Eax register x64. For output purposes, EAX .

Eax register x64 Checking for negative numbers is pointless. – Extends the precision of the accumulator (also called the data register). Some registers are designated for a certain purpose, movsbl %al, %edx # copy low-byte from register %eax, sign-extend to 4 byte long in %edx movzbl %al, %edx # copy low-byte from register %eax, zero-extend to 4 byte long in %edx It's only useful to push imm/pop reg for small values that fit in an 8-bit immediate. Q is not responsible. In general you shouldn’t expect a big performance change from using The x64 architecture is the evolution of the older x86 architecture, it kept compatibility with its predecessor (x86 registers are still available) but it also introduced new features: Registers have now a capacity of 64 bits; There are 8 more general-purpose registers; Segment registers are forced to 0 in 64 bits mode; The EAX, EBX and ECX registers on a cpu are they 64 bits for a 64-bit cpu ? Skip to main content. In this case, it zero-extends the BYTE loaded from memory at [rbp-528+rax] to the DWORD destination register, EAX. Updated Mar 1, 2024; Assembly; ThreeDeeJay / dsoal. "Scratch" registers any function is allowed to overwrite, and use for anything you want without asking anybody. In your generated code, it gets a snapshot of the stack pointer (rsp) so that when adjustments are made to rsp (i. See: Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register? Prefer using different source/destination registers, because mov-elimination fails for mov eax,eax on both Intel and AMD CPUs. The only way to get the upper half of EAX would be to do some shifting/rotating. The leading e stands for extended and means that your register is 32 bits wide. c -o cr0 $ No errors or warnings. 22 Jun, 2024. x64 extends x86's 8 general-purpose registers to be 64-bit, and adds 8 new 64-bit registers. See Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?. Some Intel and AMD processors feature a 64-bit TSC register whose value monotonously increases by one, If you just need 32-bit registers, you can safely work with them, this is OK under 64-bit. The registers starting with r as in rax, rbx, etc, are the 64-bit registers introduced with the AMD64 extension to the existing 32-bit x86 ISA. The eax register receives function return values if the result is 32 bits or smaller. punpckhqdq xmm0,xmm0 is the SSE2 integer equivalent of movhlps xmm0,xmm0. Floating-point registers like XMM0, YMM0 or ST(0) These registers are mapped to 32-bit registers on 32-bit platform, and to 64-bit registers on 64-bit platform. Some registers are designated for a certain purpose, such as %rsp being used x86-64 Register Cheatsheet. movq rbx,xmm0 ;high 64 bits. For the EAX, EBX, ECX, and EDX registers, subsections may be used. The registers are stored on the stack in the following order: EAX, ECX, EDX, EBX, ESP (original value), EBP, ESI, and EDI (if the current operand-size attribute is 32) and AX, CX, DX, BX, SP (original value), BP, SI, and DI (if the operand-size attribute is 16). bh (8 regs) encode al. GNU C only has specific-register constraints for the original 8 registers, like "=S"(rsi). esp: stack pointer (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. Any write operation on a 32 bits register zeros out the top 32 bits of the overlapped 64 bit register, e. If two operands are equal, their bitwise AND is zero when both are zero. Register 64-bit memiliki nama yang dimulai dengan "r". All of the new x64 registers begin with "r" and are all 64-bit or QWORD (quad-word) values. EDX:EAX. Stack Exchange Network. x64 Cheat Sheet Fall2013 1 x64 Registers x64 assembly code uses sixteen 64-bit registers. After shifting right some register by shr, the value in it is always positive. It's the caller who expects the return value in register eax, according to the calling convention. No, that's only if you use one-operand mul or imul, and the result goes in EDX:EAX. We can still reference EAX and its sub-registers for added control. The multiply result isn't truncated, it was always a 32-bit multiply in the first place, only reading 32-bit inputs. https://wiki. Registers can be used in your software directly with instructions such as mov, add or cmp. Нижние 32 бита, 16 битов и 8 бит каждого регистра напрямую Регистры в процессоре x86-64 можно разделить на четыре категории: регистры общего назначения, специальные регистры для приложений, сегментные регистры и специальные регистры режима ядра. Using 0x80 and eax for syscall number, ebx, ecx, edx, esi, edi, and ebp to pass parameters is just one of many possible other choices to implement a system call, but 64-bit registers using the 'r' prefix: rax, r15; 32-bit registers using the 'e' prefix (original registers: e_x) or 'd' suffix (added registers: