Jul 7, 2017 · There's nothing like mov eax, ebx that modifies only upper half of RAX the way mov ah, 1 modifies the upper half of AX. I'm arguing that you should delete the footnote, because it's more confusing than helpful.

Dec 20, 2016 · mov (%rax), %eax # AT&T syntax or, equivalently in Intel syntax: mov eax, DWORD PTR [rax] ; Intel syntax dereferences the memory address stored in rax, reads a 32-bit value from that memory address, and stores it in the eax register.

With the advent of x64 and 64-bit architecture, register size doubled once again and the EAX register was renamed RAX. However a new naming convention was also adopted, and the RAX register is also known as the R0 register:

RAX is the full 64-bit value, with EAX and its sub-components mapped to the lower 32 bits. The upper half of 64-bit registers is only accessible in 64-bit mode, unlike 32-bit registers which can be used in any mode on CPUs that support them.

Dec 11, 2024 · x64 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". For example, the 64-bit extension of eax is called rax. The new registers are named r8 through r15. The lower 32 bits, 16 bits, and 8 bits of each register are directly addressable in operands.

Mar 20, 2020 · So the accumulator is now referred to as RAX: Why R? Well, AMD wanted to streamline the register handling. They introduced eight new registers called R8 to R15. They even discussed calling the extensions of the existing eight registers as R0 to R7.

Oct 8, 2021 · 累加寄存器 (eax / rax) eax是32位累加寄存器，rax是它的64位扩展版本。 常用于算术运算和函数返回值。 基址寄存器 (ebx / rbx) ebx是32位基址寄存器，rbx是它的64位扩展版本。 通常用作基址指针，但在现代编程中用途较少固定。

Loading a value into a 32-bit register name sets the upper 32 bits of the register to zero. Thus, after movl $-1, %eax, the %rax register has value 0x00000000FFFFFFFF. Loading a value into a 16- or 8-bit register name leaves all other bits unchanged.

rax is the 64-bit, "long" size register. It was added in 2003 during the transition to 64-bit processors. eax is the 32-bit, "int" size register. It was added in 1985 during the transition to 32-bit processors with the 80386 CPU.

The sequence cmpq %rax, %rbx; jg L will jump to label L if and only if %rbx is greater than %rax (signed). The weird-looking instruction testq %rax, %rax, or more generally testq REG, SAMEREG, is used to load the condition flags appropriately for a single register.