Rotates are required for bit shifts across multiple words. When you SHL the lower word, the high-order bit spills out into the carry. To complete the operation, you need to shift the higher word(s) while bringing in the carry to the low-order bit. RCL is the instruction that accomplishes this. High word Low word CF … Read more
You can’t access it directly because there’s no legitimate use case. Having any arbitrary instruction change eip would make branch prediction very difficult, and would probably open up a whole host of security issues. You can edit eip using jmp, call or ret. You just can’t directly read from or write to eip using normal … Read more
All-zero: pxor xmm0,xmm0 (or xorps xmm0,xmm0, one instruction-byte shorter.) There isn’t much difference on modern CPUs, but on Nehalem (before xor-zero elimination), the xorps uop could only run on port 5. I think that’s why compilers favour pxor-zeroing even for registers that will be used with FP instructions. All-ones: pcmpeqw xmm0,xmm0. This is the usual … Read more
Looks like the dissembler used short-hand for SAR EAX,1 which has an opcode of 0xD1F8. when the immediate is not 1, aka SAR EAX,xx, the opcode is 0xC1F8 xx, see the Intel Instruction reference, Vol. 2B, 4-353.
In the first version (+ 1 2 3) is raw code, whereas in the second version it is data. By assuming the truth of this statement it can be argued that Lisp isn’t even homiconic. The code has the same representation as data in the sense that they are both lists/trees/S-expressions. But the fact that … Read more
I wrote librdrand. It’s a very basic set of routines to use the RdRand instruction to fill buffers with random numbers. The performance data we showed at IDF is from test software I wrote that spawns a number of threads using pthreads in Linux. Each thread pulls fills a memory buffer with random numbers using … Read more
Well, first of all, jmp simply ‘jumps’ to the label that you give to it (which is a memory address as program instructions are stored in memory) while call stores the location where it will return (below the call instruction) in the stack, jmp to the label, and then at the ret instruction, jmp back … Read more
General claim is modern systems have better branch predictors and compilers are much more advanced so their cost on instruction encoding space is not justified. This is from ARMv8 Instruction Set Overview The A64 instruction set does not include the concept of predicated or conditional execution. Benchmarking shows that modern branch predictors work well enough … Read more
It’s AT&T assembly syntax: source comes before destination mnemonic suffixes indicate the size of the operands (q for quad, etc.) registers are prefixed with % and immediate values with $ effective addresses are in the form DISP(BASE, INDEX, SCALE) (DISP + BASE + INDEX * SCALE) Indirect jump/call operands indicated with * (as opposed to … Read more
JNZ is short for “Jump if not zero (ZF = 0)”, and NOT “Jump if the ZF is set”. If it’s any easier to remember, consider that JNZ and JNE (jump if not equal) are equivalent. Therefore, when you’re doing cmp al, 47 and the content of AL is equal to 47, the ZF is … Read more