Rax register usage

Rax register usage. Two important differences between x86 and x64 are: Register Status Use; RAX: Volatile: Return value register: RCX: Volatile: First integer argument: RDX: Volatile: Second integer argument: R8: Volatile: Third integer eax, ebx, ecx and so on are actually registers, which can be seen as "hardware" variables, somewhat similar to higher level-language's variables. Some VIA CPUs expose the internal RISC instructions and R0-R4 are also mapped directly to EAX/ECX/EDX/EBX as expected. You would instead use an 8-bit register, or write the full register with a value that has the value you want in the low byte(s) you care about. Use LEA. For sub rsp,8 the top of the stack remains unaltered (so whatever was sitting there in memory will stay there). Addressing. These names will not mean much to you now, but you’ll explore the importance of each register soon. If the function binky calls winky, we refer to binky as the caller and winky as the callee. @J. 8086 also introduced the segment registers, but they were very much a separate beast. Why is it so? See the three "snapshots" below for details. (In 32 or 64-bit code, prefer a movzx eax, byte [mem] or movzx eax, word [mem] load if you don't specifically want this merging: Why doesn't GCC use partial registers?. There must be some So that's really the real answer: It's not that Intel or AMD intentionally "left out" a high 16-bit register for EAX, or a high 32-bit register for RAX: It's that the high 8-bit registers are a weird leftover historical anomaly, and replicating their design at higher bit sizes would be really difficult given the requirement that the A permanent sales and use tax license can be obtained through the Maryland Tax Connect portal. Its 16 bits are split into two 8-bit registers, AH and AL, allowing it to execute 8-bit instructions as well. the A, X, and Y in the 6510 (instead of A, B, and C). We assign the value 1 to the rax register in this example. movb $0x4,0x0(%rip) will store byte value 4 into memory at absolute address rip + 0, ie. And registers almost always counted from zero, so RBX should be R3, and AX, CX, DX would be R0, R1, R2 respectively. One-operand form — This form is identical to that used by the MUL instruction. 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). broadcasts the sign bit of eax into every bit of edx. With al, you can access the lower byte of the same register There is specific instruction movabs rax,<64b immediate> which will contain encoded value "1" as 64b integer, but common modern assembler NASM will for example mov rax,1 assemble into instruction mov eax,1 with 32b immediate (machine code b8 01 00 00 00), which will set up the final rax content in the exactly same way, but the encoding is much CR8. ebx was the base register from which you did memory address calculations; ecx was the count register which held a loop counter; edx was the data register which you could use for I/O port access; edi was the destination index register which pointed to the "destination" of a string operation But are you doing division? rax dividend/quotient rbx divisor rdx remainder Reply reply [deleted] • The registers are general purpose yes, but some instructions do use them in particular, it's most likely a DIV giving you problems here. is set up in the rax and rdx registers as shown in Table 12. it has to merge a new AL into the full RAX, for example. For example, on my machine, if you set all bits of %rax to 1, and then perform xor %eax, %eax, you will find that 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). We do this because RAX is also used to store the return value for the syscall, so the kernel stores the syscall number with offset ORIG_RAX and the return value with offset RAX. We prefix values with a dollar sign, as in $1. The 64-bit Windows ABI. We can also use special names to access the second to last byte of the registers rax, rbx, rcx and rdx, they are ah, bh, ch and dh. rcx - register c Every expression value will be loaded from the stack, manipulated using the registers %r10 and %r11, and then the result will be written back to the stack, ready to be used in another The callee-saved registers are used for local state of the caller that needs to preserved across further function calls. Essentially all x86 chips released in the last decade from AMD and Intel support this ISA. The leading e stands for extended and means that your register is 32 bits wide. assembly; architecture; x86-64; Share. Try -O3 to get optimized machine code, which will load rdi directly (and rax will contain whatever the CRT library initialization left there, i. 1 altreg: Alternate Register Names 16 Register Responsibilities Some registers take on special responsibilities during program execution. It was added in 1979, but is used in DOS or BIOS code to this day. Instead of jumping to a function within the same program, though, syscall triggers a mode switch and jumps to a routine in the kernel portion of memory. If the function binky calls winky, we refer to binky as the caller and winky as the callee This simply copies the contents of the rcx register into the rax register. LCO: . So when I compile miscompiles, choosing EDX as the input (see my answer). (generally in The simplest way is to look at the contents of the RAX register directly after the MOV operation. This is because printf uses a variable number of arguments and %rax specifies how many SSE registers are used for the arguments. global _start . Additionally, the lower bytes of some of these registers may be accessed independently as 32-, 16- or 8-bit registers. Register 64-bit memiliki nama yang dimulai dengan "r". Small leaf functions can still avoid manipulating RSP to reserve space. (adc r/m32, imm8, where the mod/rm byte encodes rax as the destination, and /2 in the reg field is part of the opcode. So, if %rax is initially 0, and you execute the instruction movl $5, %eax, then %rax will also take on the value 5. The following mapping shows that this conversion is possible: a ↦ rax, b ↦ rdx, c ↦ rdx, d ↦ rax, resulting in the In particular, the notion of the processor registers does not match reality, there is no such thing as a EAX or RAX register. For example, the 64-bit extension of eax is When calling printf, we must set the value of register %rax to 0 before issuing the call rax is the 64-bit, "long" size register. Please visit www. – Jonathon Reinhart. rcx. %eax is the 32 bit one, and %ax would be 16 bits. The later behaviour is different from MOV r/m16, Sreg where bits Addressing modes. Which could kill one of the surprising features, the use of (non repetitive) string instructions resulting in extreme performance due to their single byte encoding. gov and select “Register a Business in Maryland-Combined Registration Application” from the “Quick Links” options on This actually sets the RAX register to 60: mov eax, 60 Writing to EAX always clears the upper 32-bit half of the 64-bit register. 