What’s the difference between EAX, EBX, and ECX in assembly? eax, ebx, ecx and so on are actually registers, which can be seen as "hardware" variables, somewhat similar to higher level-language's variables Registers can be used in your software directly with instructions such as mov, add or cmp
assembly - How do AX, AH, AL map onto EAX? - Stack Overflow So AX is composed of AH:AL halves, and is itself the low half of EAX (The upper half of EAX isn't directly accessible as a 16-bit register; you can shift or rotate EAX if you want to get at it ) x86-64 CPUs extend the integer registers to 64-bit: RAX is the full 64-bit value, with EAX and its sub-components mapped to the lower 32 bits The
What does the bracket in `movl (%eax), %eax` mean? MOV sets EAX = the value from memory at that address Your re-phrasing of the existing answer isn't adding anything new, and is actually less clear because you're mis-using "address" to mean "memory at that address", which is the opposite of what "address" normally means
Assembly registers in 64-bit architecture - Stack Overflow hmm eax == 32bits ax == 16bits ah|al == 8bits, it's always like this, x64 adds new registers, back in the 16 bit days we only had ax al ah then when 32 bit addressing came round it was added in a way that didn't really effect how you address the 16bit or 8bit registers, the new registers in x64 (64 bit registers that overlap, eax that overlaps ax, etc) start with an r so rax rbx and so on
c - Why is eax used on 64-bit? - Stack Overflow %eax refers to the lower 32 bits of the 64-bit %rax register So, if %rax is initially 0, and you execute the instruction movl $5, %eax, then %rax will also take on the value 5 However, they are not entirely independent, so one should be careful
xv6: what does `popl 0(%eax)` mean? (OSTEP) - Stack Overflow I understand that movl 4(%esp), %eax moves the content inside the address 4(%esp) (first parameter of the method, or the old context address) to register eax However, the next line popl 0(%eax) seems to also pop from the stack (which points to the instruction pointer of the caller) and load it into eax
assembly - The point of test %eax %eax - Stack Overflow TEST EAX,EAX JE some_address the CPU will jump to "some_address" if and only if ZF = 1, in other words if and only if AND(EAX,EAX) = 0 which in turn it can occur if and only if EAX == 0 the equivalent C code is:
Why would one use movl $1, %eax as opposed to, say, movb $1, %eax %eax is not zeroed out unless you either movl $0, %eax, or if you xorl %eax, %eax Otherwise it holds whatever value was previously in there When you movl $1, %eax, you will end up with 0x00000001 in the register because the 32-bit instruction moves a 32-bit immediate value into the register