r__d): eax, r15d; 16-bit registers using no prefix (original registers: _x) or a 'w' suffix (added registers: r__w): ax, r15w; 8-bit registers using 'h' ("high byte" of 16 bits) suffix (original All relative jump instructions have the jump distance encoded in the instruction as an immediate (=integer constant) operand. When you use normal 2-operand imul it's just like add. References. Moreover, further down in the code, it actually uses the 64-bit register %rax to do the iterating, which never gets initialized outside of xorl %eax %eax, which would seem to only zero out the lower 32 bits of the register. In x64, any operation on a 32-bit register clears the top 32 bits of the corresponding 64-bit register too, so there's no need to use xor rax, rax which would necessitate an extra REX byte for encoding. You'll have to follow a two-step process, which may or may not involve a roundtrip to memory or the destruction of a register. xchg eax, ebx (1 byte, 3 uops on modern ret doesn't change registers, it's not like the C return statement which takes an operand. This is because “a” and “b” can point to both 32-bit (like %EAX or %EDX registers) and 64-bit memory cells (like %RAX and %RDX registers). It's hard to do this efficiently without touching a 2nd register, as requested. Writes to whole register: 32 bit eax: mov eax,ecx: mov eax,ecx: movsx eax,cx: movsx eax,cl: Top half of destination gets zeroed: 16 bit The code compiles OK for me on 32-bit x86 bit not on x86-64 - this is with gcc 4. leave 0x08048434 <main+64>: ret Implications for Other Registers. The name "AL" still refers to bits 0-7 of the first letter-named register, "AH" to bits 8-15, and "AX" to bits 0-15; the name "EAX" now refers to all 32 bits of the register. Star 106. Fastcalls are x64 calling Reference. You'll have to use indirect instruction sequences if you're interested in doing that. New instructions for dealing with 64-bit registers:; sign-extend 32-bit to 64-bit movsxd r64, r32/m32 PUSH and POP in x86-64 Assembly PUSH and POP are two of the most fundamental instructions in assembly, allowing the program to add or remove items from the stack. Also, one of your earlier versions special cased "=a" (eax) while the others used "=m" (cid->ebx) for some reason (instead of letting the asm be less bad by using "=rm"). The difference is, that the rip points at the time of evaluation to the beginning of next instruction. These 32-bit registers can be used in three ways −. The least significant byte of AX can be used as a single 8-bit register called AL, while the most significant byte of AX can be used as a single 8-bit register called AH. So while there aren't any direct instructions to specifically access the upper half of a 32- or 64-bit register, 83F800 cmp eax, 0 09C0 or eax, eax 85C0 test eax, eax The resulting code will be. The XOR eax, 1 instruction just flips the lowest bit of EAX. You can do stuff like imul esi, edi. The register names are as follows: 8-byte register Bytes 5-8 Bytes 7-8 Byte 8 %rax %eax %ax %al %rcx %ecx %cx %cl %rdx %edx %dx %dl %rbx Register. ) 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. Cool. mov dword ptr ss:[rsp],eax and mov eax,dword ptr ss:[rsp] look like they are some effects of the removed optimizations, they don’t do anything useful. The CDQE instruction sign-extends a DWORD (32-bit value) in the EAX register to a QWORD (64-bit value) in the RAX register. In the article Understanding C by learning assembly, it's written that RBP and RSP are special-purpose registers (RBP points to the base of the current stack frame and RSP points to the top of the current stack frame). If the result is 64 bits, then the result is stored in the edx:eax pair. Operand formats I studied that in x32, mul instruction multiplies the source with eax register and store the result as 64 bit value across two registers i. Use mov eax, edi to zero-extend EDI into RAX if you can't already guarantee that the high bits of RDI are all zero. Similarly, in the 64-bit version, the 'E' is replaced with an 'R' (register), so the 64-bit version of 'EAX' is called 'RAX'. Thus, because rax is 64-bits it can serve as a pointer, and eax is the right 32-bits of it, it cannot. It seems logical to me to take Rax in x64 in place of Eax in x86 and Rbx in x64 in place of Ebx in x86. This will put the upper 16-bits of EAX into DX and then restore EAX back to it's original value. That ISA extension was subsequently adopted by Intel and is often known by the more neutral name x86-64. x86-64 System V passes args in registers, which is more efficient than i386 System V's stack args convention. Are EAX , EBX registers 64 bit in length in modern processors [closed] Ask Question Asked 3 years, 7 months ago. Aaaah, okay. Each register is 64 bits wide; the lower 32-, 16- and 8-bit portions