0x8048d8eor 48, • E x refers to a register, e. •%rax stores the return value •%rdi stores the first parameter to a function •%rsi stores the second parameter to a function •%rdx stores the third parameter to a function •%rip stores the address of the next instruction to execute •%rsp stores the address of the current top of Note that register rax is live because it is the return result for the ret instruction. Register baru diberi nama r8 hingga r15. The rip register will be of particular interest to us once we When calling printf, we must set the value of register %rax to 0 before issuing the call instruction. It is also possible to address the first four registers (AX, CX, DX and BX) in their size of 16-bit as two 8-bit halves. Modifying a 32-bit register name sets the upper 32 bits of the register to zero. The value stored in memory at location 0x1000 is 0x500. %rdi, %rsi, %rdx, %rcx, %r8, %r9: Used to pass arguments, and can be modified by the procedure. For example: For example: movb $1, %al # AL = 1, merged with existing 3/7 bytes of previous EAX/RAX movl $1, %eax # AL = 1, AX = 1, EAX = 1, RAX = 1 I'm not sure whether you're talking about the case where the segment register is the destination or source operand of the MOV instruction, but the Intel manual lists both REX. Examine the register values using the info registers command. It is subject to change with different architectures, but for Intel and AMD it CS107 x86-64 Reference Sheet Common instructions mov src, dst dst = src movsbl src, dst byte to int, sign-extend movzbl src, dst byte to int, zero-fill cmov src, reg reg = src when condition holds, using same condition suffixes as jmp lea addr, dst dst = addr ZF add src, dst dst += src sub src, dst dst -= src imul src, dst dst *= src neg dst dst = -dst (arith inverse) This was surprising since the expectation was that rax would turn into 0x0000000000000001, based on the well-known behavior of zero-extension when writing to 32-bit sub-registers like eax in x86-64 architecture. The destination operand is written back if the movb $0x4,0x0 will store byte value 4 into memory at absolute address 0. simple direct al register access (a. One 32 bit variant works like the 16 bit multiplication but writes the register into EDX:EAX. Any register can be used in addressing, and it is generally more efficient to use a full 32-bit register instead of a 16-bit register part. I'm not sure whether you're talking about the case where the segment register is the destination or source operand of the MOV instruction, but the Intel manual lists both REX. x64 Registers. Writing EAX zero-extends into RAX. This instruction has three forms, depending on the number of operands. CR8 is a new register accessible in 64-bit mode using the REX prefix. To know what it really tests for you need to check the following instruction(s). So, your understanding is incorrect. There are also special registers for branching, exception handling, and other purposes. As we can see there is an inconsistent pattern to these register names. During instruction execution, this value is sign-extended to 64 bits and loaded into register RAX. The names in the 8086 are chosen to reflect their usage as well. Since callouts can only return an single integer value, we have provided When performing a syscall, RAX register contains the syscall number. 1:. A value in the range between -4095 and -1 indicates an error, it is -errno. Follow edited Nov 2, Some registers are designated for a certain purpose, such as %rsp being used as the stack pointer or %rax for the return value from a function. at relative-to-RIP address 0. lea rcx, hello_msg call printf ; printf(rcx, rdx, r8, r9, stack) printf on 64-bit Windows ignores RAX as an input; RAX is the return-value register in the Windows x64 calling convention (and can also be clobbered by void functions). Preserved It covers topics such as the calling convention, type layout, stack and register usage, and more. Abstract x86-64 Preserved accross form Register Usage function calls res 0 %rax return value No arg 1 %rdi argument 1 No arg 2 %rsi argument 2 No arg 3 %rdx argument 3 Use the stepi command to step through the assembly instructions one by one. System Calls vs. Function Calls¶. github. This is the code of the process I'm trying to trace: #include <stdio. Is register %eax is part of %rax? yes -- it is the lower 32 bits of the 64-bit %rax. Even though the instruction bytes actually come from 7 different addresses. There is a nice set of instructions that set up the dividend in the rdx and rax registers, using the contents of the rax register, for signed division. smbSign smbSign Public. The notation dx:ax, edx:eax, and rdx:rax, means that the high-order portion of I want to copy the value from YMM to RAX register, but I didn't find the correct instruction to do that. The algorithm manages to register-allocate this code using three registers, plus the preallocated rax register. The eax register is just the lower four bytes of the rax Register. Writing AL, AH, or AX leaves other bytes unmodified in the full AX/EAX/RAX, for historical reasons. Let's suppose I have two registers (say RAX and RDX) that I can freely use without impacting sourrounding code. The rax register points to the full 8. An integer or pointer return value is returned in the rax register, NOTE: A floating-point return value However this feature requires the use of register keyword which was deprecated in C++11 and removed in C++17. Registers can be used in your software directly with instructions such as mov, add or cmp. Now you may want to use the address multiple times. g. The AMD64 architecture allows software to define up to 15 external interrupt-priority classes. So, fun fact, movabs 0x123456789abcdef0, %si or %dil don't have an exact drop-in "replacement" / implementation. h> int x=0; int main(){ while(1){ x++; For example, in multiplication operation, one operand is stored in EAX or AX or AL register according to the size of the operand. If the function binky calls winky, we refer to binky as the caller and winky as the callee These registers are RAX, RBX, RCX, RDX, RDI, RSI, RSP, RBP and R8 through R15. It looks at the rax register, finding the value of ’60’ indicating that we want to exit the program. But x86 in 16 and 32-bit mode does have an immediate-division instruction, and it's actually slightly faster than div r/m8 on Intel CPUs %eax refers to the lower 32 bits of the 64-bit %rax register. 