Monday, March 28, 2011


Its almost impossible to ignore the entire debate sparked off from a post by Bob Harper of CMU:
First, it was on hacker news, then it was slashdotted with over 600+ comments.

Thursday, March 24, 2011

Intel Trickery

STI - Set Interrupt Flag
If protected-mode virtual interrupts are not enabled, STI sets the interrupt flag (IF) in the EFLAGS register. After the IF flag is set, the processor begins responding to external, maskable interrupts after the next instruction is executed. The delayed effect of this instruction is provided to allow interrupts to be enabled just before returning from a procedure (or subroutine).

Source: Intel Developer's Manual, Vol 2B.

Serializing Instruction
Starting with the Pentium Pro, Intel processors have supported out-of-order execution, where instructions are not necessarily performed in the order they appear in the executable. This can cause RDTSC to be executed later than expected, producing a misleading cycle count.[3] This problem can be solved by executing a serializing instruction, such as CPUID, to force every preceding instruction to complete before allowing the program to continue or by using RDTSCP instruction, which is a serializing variant of the RDTSC instruction (starting from Core i7[4]).

Source: Wikipedia: Time Stamp Counter

Atomic Ops

The LOCK prefix can be prepended only to the following instructions and only to those forms of the instructions where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCH8B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG. The XCHG instruction always asserts the LOCK# signal regardless of the presence or absence of the LOCK prefix.

Source: Intel Developer's Manual, Vol 2A.