# What about Registers? In computer architecture, 32-bit registers are registers that can hold 32-bit (4-byte) values. These registers are used to temporarily store data that is being operated on by the CPU. The 32-bit registers are a subset of the general-purpose registers in a CPU and are used to store data that is being manipulated by the CPU. The most common use of 32-bit registers is to hold memory addresses, but they can also be used to hold data that is being operated on by the CPU. Some common 32-bit registers include: - EAX (extended accumulator register) - EBX (base register) - ECX (counter register) - EDX (data register) - EBP (base pointer register) - ESP (stack pointer register) - EIP (instruction pointer register) - EFLAGS (flags register) These registers are used by the CPU to perform various operations and are typically used in conjunction with other registers and memory locations to perform more complex operations. - Refer to Intel's Architecture Documentation for latest information: https://software.intel.com/en-us/articles/intel-sdm 64-bit registers are registers that can hold 64-bit (8-byte) values. These registers are used to temporarily store data that is being operated on by the CPU. The 64-bit registers are a subset of the general-purpose registers in a CPU and are used to store data that is being manipulated by the CPU. The most common use of 64-bit registers is to hold memory addresses, but they can also be used to hold data that is being operated on by the CPU. Some common 64-bit registers include: - RAX (register A extended) - RBX (register B extended) - RCX (register C extended) - RDX (register D extended) - RBP (base pointer register) - RSP (stack pointer register) - RIP (instruction pointer register) - RFLAGS (flags register) - R8-R15 (additional general purpose registers) These registers are used by the CPU to perform various operations and are typically used in conjunction with other registers and memory locations to perform more complex operations. It's worth noting that, the registers may differ depending on the CPU architecture or instruction set architecture (ISA) used in the computer, some architectures may have more or less registers and different names. 64-bit architecture is more common in modern processors and operating systems, as it allows more memory addressability, and faster computation. ### Additional Notes: The x64 architecture extends x86's 8 general-purpose registers to be 64-bit, and adds 8 new 64-bit registers. The 64-bit registers have names beginning with "r", so for example the 64-bit extension of **eax** is called **rax**. The lower 32 bits, 16 bits, and 8 bits of each register are directly addressable in operands. This includes registers, like **esi**, whose lower 8 bits were not previously addressable. The following table specifies the assembly-language names for the lower portions of 64-bit registers.
64-bit register Lower 32 bits Lower 16 bits Lower 8 bits
**rax** **eax** **ax** **al**
**rbx** **ebx** **bx** **bl**
**rcx** **ecx** **cx** **cl**
**rdx** **edx** **dx** **dl**
**rsi** **esi** **si** **sil**
**rdi** **edi** **di** **dil**
**rbp** **ebp** **bp** **bpl**
**rsp** **esp** **sp** **spl**
**r8** **r8d** **r8w** **r8b**
**r9** **r9d** **r9w** **r9b**
**r10** **r10d** **r10w** **r10b**
**r11** **r11d** **r11w** **r11b**
**r12** **r12d** **r12w** **r12b**
**r13** **r13d** **r13w** **r13b**
**r14** **r14d** **r14w** **r14b**
**r15** **r15d** **r15w** **r15b**
* Operations that output to a 32-bit subregister are automatically zero-extended to the entire 64-bit register. * Operations that output to 8-bit or 16-bit subregisters are *not* zero-extended (this is compatible x86 behavior). * The high 8 bits of **ax**, **bx**, **cx**, and **dx** are still addressable as **ah**, **bh**, **ch**, **dh**, but cannot be used with all types of operands. * The instruction pointer, **eip**, and **flags** register have been extended to 64 bits (**rip** and **rflags**, respectively) as well. The x64 processor also provides several sets of floating-point registers: * Eight 80-bit x87 registers. * Eight 64-bit MMX registers. (These overlap with the x87 registers.) * The original set of eight 128-bit SSE registers is increased to sixteen. The addressing modes in 64-bit mode are similar to, but not identical to, x86. * Instructions that refer to 64-bit registers are automatically performed with 64-bit precision. (For example **mov rax, \[rbx\]** moves 8 bytes beginning at **rbx** into **rax**.) * A special form of the **mov** instruction has been added for 64-bit immediate constants or constant addresses. For all other instructions, immediate constants or constant addresses are still 32 bits. * x64 provides a new **rip**-relative addressing mode. Instructions that refer to a single constant address are encoded as offsets from **rip**. For example, the **mov rax, \[***addr***\]** instruction moves 8 bytes beginning at *addr* + **rip** to **rax**. Note: Instructions, like **jmp**, **call**, **push**, and **pop**, that implicitly refer to the instruction pointer and the stack pointer treat them as 64 bits registers on x64. ## ARM-based Registers In the ARM architecture, there are several different types of registers. These include: - General-purpose registers (GPRs): These are the main registers that are used for data manipulation and are also used to hold memory addresses. There are 16 GPRs in the ARM architecture, numbered R0-R15. - Program Counter (PC): This register holds the address of the next instruction to be executed. It is also used to hold the return address when a subroutine is called. - Stack Pointer (SP): This register holds the address of the top of the stack. - Link Register (LR): This register holds the return address when a subroutine is called. - Condition Code Registers (CCRs): These registers hold the current state of the processor's condition codes. They are used to determine the outcome of a comparison operation and to control the execution of instructions based on certain conditions. - Floating-point registers: These registers are used to hold floating-point values. There are 32 single-precision registers and 16 double-precision registers in the ARM architecture. - Vector registers: These registers are used to hold SIMD (Single Instruction, Multiple Data) data. There are 32 vector registers in the ARM architecture.