3) long rotate ( long op1, long direction, long number_of_bits ) I'm in the habit of using this register size, since they also work in 32 bit mode, although I should probably use the longer rax registers for everything. %rax, %edi) Memory: use the data at the memory address specified by the addressing mode D(Rb,Ri,S) The operation determines the effect of the operands on the processor state and has a suffix (“b” for byte, “w” for M3-L1: x86-64 Programming I CSE351, Winter 2021 Reading Review Terminology: Instruction Set Architecture (ISA): CISC vs. Assembly - Registers - Processor operations mostly involve processing data. If I have a variable in my C code, and I want to move the contents of RAX into this variable, which form of the MOV instruction should I use? Even adc $3, %rax can't usefully use the special rax-only encoding REX. Without they wouldn't be much useful - or always needing a segment prefix. Just like how %rdi stores This simply copies the contents of the rcx register into the rax register. So it can be use for global variables only. Preserved RAX is the 64-bit extension to the 32-bit EAX accumulator register. It doesn't use shadow-space; it defines a red-zone below RSP that's safe from being asynchronously clobbered. EAX, EBX are names of the 32-bit registers, and you may see these in 32-bit code or 64-bit code. Loading such an addres into a register is never intended to set status flags and luckily it doesn't. This needs to be taking However in x86 the largest sized registers we can use are the 4 byte registers like ebp, esp, eip etc. This clobbers %rcx and %r11 as well as the %rax return value, but other registers are preserved. I'm just playing around with allocating some executable memory and manually assembling some x86 code to run in it. The 64-bit registers have names beginning with "r". Here is an assembly version:. The code below demonstrates how to write the immediate test is a non-destructive and, it doesn't return the result of the operation but it sets the flags register accordingly. In nasm you can also use those numbered registers with %use altreg. The difference is, that the rip points at the time of evaluation to the beginning of next instruction. True to its CISC nature, x86-64 supports a variety of According to Intel, in x64, the following registers are called general-purpose registers (RAX, RBX, RCX, RDX, RBP, RSI, RDI, RSP and R8-R15). In modern x86 code, the trap instruction is syscall, which acts in a manner analogous to call. return value, number of vector registers used for functions with variable arguments, (lower) result of multiplication and division. To access the bottom four bytes we replace ‘r’ with ‘e’, to access the lower two byte we remove the initial ‘r’. Each GPR is 32 bits wide, and each FPR is 64 bits wide. code_examples code_examples Public. rodata broadcast_low32_multiplier: dq 0x100000001 Data registers: Data registers consists of four 32-bit data registers, which are used for arithmetic, logical and other operations. E. 1. I'm trying to read the register values of a running process. At most two variables are live at a given time, suggesting that we might be able to convert this to code that uses only two registers. (adc rax, imm32)REX. Many instructions exist in x86 that allow you to do all the normal math operations on registers_use and memory. the accumulator HTML. In the registers’ case, it is clear from their names whether we use 64-bit %RAX or 32-bit %EAX. For push rax rax value is pushed on top of the stack. We use a different instruction mnemonic for each different byte size. 2) long xor ( int op1, int op2 ) xor will return the result of bit exclusive OR of op1 / op2. There is no corresponding instruction with an ymm register because the lower half of an ymm register is just the So that's really the real answer: It's not that Intel or AMD intentionally "left out" a high 16-bit register for EAX, or a high 32-bit register for RAX: It's that the high 8-bit registers are a weird leftover historical anomaly, and replicating their design at higher bit sizes would be really difficult given the requirement that the Register: use the data stored in one of the 16 general purpose registers or subsets (e. io Public. It covers x86 assembly language is the name for the family of assembly languages which provide some level of backward compatibility with CPUs back to the Intel 8008 microprocessor, which was launched in April 1972. x64 calling conventions. Don't know? 7 of 80. If there are overflow, return the overflowed values in RAX register. 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. 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. 1. Addressing modes. Assume that %rax stores the memory address 0x1000. Index: Any general purpose register (rax, rbx, &c) 1. It was added in 1985 during the transition to 32 The rcx register is used implicitly as the cycle counter by the loop instruction (c stands for cycle). Or for data, mov rax, [rdi] does an 8-byte load from the address in RDI. The complete EABI definitions currently live here on ARM's infocenter. R5 is RBP. At the level of assembly language, a system call involves executing a trap instruction. Not to be confused with cdqe, the 64-bit instruction that is a more compact form of movsxd rax, eax. Note that the above example was similar to the other example from your question: mov %rax, %eax # AT&T syntax (INVALID!) mov eax, rax ; Intel syntax (INVALID!) In the first example, the mov rax,100 instruction loads an immediate value into the RAX register. Displacements in asm source are always in bytes. In x64 there is the rax, eax, ax, and al register. This is not like AH and AL, where writes preserve the rest of the register. And yes, all the x86 ABIs use eax / rax as the register for return values. It would appear that the segfault occurs due to a null pointer dereference ( %rax is being dereferenced, but it is an invalid dereference because %rax is zero). The destination register is specified after the source in the AT&T syntax. The least significant bits are shown in this diagram on the rightmost side. 4. can use XOR instruction. Instead we discover a web application vulnerable to a remote authenticated command injection that requires us to first find valid credentials. This data can be stored in memory and accessed from thereon. You cannot fill part of rax with a mov. Workarounds. They only take one explicit operand (register or memory), with the dividend being implicit in AX, or DX:AX, EDX:EAX, or RDX:RAX. Then it exits the program, providing the user the exit code specified. To access the lower 16 bits of each of these eight registers, reference the last two letters of the register’s name. %rax, • R[E Short answer. There is no dereferencing being done. However when I looked at the value of rax it's 0xffffffffffffffda. long 1084227584 _start: mov . On Linux, there are two ARM ABIs; the old one and the new one. g i r rax, i r eax i r <register_name_1> <register_name_2> : print multiple registers, e. True to its CISC nature, x86-64 supports a variety of addressing modes. x0 through x30 - for 64-bit-wide access (same registers) w0 through w30 - for 32-bit-wide access In this article. g: The div and idiv instructions don't have forms that take an immediate. The values in the register(s) are acted on directly. Use tax may also apply to purchases shipped to a California consumer from another state, including purchases made by mail order, telephone, or Internet. See this answer for how to use them. Most have zero or one source General-Purpose Registers For AArch64. ; i r f: print all FPU floating registers (st0-7 *1 - The usage of ES for string destination is an exception, as simply two segment registers are needed. R7 