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
Something I found: * EAX – Accumulator Register * EBX – Base Register * ECX – Counter Register * EDX – Data Register * ESI – Source Index * EDI – Destination Index * EBP – Base Pointer * ESP – Stack Pointer
CPU register is always faster than the L1 cache. It is the closest. The difference is roughly a factor of 3. Trying to make this as intuitive as possible without getting lost in the physics underlying the question: there is a simple correlation between speed and distance in electronics. The further you make a signal … Read more
I think it’s just R for “register”, since there are additional registers R8 – R15 on x86-64, and R is a common prefix on many CPU architectures where registers are numbered.
There is no good example of register usage when using modern compilers (read: last 15+ years, and note that this answer is from 2008) because it almost never does any good and can do some bad. When you use register, you are telling the compiler “I know how to optimize my code better than you … Read more
ESP is the current stack pointer. EBP is the base pointer for the current stack frame. When you call a function, typically space is reserved on the stack for local variables. This space is usually referenced via EBP (all local variables and function parameters are a known constant offset from this register for the duration … Read more
R just stands for “register”. The AMD64 ISA extension added 8 additional general-purpose registers, named R8 through R15. The 64-bit extended versions of the original 8 registers had an R prefix added to them for symmetry. E stands for “extended” or “enhanced”. (Wikipedia says “extended”.) They are the “extended” versions of the 16-bit registers, in … Read more
Don’t take that paragraph too seriously. It seems to be making excessive assumptions and then going into excessive detail, which can’t really be generalized. But, your questions are very good. For a computer to do some operations on some data which are residing in the main memory, is it true that the data must be … Read more
Editor’s note: this way of using a local register-asm variable is now documented by GCC as “not supported”. It still usually happens to work on GCC, but breaks with clang. (This wording in the documentation was added after this answer was posted, I think.) The global fixed-register variable version has a large performance cost for … Read more
General purpose means all of these registers might be used with any instructions doing computation with general purpose registers while, for example, you cannot do whatever you want with the instruction pointer (RIP) or the flags register (RFLAGS). Some of these registers were envisioned to be used for specific use, and commonly are. The most … Read more