are selectable by a pseudo-register name. Understanding this behavior rbp is the frame pointer on x86_64. The eax register is used to return values from functions. and God knows x64 has more than enough instructions already. and EAX support via OpenAL Soft. It is also possible to address the first four registers (AX, With the advent of x64 and 64-bit architecture, register size doubled once again and the EAX register was renamed RAX. Intel Core 2 Duo, an example of an x86-compatible, 64-bit multicore processor AMD Athlon (early version), a technically different but fully The problem, in this scenario, is that eax is a 32-bit register, so the constant is 32-bits, so we end up with a five-byte In general 64-bit code is larger than 32-bit code, except when the extra registers, 64-bit registers, or cleaner ABI lead to smaller code. 4. It avoids the latency and extra instructions of The x64 architecture extends this by providing 64-bit registers that supersede the 32-bit x86 registers. The general registers are further divided into the following groups −. e. Register baru diberi nama r8 hingga r15. Several of the movb $0x4,0x0 will store byte value 4 into memory at absolute address 0. The 64 bit registers are shown in red. Also, I see that 64-bit Cheat Engine is aware of this when opening the 32-bit process (notice the migration of the values eax ↔ ecx and ebx ↔ edx : In the x86-64 Tour of Intel Manuals, I read. Like push 1 (2 bytes) / pop eax (1 byte) for 3 bytes total, vs. When CPUID executes with EAX set to 02H, the processor returns information about the processor’s internal TLBs, cache, and prefetch hardware in the EAX, EBX, ECX, and EDX registers. No reason for that; there was no input from C in the EAX register, just a hard-coded mov-immediate instruction inside the template. (On processors that support the Intel 64 architecture, the high-order 32 bits of each of RAX and RDX are cleared. The IA-64 general-purpose registers are named with an r, followed by the register number. Making it otherwise would require extending the instruction-set. Namely, it sets the zero flag if the difference is zero (operands are equal). text section of the assembly code, and right one shows the fastcalls in x64 assembly. Move the value 42 into the eax register pop rbp ; Restore the base pointer ret ; Return All registers of all sizes, except floating-point registers (eg: RAX, EAX, AL) can be used as variables. ; the function return value is in rax. With the advent of x64 and 64-bit architecture, register size doubled once again and the EAX register was renamed RAX. Read about it on msdn. Why does this code execute more slowly after strength-reducing multiplications to loop-carried additions? It should be noted that it was not Intel that increased the number of registers in x64 and named them, but AMD, as the 64-bit extensions to x86 were created by AMD. EDIT: Linux irrelevant parts removed. This behavior is consistent with how GDB handles other registers. movzlq %eax, %rdx might make logical sense, but it's not supported since it would be redundant. mov eax,ecx – Will save the same value (the sum) into EAX register. For output purposes, EAX The x86 architectures were based on the Intel 8086 microprocessor chip, initially released in 1978. Notes for x86-64 registers and useful pwning info. Essentially all x86 chips released in the last decade from AMD and Intel support this ISA. But yes, mov r64,imm64 to feed a 32-bit operand-size imul is totally pointless. audio spatial sound hrtf eax surround binaural openal-soft directsound directsound3d dsoal 32 bit inputs (one in EAX) give a 64 bit output (in EDX:EAX) Note that the relevant width is always the width of the argument (and the implicit A register), not the minimum width to hold either of the values. push reg means push native_register_size. %ebx, %ecx, %edx, %esi, %edi, %ebp are used for passing 6 parameters to system calls. collection research modern knowledge register topic assembly x86-64 templates reverse-engineering intel bytes pointer x86 x86-32 eax x86-x64 dword eflags. See the question and answer for x86_64 registers rax/eax/ax/al overwriting full register contents as well as Why do most x64 instructions zero the upper part of a 32 bit register which provide some explanation about how the 64 bit register operation differ from the 32 bit register operation. Right now, Writes the contents of registers EDX:EAX into the 64-bit model specific register (MSR) specified in the ECX register. On the 80386, rather than combining 16-bit registers into 32-bit registers, Intel added 16 bits to each register. If a register starts with an E (or ends with a d) it's a 32 bit register (the lower 32 bits). But instead of Eax → Rax and Ebx → Rbx we see Eax → Rcx and Ebx → Rdx . The EDX register is loaded with the high-order 32 bits of the MSR and the EAX register is loaded with the low-order 32 bits. They changed the meaning of some of the opcodes to those new instructions. r15l. test rax, rax jz is_zero Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register? 