is RDI. REX. It multiplies the AX register with whatever you pass as the argument to imul and stores the result in DX:AX. These are shown in Table 12. In the following table, • Immrefers to a constant value, e. python3 scanner to detect smb security mode Python. eax is the 32-bit, "int" size register. Un-prefixed operands are treated as memory addresses, The CDQE instruction sign-extends a DWORD (32-bit value) in the EAX register to a QWORD (64-bit value) in the RAX register. DX is known as the data register A mov instruction will always fill the destination operand with a complete value, of exactly the same size as the destination operand. You can mov a register value into RSI and expect it to store data in the previous register - that doesn't make any sense. These instructions perform computation on values, typically values stored in registers. You need a full 64-bit register to hold the address (if RIP-relative or 32-bit absolute won't reach), but mov to an 8 or 16-bit register only modifies that 8 or 16 bits of register state, merging into the full reg. 10 12 What does this instruction do? Here's the full list of ordinary integer x86 registers. If we don’t have any pop rsp instructions, we can still control rsp indirectly by controlling the rax register and then using the xchg rax, rsp instruction to move the content from rax to rsp. Preserved 4 Register Convention We extract from [MHJM09] the relevant information on register usage. I want to do something like: mov rax, ymm0 for example for XMM I can do: movq rax, xmm0. Virtual Address of Entry Point in Eax register;; Virtual Address of Process Environment Block structure in Ebx register. However, they are not entirely independent, so one should be careful. Similarly, we prefix registers with a Checking rcx and rax using printf return 1 and 0 respectively after inputting a char or string (only way the program doesn't segfault). Be familiar with x86 assembly as the differences between x86 and x64 assembly are minor. If the function binky calls winky, we refer to binky as the caller and winky as the callee uint64_t rax = 0, rbx = 0; __asm__("" : "=a"(rax), "=b"(rbx) ::); /* make rax and rbx take on the current values in those registers */ Note that you don't need any actual instructions -- the constraints tell gcc that after doing nothing, the value rax will be in Some registers are designated for a certain purpose, such as %rsp being used as the stack pointer or %rax for the return value from a function. 3. We will tell you which registers_use are set beforehand and where you should put the result. sub rsp,64 ;reserve 64 bytes for local variables mov rax,[rbp+8]; rax = the last stack-passed qword parameter (if any In the following code: !6 movl $5, %ebx The register shows the value of: rbx 0x0000000000000005 Does ebx automatically convert to rdx on a 64-bit architecture? Is writing ebx "wrong" or is m The names of the 64-bit registers are RAX, RBX, RCX, and RDX. This workaround is based on Peter Cordes's comment below. Performs a signed multiplication of two operands. When gcc compiles a C source file into an object file, it first translates the C code into assembly code, and then invokes the assembler to translate the assembly code into the machine code. Improve this question. The change to the rsp value is exactly the same for both instructions. 4th argument, Regarding your comment on usage of al, ax and eax: when using the x86-64 architecture the requirement is to specify the number of syscall in rax, using any other part of the register is based on luck - if all the bits in the other parts of the register were zero, then you can use the lower bits - but you should not trust it. What does this accomplish? Well, %rax is also a special register. h> int x=0; int main(){ while(1){ x++; 16 Register Responsibilities Some registers take on special responsibilities during program execution. The XOR eax, 1 instruction just flips the lowest bit of Description ¶ . One primary job of the instruction decoder is to convert the legacy x86/x64 instructions into micro-ops, instructions of a RISC-like processor. I call malloc to get 40 bytes of space. If you use the loop instruction and the rcx register simultaneously things will The basic kinds of assembly instructions are: Computation. ; cdq sign-extends eax into edx:eax, i. code examples from hack the box and other online challenges/ctfs. 0:000> uf msvcrt!printf msvcrt!printf: 000007fe`fe667e28 mov rax,rsp 000007fe`fe667e2b mov qword ptr [rax+8],rcx 000007fe`fe667e2f mov qword ptr [rax+10h],rdx 000007fe The general-purpose register rax is the return value register. Also note that anytime that bits are labeled, bit 0 is the least significant bit (furthest right) and bit 63 is the most significant bit (furthest left). If you absolutely want a move to RAX, you need to use something like this: And yes, all the x86 ABIs use eax / rax as the register for return values. It's same as using other registers for addressing, like movb $4,0(%edi). The new SP is stack pointer, BP is base pointer, SI is source index, DI is destination index. The aarch64 registers are named: r0 through r30 - to refer generally to the registers. That's the story of the general registers in the 8086 family. movq $3, %rax. highly likely something else). RAX is 64 bits wide; EAX is the lower 32 bits of RAX; AX is the lower 16 bits of RAX; AL is the lower 8 bits of RAX; and AH is bits 9-16 of RAX. It's possible to imagine cases where that was actually Gdb commands:. There is specific instruction movabs rax,<64b immediate> which will contain encoded value "1" as 64b integer, but common modern assembler NASM will for example mov rax,1 assemble into instruction mov eax,1 with 32b immediate (machine code b8 01 00 00 00), which will set up the final rax content in the exactly same way, but the encoding is much If that was written with assembly, there would be no problem, as the value could be accessed simultaneously with e. Which of the following operand specifiers could NOT be used to access the data? Choose matching term %rip %rbx %rbp %rax. The idea is to The variant you've stumbled upon is a 16 bit multiplication. Question: Given the following information: Register Value %rax Ox08 Memory Address Value Ox3028 Ox25 %rbx Ox02 Ox3020 Ox50 %rcx Ox3018 Ox3018 Ox60 %rdx Ox40 Ox3010 Ox6138 Ox6128 %rsi Ox18000x3008 %rdi Some registers are designated for a certain purpose, such as %rsp being used as the stack pointer or %rax for the return value from a function. Continue stepping through the code until the program exits or you have debugged the issue. The variant you've stumbled upon is a 16 bit multiplication. But in the case of memory addresses “a” and “b,” it is not clear whether they refer to 64-bit When calling printf, we must set the value of register %rax to 0 before issuing the call instruction. The Explanation. As Jester has already said in a comment, precisely what will happen as a result of that mov rax, dwordVal depends on the conventions followed by your assembler, and some Here's an example regarding the rax 64-bit register and its subsequent divisions: As time passed more registers were made available, the need of better usage of the exact register size decreased. add al, popq %rax What is the value of %rax? What would this be useful for? PRACTICE PROBLEM 13 The “call” will cause the address of the “popq” instruction to be pushed onto the stack. Misalnya, ekstensi eax 64-bit disebut rax. Since callouts can only return an single integer value, we have provided It isn't required, this code just wastes an instruction by doing an lea into RAX and then copying to RCX, when it could do. Commented Apr 26, 2012 at 1:37. For shorthand, when we say A += B, it really means, A = A + B. (Fun fact: the machine encoding for AVX512 disp8 displacements is scaled by the operand-size, so you can reach +127 * 64 bytes with a compact displacement, but vcmpeqps 13(%rax), %zmm1, %k1 (512-bit memory operand) would require a disp32 because 13 is not a info all-registers gives you all the register values including FPU register stack, xmm registers. 