cmp arg2, arg1 performs the same operation as sub arg2, arg1 except that none of the operands are modified. The difference is not stored anywhere. However, the flags register is updated and can be used in a conditional jump, like jump-if-equal (JE), most often as the next instruction after the cmp. The advantage over other … Read more
As has already been stated, wrapping brackets around an operand means that that operand is to be dereferenced, as if it were a pointer in C. In other words, the brackets mean that you are reading a value from (or storing a value into) that memory location, rather than reading that value directly. So, this: … Read more
Main differences: ARM is a RISC style architecture – instructions have a regular size (32-bit for standard ARM and 16-bits for Thumb mode, though Thumb has some instructions that chew up 2 instruction ‘slots’) up through at least ARM v5 architecture (I’m not sure what v6 does), the interrupt model on ARM is vastly different … Read more
The answer to your question is – it depends. That’s strictly a CPU design decision, which balances over the tradeoff between performance and complexity. Take for example recent Intel Core processors – they’re physically tagged and virtually indexed (at least according to http://www.realworldtech.com/sandy-bridge/7/). This means that the caches can only complete lookups in pure physical … Read more
It’s about locking the memory bus for that address. The Intel 64 and IA-32 Architectures Software Developer’s Manual – Volume 3A: System Programming Guide, Part 1 tells us: 7.1.4 Effects of a LOCK Operation on Internal Processor Caches. For the Intel486 and Pentium processors, the LOCK# signal is always asserted on the bus during a … Read more
The distinction is in what instructions are used to manipulate the data, not the data itself. Modern computers (since circa 1970) use a representation of integer data called two’s-complement in which addition and subtraction work exactly the same on both signed and unsigned numbers. The difference in representation is the interpretation given to the most … Read more
This may get you started. Is that more like what you’re looking for? Just had a quick search – it looks like there are a few different sorts of assembly syntax highlighting built in. Which one are you using? Copy-pasted from :help syntax Currently these syntax files are included: asm GNU assembly (the default) asm68k … Read more
This is how the C library function abs() does it in assembly without branching: abs(x) = (x XOR y) – y where y = x >> 31 (assuming 32-bit input), and >> is arithmetic right shift operator. Explanation of the above formula: We want to generate 2’s complement of negative x only. y = 0xFFFFFFFF, … Read more
Per-element atomicity of vector load/store and gather/scatter?
TL;DR The discrepancy you are observing between RDTSC and REFTSC and is due to TurboBoost P-state transitions. During these transitions, most of the core, including the fixed-function performance counter REF_TSC, is halted for approximately 20000-21000 cycles (8.5us), but rdtsc continues at its invariant frequency. rdtsc is probably in an isolated power and clock domain because … Read more