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
You will need both always. The program counter (PC) holds the address of the next instruction to be executed, while the instruction register (IR) holds the encoded instruction. Upon fetching the instruction, the program counter is incremented by one “address value” (to the location of the next instruction). The instruction is then decoded and executed … 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
These registers were originally implicitly used in repetitive instructions, for instance MOVSB, which copies a byte from DS:SI (DataSegment:SourceIndex) to ES:DI (ExtraSegment:DestinationIndex), at the time of the 16-bits computers with segmented memory in real mode. And also as index registers in 16-bit addressing modes like [bx + si]. Right now, these registers are for example … Read more
Yes, CPU register is just a small amount of data storage, that facilitates some CPU operations. CPU cache, it is a high speed volatile memory which is bigger in size, that helps the processor to reduce the memory operations.
No, that’s not quite right. EAX is the full 32-bit value AX is the lower 16-bits AL is the lower 8 bits AH is the bits 8 through 15 (zero-based), the top half of AX So AX is composed of AH:AL halves, and is itself the low half of EAX. (The upper half of EAX … Read more