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 . When you call a function in x64, the manner and use of the registers follows a very specific convention. rax on x86-64, eax on x86, etc). Enable x64 compiler optimization. To simplify the interface to the processor’s bus, the destination operand receives a write cycle without regard to the result of the comparison. text . malloc returns the starting address of this space in rax (the 64-bit version of eax). Since we Netmon provides a common Windows attack surface with a few well-known ports open. g: My program works correctly most of the time, but about every 800 runs or so I get an unexplained segfault within the 3rd party library at the instruction indicated below. Thus, after movl $-1, %eax, the %rax register has value 0x0000'0000'FFFF'FFFF. An addressing mode is an expression that calculates an address in memory to be read/written to. 5. By manipulating eax, the first 32-bits of rax are modified, just like ax ebx was the base register from which you did memory address calculations; ecx was the count register which held a loop counter; edx was the data register which you could use for I/O port access; edi was the destination index register which pointed to the "destination" of a string operation It explains the different registers and their evolution. Just use vmovq rax,xmm0. The above assembly is meaningless - the compiler will throw it out. ; i r a: print all register, include floating point & vector register (xmm, ymm, zmm). %eax refers to the lower 32 bits of the 64-bit %rax register. Other registers are all-purpose, but have a conventional use depending on whether caller-owned or callee-owned. These are rax, rbx, rcx and rdx all of which extend the x86 registers eax, ebx, ecx and edx, respectively. [1] [2] It is used to produce object code for the x86 class of processors. A player/team card is a pass that allows you to earn rax based on a players performance, or a teams score differential. Un-prefixed operands are treated as memory addresses, For push rax rax value is pushed on top of the stack. For our purposes the value will always be 0. There's no form of div / idiv that ignores edx in the input. The number of the syscall has to be passed in register %rax. System-calls are limited to six arguments, no argument is passed directly on the stack. With the x86 assembly essentials we’ve just covered, we can now dive into compiler-generated x86 assembly for this simple C program. To access 64-bit integer registers you use the new names with R-prefix such as rax, rbx Register names don't change so you just use the byte registers (a l, bl, cl, dl, ah, bh, ch, dh ) for the LSB and MSB of ax, bx, cx, dx like before. ; But when I do the same for x86_64 process then CONTEXT contains:. This little diagram was taken from another answer to the same question, but it shows it rax-register. This instruction now takes the value 8 that we just stored in memory on the stack, and puts it in the register %rax. (gdb) i all-r rax 0x2aaaaace62ce 46912498459342 rbx 0x2aab18e71290 46914345570960 rcx 0x2aaab2020d60 46912619285856 rdx 0xffffffffffd934ee -2542354 rsi 0x2aab18ec7a40 46914345925184 rdi 0xa 10 rbp 0x2aab18e6f000 0x2aab18e6f000 rsp 0x2aab18e6f000 I'm trying to read the register values of a running process. Additionally, segment registers are generally unused in flat mode, and it is generally a bad idea to touch them. Caller-Saved Registers: %rax: Holds return value and can be modified by the procedure. This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically. This topic describes the basic According to the calling convention, a function must place its return value in the %rax register, so foo has succeeded in returning x + 3. There's a little bit of Assembly in that header file that's obviously failing, to be converted to machine code, on your System. x64 type and storage layout. In the first column is a suggested numbering for the purpose of register allocation. Other registers are all-purpose, but have a conventional use depending on whether caller-saved or callee-saved. PowerShellTools What's wrong with the code there's never a need to explicitly move anything into %rax directly, because you can instead do asm("" : : "a"(segment)); - i. The 64 bit registers are shown in red. g i r rdi rsi, i r: print all register except floating point & vector register (xmm, ymm, zmm). The 3 Resources. Python. But add %dl, (%rax) is an RMW on (%rax), so the dl data isn't needed until a load from (%rax) has that data ready. Virtual Address of Entry Point in Rcx register For example, a movq instruction which sets the value of the 64-bit %rax register to the immediate value 3 can be written as. W + 83 mod/rm 03 is 4 bytes. The byte ordering is little endian on x86, so the 8 bytes will be loaded from memory in reversed order, resulting in rax having 0x0000000000000A73. Therefore we The best solution (especially if you want to stick to SSE2 - i. i r <register_name>: print a single register, e. Your number is 1111, and you need complement of 0 and 3 position like this 0110, so: mov al, 1111 xor al, 0110 not al int 3 This should change al register of 1111 to 0110 Register names (on x86, both register and %register are accepted, such as rax or %rax) The special name cc, which specifies that the assembly altered condition flags. With 4 general purpose registers, it was logical to General-Purpose Registers For X86 64. Since callouts can only return an single integer value, we have provided If we take a look at the first instruction, it seems that it takes the value from the rcx register and write it in the memory address pointed by rax register. BX is known as the base register, as it could be used in indexed addressing. R6 is RSI. to avoid using AVX) to initialize two registers (say, xmm0 and xmm1) with the two 64-bit halves of your immediate value, do MOVLHPS xmm0,xmm1 In order to initialize a 64-bit value, the easiest solution is to use a general-purpose register (say, AX), and then use MOVQ to transfer