320. movb $0x4,0x0(%rip) will store byte value 4 into memory at absolute address rip + 0, ie. By manipulating eax, the first 32-bits of rax are modified, just like ax @AndersonGreen: not directly (see the chart in Mike Thompson's answer). On x86-64, this performs the same calculation, but since the destination is the 32-bit partial register eax, the operation also zeroes out the upper 32 bits of rax. Modified 3 years, 7 months ago. callee-saved register: if P calls Q, and Q wants to modify the register, Q is responsible for saving it on entry and restoring it on exit. For r8. ; register rbx must not be modified by a function; GCC saves/restores it as required, so it can keep a copy of x there across the call to bar. Data Registers. (On processors that support the Intel 64 architecture, the high-order 32 bits of RCX are ignored. The following is a list of calling conventions used on the x86 architecture: Win32 (__stdcall) actuallyprint: mov rdi, format ; pass pointer to format movzx rsi, ecx ; pass register contents by value mov eax, 0 ; probably don't need this call printf mov rdi, LC8 mov eax, 0 call printf add edx, 1 It's from the same bag of tricks that holds the idea of clearing a register with XOR EAX,EAX (which i saw on someone's license plate in Colorado!) rather than the obvious but maybe slower MOV EAX, #0 (i use an older notation). %eax for syscall_number. ecx has special semantic for looping functions. You will see EAX and EDX combined in 64-bit operations using 32-bit code. See Tips for golfing in x86/x64 machine code. to perform a conditional jump (think of the if operator in many high level programming languages), an In x86-32 parameters for Linux system call are passed using registers. 1. This handout only mentions a small subset of the rich possibilities provided by the x86-64 instruction set and architecture. For example, the 64-bit Например, 64-разрядное расширение eax называется rax. The Intel documentation (3. Correction. rodata broadcast_low32_multiplier: dq 0x100000001 The top right window contains the values of the registers. A 32-bit multiply by zero will (slowly) set EDX:EAX to all zeros. In the registers’ case, it is clear from their names whether we use 64-bit %RAX or 32-bit %EAX. I'm afraid you are mistaken: the function argument is passed in rdi, as per the x86-64 System V calling convention. Using 8-bit, 16-bit, and 32 parts of 64-bit rNN To begin with, the IA-64 has 128 general-purpose registers, each 64 bits wide. For example, AMD needed some room to add new opcodes for REX prefixes and some other new instructions when they developed the 64-bit x86 extensions. osdev. However a new naming convention was also adopted, and the RAX register is also known as the R0 register: Each register is 64 bits wide; the lower 32-, 16- and 8-bit portions are selectable by a pseudo-register name. The x64 Registers. The upper half of 64-bit registers is only accessible in 64-bit mode, unlike EAX x86 Register Meaning and History. Thanks! – General part. Monikers Description 64-bit 32-bit 16-bit RIP EIP IP Instruction Pointer Note: The instruction pointer can only be used in RIP-relative addressing eax, ebx, ecx and so on are actually registers, which can be seen as "hardware" variables, somewhat similar to higher level-language's variables. Be familiar with x86 assembly as the differences between x86 and x64 assembly are minor. The middle two windows, left one shows you the . mov [reg], MyByte would be a memory-to-memory move which is not supported and would not do what you wanted anyway. Vladimir Keleshev • 2020-03-20. Новые регистры называются с r8 по r15. The MOVZX instruction zero-extends the source to the destination. An earlier version of this blog post stated that the X in EAX AX AH AL Accumulator RBX EBX BX BH BL Base RCX ECX CX CH CL Counter RDX EDX DX DH DL Data (commonly extends the A register) when an operand contains a 64-bit register. it has also a special semantic for arithmic functions. The stack starts at a relatively high memory address and grows down toward lower addresses. mov eax, 1 (5 bytes total, with 3 zero bytes in the imm32 so it's also a problem for shellcode). when we multiply it stores in eax. . For example, on x64 eax is a 32-bit sub-register consisting of the lower half of the 64-bit rax register. EDX stores most significant bit of 32 bits of operation and EAX do the same. While not totally wrong, narrowing down to int 0x80 and syscall oversimplifies the question as with sysenter there is at least a 3rd option. It is well known that under x86-64, using 32-bit operands clears the higher bits of the 64-bit register. 