GCC sucks at optimizing ARM code, particularly weak at 64-bit integer arithmetic.