How many registers in a cpu

The storage time of internal registers is extremely temporary, as they often hold data for less than a millisecond. User-accessible registers are larger than internal registers and typically hold data for a longer time.

For example, a data register may store individual values referenced being by a currently running program. An address register contains memory addresses, which reference different blocks of memory within the system RAM. Registers vary in both number and size, depending on the CPU architecture.

Some processors have 8 registers while others have 16, 32, or more. For many years, registers were bit, but now many are bit in size. A bit register is necessary for a bit processor, since it enables the CPU to access bit memory addresses. In long mode all but FS and GS are treated as having a base address of zero and a bit extent, effectively producing a flat address space.

More on those later. The x86 family has gone through several generations of SIMD and floating-point instruction groups, each of which has introduced, extended, or re-contextualized various registers:. Originally a discrete coprocessor with its own instruction set and register file, the x87 instructions have been regularly baked into x86 cores themselves since the Because of its coprocessor history, x87 defines both normal registers 6 akin to GPRs and a variety of special registers needed to control the FPU state:.

For design reasons that are a complete mystery to me, the MMX registers are actually sub-registers of the x87 STn registers: each bit MMn occupies the mantissa component of its corresponding STn. AVX also introduces eight opmask registers, k0 through k7. Intel added these with MPX , which was intended to offer hardware-accelerated bounds checking. These are what they sound like: registers that aid and accelerate software debuggers, like GDB.

DR0 through DR3 contain linear addresses, each of which is associated with a breakpoint condition. Showing results for. Search instead for. Did you mean:. How to know how many registers in a CPU? As I know, the number of registers in a CPU is limited. How to know how many registers can be allocated in a program? Lot of thanks chiungliang.

Bernard Black Belt. Copy link. Improve this question. NirIzr 11k 1 1 gold badge 34 34 silver badges 83 83 bronze badges. Although OP is learning reverse engineering, this question in particular has little specifically related to reverse engineering. How many registers does a x processor have? Add a comment. Active Oldest Votes. EDIT Note that most of the registers are specialized and have been introduced to support a specific instruction set the best example are the FPU registers ST0-ST7, introduced to support floating point computation.

Improve this answer. I like the fact you say "known" registers. So it looks like nobody really knows because it's not public information? Is it true that when most people say 'register', they really mean general purpose register? First of all, knowing what is inside hardware is really hard see the Intel God-mode talk given at BlackHat for example , so there is always the possibility that unknown feature are here unknown of everybody it may be a register or an instruction.

And, no, they are not all GPR, most of the registers are linked to an instruction set. R15 so 12 registers in total. Other things inside are just unnecessary implementation details. For example with register renaming there are actually a lot more registers that maps to the running program's registers.

There are also a lot of internal hardware registers to store data temporarily, like between each stage in the pipeline. Those will change between every implementation that we have no way to know — phuclv.