32 bit yang lebih rendah, 16 bit, dan 8 bit dari setiap register dapat diatasi secara langsung It is not clear why would you need to put contents of a specific register into a variable, given a volatile nature of the most of them. Four 32-bit data registers are used for arithmetic, logical, and other operations. caller-saved register: if P calls Q, and P has something in the register it will need later, then P must save it first. Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?. As complete 32-bit data registers: EAX, EBX, ECX, EDX. Pointer Registers. It's same as using other registers for addressing, like movb $4,0(%edi). Because I'm on a 64-bit platform, memory addresses are 64-bit and require a 64-bit pointer to point to them. The only suggestion in comments that won't suck for performance is using a 64-bit constant for imul:; mov eax,eax ; if eax isn't already zero-extended into rax imul rax, [rel broadcast_low32_multiplier] section . For a more complete (but still readable) introduction, consult The AMD64 Architecture Programmer’s Manual, add ecx,eax – Will add to ECX the value of the EAX register, so ECX will become 7. @RudyVelthuis, The reason you can't push eax in x64 is that the encoding for push r64 in x64 is the same as the encoding for push r32 in x86. This instruction fills bit 31 with the same bit that was there, this way preserving the sign of the number. Misalnya, ekstensi eax 64-bit disebut rax. ECX. So, my question is what happens in x64? how the values are stored in RAX:RDX after multiplication. in registers (SSE2) movq rax,xmm0 ;lower 64 bits movhlps xmm0,xmm0 ;move high 64 bits to low 64 bits. However a new naming convention was also adopted, and the RAX register is also known as the R0 register: Fig 4: The 64-bit RAX register further extends EAX. Thus, r0 is the first general-purpose register, and r127 is the last general According to Intel, in x64, the following registers are called general-purpose registers (RAX, RBX, RCX, RDX, RBP, RSI, RDI, RSP and R8-R15). On the other hand, 64-bit registers begin with r. Data registers, Pointer registers, and; Index registers. Some Any lvalue works. Also, swapping registers that way is insane vs. These are rax, rbx, rcx and rdx all of which extend the x86 registers eax, ebx, ecx and edx, respectively. 1 extra encoding bit from the rex prefix. ) The contents of the EDX register are copied to high-order 32 bits of the selected XCR and the contents of the EAX register are The following instructions are illegal and can't be encoded in 64-bit mode: push al push eax push BYTE PTR[0] push DWORD PTR[0] push 0100000000h ; 64-bit immediate value isn't supported Pushing an 8-bit or 32-bit value on the stack requires loading the value into a register, extending it and then using a 64-bit PUSH, just like you should do If you want to change only the higher bytes of a register, simply do: movzx eax,cl //mov al,cl, but faster shl eax,24 //mov al to high byte. (Actually eax; a 32-bit int only uses the low half); You can verify the Description ¶ . Viewed 206 times add ecx,eax – Will add to ECX the value of the EAX register, so ECX will become 7. I. 16 regs incl. These registers are conceptually similar to the general-purpose registers such as EAX on the x86. From an 8-bit A of 8008, to 16-bit AX of 8086, to 32-bit EAX of 80386, to 64-bit RAX. Relative jumps (and calls) exist to divert execution from one known location in the code to another in the normal course of execution, e. ) The contents of the EDX register are copied to high-order 32 bits of the selected MSR and the contents of the EAX register are CMP subtracts the operands and sets the flags. Both eax and rax are interesting, as they are used by x86 and x64, respectively, for pointers or integers returned by a function. Likewise BX was BH and BL, etc. The 64-bit registers have names beginning with "r". The least significant byte of AX can be used as a single 8-bit Writes the contents of registers EDX:EAX into the 64-bit extended control register (XCR) specified in the ECX register. 