its value to the XMM The rax register is where our integer and pointer return values are stored when returning from any functions (this is different for floating point return values - which will need to be covered in a future post). Definition. There are many instructions which take arguments or return results in particular registers - for example, the variable shift instructions only take their argument in cl. There are other registers with various specialized roles, and their sizes are mov rax, 60 ; exit syscall is number 60 mov rdi, 0 ; exit code of 0 indicates success syscall. Returning from the syscall, register %rax contains the result of the system-call. This dictates where the parameters to the function should go You can use a XOR and NOT instruction respectively, for example. i. rax) register that represents a sequence of ascii chars (each of them taking up one byte) and I want to determine if there is a zero byte somewhere. The XOR eax, 1 instruction just flips the lowest bit of Yes, every byte has its own address, but you can still say a 7-byte instruction like mov rax, -123 has an address, that being the address of its lowest byte. CX is known as the count register, as the ECX, CX registers store the loop count in iterative operations. rsp (and esp, etc) may not be used as an index register. Register Usage; Green registers are callee-saved More on this will be explained on the page about Calling Subroutines. – Jason. Similarly, we prefix registers with a When I create a Windows x86 process in a suspended state (CREATE_SUSPENDED) its CONTEXT contains:. Syscall to Save GID, UID, and GidReceived. PRACTICE PROBLEM 3. CR8 is used to prioritize external interrupts and is referred to as the task-priority register (TPR). The syscall to save GID, UID, and GidReceived is used to save the current process's real, effective, and saved group and user IDs and the received group ID. x64 register usage. %ah refers to the high 8 bits of the 16 bits in the register, and %al refers to the lower end. The General-Purpose Register Convention RIP addressing is always relative to RIP (64bit Instruction Pointer) register. From the AAPCS, §5. For example, if we want to assign a value to a register using the AT&T syntax, we use the mov instruction: mov $1, %rax. So the other major x86-64 calling convention does use more arg-passing registers. The 64-bit versions of the 'original' x86 registers are named: rax - register a extended; rbx - register b extended; rcx - register c extended; rdx - register d extended; rbp - register base pointer (start of stack) rsp - register stack pointer (current location in stack, growing downwards) 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. rax. W + 15 03 00 00 00 is 6 bytes. 2 Operand Specifiers The basic types of operand specifiers are below. In this case, the %rax register is a 64 bit register. The special name When calling printf, we must set the value of register %rax to 0 before issuing the call instruction. optionally used as base pointer. The 64-bit Windows Application Binary Interface specifies exactly how code should run when running under 64-bit Windows. Small instructions that are easy to execute concurrently and being able to Here's the full list of ordinary integer x86 registers. What is better (suppose I don't need the temporary value from 0xabcd afterwards): #1 MOV RAX, [0xabcd] MOVSX RAX, BYTE PTR [RAX] MOV [0xbcde], RAX or #2 MOV RAX, [0xabcd] MOVSX RDX, BYTE PTR [RAX] MOV [0xbcde], RDX The use tax generally applies to the storage, use, or other consumption in California of goods purchased from retailers in transactions not subject to the sales tax. Is there some way to access the low byte (especially for performing additions) of a uint64_t variable with C++ so that MSVC would be smart enough to compile it with e. Base: Any general purpose register. That’s it! The ‘syscall’ instruction executes the system call. We will not use Metasploit for this one. 3. Data registers are again classified into 4 types they are: AX: This is known as the accumulator register. System-calls are limited to six arguments,no argument is passed directly on the stack. This means that if anything is to be returned to the caller using the System V AMD64 ABI the return value will be stored in rax. So for example you could do either: sub rsp,16 or. In most cases, its rax. The MOVZX instruction zero-extends the source to the destination. Thus, the register corresponding to the lower 32 bits of register %rax is register %eax. Commented Nov 23, 2014 at 22:44. float a = 5; in this case, a is stored as 1084227584 in the XMM register. Can you provide a full build log with: The x86-64 string instructions use registers RSI and RDI for the source and destination buffers, respectively; string instructions that employ a repeat (or length) count must load this value into In this example, implicit register RAX and explicit register RBX are used as the source operands, and implicit register pair In particular, we can use 32-bit registers instead of 64-bit ones by including the address prefix byte (0x67) in our encoding. globl main main: enter $(8 * 3), $0 mov $0, %rax Every expression value will be loaded from the stack, manipulated using the registers %r10 and %r11, and then the result will be written back to the stack, ready to be used in another The x86_64 instruction set architechture includes 16 general purpose regisers, each of which can be addressed in full, or by the lower 32, 16 and 8 bits. If the function binky calls winky, we refer to binky as the caller and winky as the callee Note that unlike primary memory (which is what we think of when we discuss memory in a C/C++ program), registers have no addresses! There is no address value that, if cast to a pointer and dereferenced, would return the contents of the %rax register. Also, immediate data doesn't have to be fetched, other than as a pre-requisite for instruction decoding. The preceding register layout shows how %rax has embedded within it the smaller registers %eax (32 bits), %ax (16 bits), %ah (8 bits), and %al (8 bits). io rax-register. What is the meaning of the second line in assembly code? (movl $0, %eax) It loads a 0 into those lower 32 bits of %rax and then sets the upper 32 bits to 0 (all instructions with a 32-bit destination set the upper 32 bits of the 64-bit register to 0) To access the lower 32 bits of the first eight registers, simply replace the r in the register name with e. Addresses are integers, as a rule of thumb, you want to keep them in general purpose registers like rax, rbx, etc. Fun fact: R10 and R11 are also non-arg-passing call-clobbered registers in x86-64 I know that EAX is essentially Extended AX register, but what is the RAX register called? My computer architecture professor was stumped, and I can't find the answer anywhere. One idea would be to make TLS_base_addr64 just define a virtual register, then CSE could work with it, and during MC lowering for TLS_base_addr64, we emit a mov from RAX to that virtual You can use rax to buy player and team cards, play card packs, and enter play card raffles in game. Please clarify what you want to do Register usage and conventions. The PowerPC® 32-bit architecture has 32 GPRs and 32 FPRs. It is probably a good idea to get familiar with the fundamentals of x86 assembly before starting to program the x87 It depends on the ABI for the platform you are compiling for. ax is the 16-bit, "short" size register. Here are some useful . R1 is RCX. RISC Instructions: data transfer, arithmetic/logical, control flow • Size specifiers: b, w, l, q Operands: immediates, registers, memory • Memory operand: displacement, base register, index register, scale factor Ed Discussion post for questions: Readers are expected to have a good understand of how things work on the X86 CPU in terms of register usage, stack usage and function layout to make most of this tutorial. This can also be used with a general register class to obtain a scratch register for use inside the asm code: #![allow(unused)] By default the compiler will always choose the name that refers to the full register size (e. c:15: Error: bad register name `%%rax' WRITE has a system call number of 4 and EXIT is 1, I have no idea what is the difference about the use of "rax"? mov rax,[register+offset] //store value that you want to use for your manipulation mov [value],rax pop rax //code address_script: push rax lea rax,[register+offset] //store address that contains the value that you want to manipulate mov [address],rax pop rax //code manipulation: push rax push rcx mov rax,[address] //now rax will contain the address The mov rax, [buffer] instruction will copy 8 bytes of memory starting at the address of buffer to the rax register. It's hard to do this efficiently without touching a 2nd register, as requested. This is a rather naive question. In this case, it zero-extends the BYTE loaded from memory at [rbp-528+rax] to the DWORD destination register, EAX. r0-r3 are the argument and scratch registers; r0-r1 are also the result registers; r4-r8 are callee-save If you use a complicated address, AX xor:= mem[0x333 +BX + 8*CX] the flags are set according to the xor operation. . rbx - register b extended. R4 is RSP. Immediate operands are always prefixed by $. push rax push rax Register Value %rax 0x100 %rcx 0x1 %rdx 0x3 Instruction Destination Result addq %rcx, (%rax) 0x100 0x100 subq %rdx, 8(%rax) 0x108 0xA8 Register Use Register Use %rdi Argument x %rax t1, t2, rval %rsi Argument y %rdx t4 %rdx Argument z %rcx t5. For example, using rax (as in the code above) would address all the 64 bits in the rax register. However, we use ORIG_RAX offset to grab that number instead of the also existing RAX offset. x64 memperluas 8 register tujuan umum x86 menjadi 64-bit, dan menambahkan 8 register 64-bit baru. Often out is used to check a register against 0, possibly coupled with a jz conditional jump. That is, the rax register is acting like a pointer. That ISA extension was subsequently adopted by Intel and is often known by the more neutral name x86-64. Registers. On the other hand, 64-bit registers begin with r. TinyHelloWorld. R2 is RDX. The 0 offset is equal to address of the following instruction after the RIP-addressed instruction. "Scratch" registers any function is allowed to overwrite, and use for anything you want without asking anybody. If you see RAX, RBX, RSP, RBP, these are names of registers in the 64-bit extension of this architecture. All of the new x64 registers begin with "r" and are all 64-bit or QWORD (quad-word) values. Originally (8086), there was just cbw (ax = 2. AFAIK, the new one (EABI) is in fact ARM's AAPCS. Note that the above example was similar to the other example from your question: mov %rax, %eax # AT&T syntax (INVALID!) mov eax, rax ; Intel syntax (INVALID!) This frees up %rbp for use as another general-purpose register. In x86-64, assume the %rax register stores the address of the data you want to access. The later behaviour is different from MOV r/m16, Sreg where bits The CDQE instruction sign-extends a DWORD (32-bit value) in the EAX register to a QWORD (64-bit value) in the RAX register. rbx. General-Purpose Registers. •%rax stores the return value •%rdi stores the first parameter to a function •%rsi stores the second parameter to a function •%rdx stores the third parameter to a function •%rip stores the address of the next instruction to execute •%rsp stores the address of the current top of If you want to actually use that register value, do it from inside the inline asm statement, or else the register may be overwritten with something else. Poletto and Sarkar report that this fast, simple algorithm usually achieves performance within 12% of optimal register allocation on a machine with 31 general purpose registers, which means that most of the benefit of register allocation has been achieved. I can then read and write from the pointed-to memory using the usual assembly bracket syntax: In this guide, we will use x86-64 assembly which can be assembled and executed on most personal computers. rax and al registers as needed. This syscall is For example, if we want to assign a value to a register using the AT&T syntax, we use the mov instruction: mov $1, %rax. Some registers are designated for a certain purpose, such as %rsp being used as the stack pointer or %rax for the return value from a function. This choice should ease running and tinkering with the snippets along the way. The first one is taken on stop-on-enter for the syscall (see ptrace(2)), the second one is taken on stop-on-exit. The x64 Registers. Show 6 more. These expressions are used as the source or destination for a mov instruction and other instructions that access memory. LCO, %eax movss . Note that the machine code uses only 32 bits to encode the immediate value 100, which is underlined. Specifically take note of the rip register that is our current instruction pointer and the rax register the return value register. rbp is the frame pointer on x86_64. R3 is RBX. Keep in mind that xchg , unlike xor , swaps the content of registers, which means that rax will receive whatever used to be in rsp. For example, a mov instruction which sets the value of the 64-bit %rax register to the immediate value 3 can be written as. Run gdb -q /bin/ls and then $ catch syscall (not single stepping), the first syscall would be brk, which has sysno 12. 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. However, reading data from and storing data into memory slows down the processor, as it involves complicated processes of sending the data request across the control bus and into the mem The mnemonics are: BX is base register, CX is count register, DX is data register, and AX is still the accumulator. The 64-bit versions of the 'original' x86 registers are named: rax - register a extended. Any given program should be either intended for the 64-bit architecture (x64) or the 32-bit (x86) architecture but not both mixed together. W forms as having an a r/m64 operand as the other operand, and zero extending the result when the source is segment register. Similarly, in the 64-bit version, the 'E' is replaced with an 'R' (register), so the 64-bit version of 'EAX' is called 'RAX'. xor is shorter than mov, and even leaves more spare space in the uop cache line it's in, but neither of those effects are well described as "not having to fetch". 