1 General-Purpose Registers in 64-Bit Mode in the manual Basic Architecture) quoted at the same source tells us: The eax register is used to return values from functions. Additionally, the lower bytes of some of these registers may be accessed independently as 32-, 16- or 8-bit registers. Some Intel and AMD processors feature a 64-bit TSC register whose value monotonously increases by one, 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. If you want to use shift as a dividing by 2 and want to keep the sign of the result, you must use the sar instruction, that means "shift arithmetically right". TEST sets the zero flag, ZF, when the result of the AND operation is zero. For instance, when dealing with SIMD registers on x86-64, such as xmm0, the upper parts of ymm0 or zmm0 remain unchanged unless explicitly modified, matching more with legacy-SSE rather than zero-extension in VEX/EVEX operations. But in the case of memory addresses “a” and “b,” it is not clear whether they refer to 64-bit Oh, and you have different names for different portions of a register: so for example %dl is the lower 8 bits of the "d" register (edx on 32 bit machines, rdx on 64 bits) register, what does the register %eax do at the end of of the function lk and what is the C prototype of the lk function – user2504287. The information is reported in encoded form and fall into the following categories: The least-significant byte in register EAX (register AL) will always return 01H. r15, your only option (to avoid needing a mov instruction inside the asm statement) is a register-asm variable. (It's faster because it doesn't have to write the high half anywhere, that's why this compiler, MSVC it looks like, chose to use imul eax, [esp-4 + _b$] instead of Here's the full list of ordinary integer x86 registers. Personally, I would have loved to have seen register It is impossible to access the higher parts of the EAX and RAX registers, or of any other 32 and 64-bit registers, directly. org/CPU_Registers_x86-64; CPU Registers Extended Feature Enable Register (EFER) is a model-specific register added in the AMD K6 processor, to allow enabling the SYSCALL/SYSRET instruction, and later for entering The eax register is used to return values from functions. 64-bit Registers RIP, RBP, RSP, RDI, etc. mov eax,1 sets rax to 1; mov eax,-1 sets rax to $0000 0000 FFFF FFFF; mov eax,2 and mov rax,2 do the exact same thing. 2. RAX is a single architectural register, though, and however sub-registers are implemented internally, if you do read RAX after writing AL you have to see the change. You probably want mov Short answer. x64 memperluas 8 register tujuan umum x86 menjadi 64-bit, dan menambahkan 8 register 64-bit baru. RAX isn't a single physical register; x86-64 CPUs do register renaming (except I think there was an in-order Atom that supported 64-bit). Intel's 64 I'd say that the other names for registers (AX, Since the 64-bit registers allow access for many sizes and locations, we define a byte as 8 bits, a word as 16 bits, a double word as 32 bits, a quadword as 64 bits, and a double quadword as 128 ADD EAX, 14 ; add 14 into 32-bit EAX Register to register ADD R8L, AL ; add 8 bit AL into R8L Indirect: this allows using an 8, 16, or 32 bit So it seems to me that this should actually be xorq %rax, %rax, particularly since this is holding a 64-bit long int. A lot of compilers offer frame pointer omission as an For 8-bit registers, the L suffix means "low", and the H suffix means "high". But why not two (say r8dl and r8dh) If there is a REX prefix (denoting x64 registers) then al. g. Perhaps the most surprising fact is that an instruction such as MOV EAX, EBX automatically zeroes upper 32 bits of RAX register. And also as index registers in 16-bit addressing modes like [bx + si]. For example, the least significant 2 bytes of EAX can be treated as a 16-bit register called AX. But if you just need 16-bit or 8-bit registers, try to avoid them or always use movzx/movsx to clear the remaining bits. Therefore, taking one particular register as an example, you have the 8-bit AL and AH registers, which are the low and high bytes of the 16-bit AX register, which is the low word of the 32-bit EAX register, which is the low double-word of the 64-bit RAX register. ) If fewer than 64 bits are implemented in the MSR being read, the values returned to EDX:EAX in Writing a 32-bit register (EAX) always zero-extends into the full 64-bit register. For example: ROR EAX, 16; MOV DX AX; ROR EAX, 16. For example, 'EAX' is the accumulator register as a 32-bit value. at relative-to-RIP address 0. 1 on Mac OS X: $ gcc -Wall -m32 cr0. reserving space for local variables or pushing values on to the stack), local variables and function parameters are still accessible from a constant offset from rbp. Pushes the contents of the general-purpose registers onto the stack. Since we are debugging a x64 binary, the values of x86 registers for example EAX or ECX will be inside of RAX or RCX itself. You can take benefit of the fact that in 64-bit mode, modifying 32-bit registers also clears highest bits (63-32), but, anyway, you cannot encode the ah register with movzx instruction under 64-bit even to a 32-bit part of a new register appeared in 64-bit mode (movzx r13d, ah would not work). The effect you want is achieved by movl %eax, %edx since writes to a 32-bit register always zero-extend into the corresponding 64-bit register. pxmlndf aebrg hgtqd ifoyz dih wrrrj orxxa jjtlqqj sad csgtiqz