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 For example, 'EAX' is the accumulator register as a 32-bit value. To move eight bytes we use movq; To move four bytes we use movl; For example, a movq instruction which sets the value of the 64-bit %rax register to the immediate value 3 can be written as. Now we can also use smaller registers, than the maximum sized registers for the architecture. push rax push rax RAX register is available only in 64-bit mode. mdtaxconnect. ) is v1 guaranteed to be at r6, or is the compiler free to make an optimization to use another register? Is there a way to tie a name to a particular register? (without manually specify %r6 everywhere?) Also, is there a difference between using an output constraint vs a clobber constraint for a temporary variable? We can also use special names to access the second to last byte of the registers rax, rbx, rcx and rdx, they are ah, bh, Normally we use movq to move a value into a register. But we won’t go into details about them here. so first you might ask yourself, what is the value of rax, now that value suppose to be a memory address where I suppose to write the value that's in rcx register. LCO, %xmm0 If we don’t have any pop rsp instructions, we can still control rsp indirectly by controlling the rax register and then using the xchg rax, rsp instruction to move the content from rax to rsp. Here, the source operand (in a general-purpose register or memory location) is multiplied by the value in the AL, AX, EAX, or RAX register (depending on the operand Register Usage in x86-64 Linux. Then, the popq instruction will pop that address (its own address) off, and store it into rax. If this means absolutely nothing to you, I encourage you take a look at the structure of the x86_64 registers. The phrase "load effective address" makes the programmer aware of that. segmented addressing is no longer used — this is not true, the segmented addressing is still used 100%, although the usage shifted dramatically to become a virtual space instead of a segment. W + 15 id ADC RAX, imm32. You can also declare variables the must be allocated in a specific register, but I think using them without initialization makes the compiler unhappy. use the register modifier "a" to say "I'm expecting this value to be passed/returned in [ER]AX". It was added in 2003 during the transition to 64-bit processors. In your generated code, it gets a snapshot of the stack pointer (rsp) so that when adjustments are made to rsp (i. Regarded as a programming language, assembly is machine-specific and low-level. Upon further analysis, this behavior is attributed to how GDB manipulates register values. 32 bit yang lebih rendah, 16 bit, dan 8 bit dari setiap register dapat diatasi secara langsung dalam operation with registers_use. On platforms that keep multiple sets of condition flags as separate registers, it's also possible to name indvidual registers: for instance, on PowerPC, you can specify that you clobber cr0. If you were talking Right now, TLS_base_addr64 defines RAX (because TLS_base_addr64 ends up being lowered to a call), and subsequent instructions use that, so MachineCSE won't touch it. Nowadays, you can use 64 bit registers for only their lower byte or word without "worrying" about not having enough registers. General Purpose Registers. x64 assembly code uses sixteen 64-bit registers. In -O0 it will use rax to prepare the pointer value into memory and into rdi as argument for function call, so they accidentally contain identical value. Suppose the address of the datum you want to load is in rax, you just write fld qword [rax] to load a double from that address. Un-prefixed operands are treated as memory addresses, Why doesn't GCC use partial registers?. Having the RAX address ready early is more valuable than x64 extends x86's 8 general-purpose registers to be 64-bit, and adds 8 new 64-bit registers. The arithmetic instructions (and test) have short encodings for rax plus a 32 *1 - The usage of ES for string destination is an exception, as simply two segment registers are needed. One can generate the Why is the %rax register used in the assembly for this procedure with 8 args? Hot Network Questions How to dry a hard-to-access space? Can I directly connect an MCU ADC pin to shunt resistor without worrying about damaging MCU from shunt resistor voltage? Should I follow my processor manual or system motherboard specified max RAM? div / idiv: divides edx:eax by the src. Malware writers like to do this, because it lets them figure out where in the (virtual memory space) they are. Edit online. The immediate-src operations share the first opcode byte. 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. Remember that debugging assembly code might be more challenging than debugging high-level languages, as you'll be working with raw The syscall instruction requires a specific register, RAX, to contain the syscall number, which identifies the requested service. And I guess choosing RAX only by chance because it's the first register: changing the "rax" string probably wouldn't change the code. Into the fray, intercepting registers. B. quotient in eax, remainder in edx. c:14: Error: bad register name `%%rbx' TinyHelloWorld. A lot of compilers offer frame pointer omission as There are some restrictions, and some differences in encoding. Can’t use the MUL/IMUL instructions, meaning you use ADD repeatedly. As registers had different usages, mnemonics were chosen reflecting their usage. Here, the source operand (in a general-purpose register or memory location) is multiplied by the value in the AL, AX, EAX, or RAX register (depending on the operand Here's the full list of ordinary integer x86 registers. Here's a complete example of assembly memory access. R0 is RAX. Displacement: An integral offset. Description ¶ . For example, on my machine, if you set all bits of %rax to 1, and then perform xor %eax, %eax, you will find that As I understand, floating points are stored in XMM registers and not the general purpose registers such as eax, so I did an experiment:. I'm a bit confused by how addressing works in 64-bit mode. The exact formula for rax a player earns isn’t stated anywhere, but typically the better a player does the more rax they get. The relationship between different register bit widths is a little weird. No, the displacement (0x47) is not scaled by the operand-size. e. I have a general-purpose (e. . To use this variant all you have to do is to use a 32 bit source operand. Registers live in a separate world from the memory whose contents are partially prescribed by the C abstract machine. It's the address of a byte, but also For more details of register usage, see Register Usage, below. nasc jyhoc kighfj mvwrj bdddz twnxpm qmd ohiosg dhga yaayg