Eax register example. It’s a register.

Eax register example I will omit this weird form of addressing mode. Moving the System Call Number to Register EAX: At the memory address corresponding to the NtCreateFile function, the hexadecimal value 55h is loaded into the EAX register. When learning x86 assembly, you’re usually told something along the lines: Here’s EAX. Registers have different names depending on the size of data they're handling: unsigned char *p = (unsigned char *) &i; for (j=0; j<4; j++) . 11. As long as on this address there is no any memory allocated by the OS, the program crashes with GPF. For example, on my machine, if you set all bits of %rax to 1, and then perform xor %eax, %eax, you will find that ALL of %rax is set to 0, not just the lower 32 x64 extends x86's 8 general-purpose registers to be 64-bit, and adds 8 new 64-bit registers. These 32-bit registers can be used in three ways −. 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. 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. and more. This ensures that the debugger recognizes ax as a register rather than a symbol. Store the arguments to the system call in the registers EBX, ECX, etc. It is important to remember that when using ADDR with local variables, the EAX register is modified rather than leaving it available for other usages within There is no eax or %eax register in 8086. -- register direct mode -- The effective address is in a register. Look at the compiler-generated asm output (gcc -S -fverbose-asm) to see what asm it generated around your asm, and which register it substituted into your asm template. The letter fiEfl in each name indicates that the fiextendedfl version of the register is desired (extended from 16 bits). I do have following questions; Are these (EAX/AX/AH/AL) real physical registers or a mapping/notation to access a part of RAX? What is the use of these mapping registers? A few arithmetic instructions take only one register as an argument, for example: notl %eax means eax = ~eax;. XORring a register with itself does it. Thus, the AX register corresponds to the lower 16 bits of the new 32-bit EAX register, SI corresponds to the lower 16 bits of ESI, and so on. mov eax, 1 (5 bytes total, with 3 zero bytes in the imm32 so it's also a problem for shellcode). Arguments are assigned to registers from left to right. May 23, 2018 · The only point where the register is guaranteed to be used is as an input,,output, input/output operand of an extended inline assembly template. And Dec 13, 2024 · For example, storing to the ax register leaves the high 16 bits of the eax register unchanged. Software should ignore this value and not interpret it as an informational descriptor. In pseudo C EAX <<= 2 SHL EAX, 0x2 ; Add the DWORD value at address (value in EBP register)-0x8 ; to the value in EAX and store the result in EAX ADD EAX Jan 11, 2015 · The idea is to use a generic register name so the compiler or assembler picks AX on 8086 (-m16?), EAX on i686 (-m32), RAX on x86_64 (-m64), and so on. The value in EAX is then incremented by 5. For example, the least significant 2 bytes of EAX can be treated as a 16-bit register called AX. The reason you typically don't see this in examples or compiled code is that moving from one register to another is typically avoided, just use the value in EAX. AX) • Each 16-bit register can be addressed as two 8-bit registers (e. The character fi%fl is used to denote a register in assembly code and is not considered a part of the register name itself; note also that register names are not case sensitive. and when we multiply it stores in eax. May 21, 2009 · The history isn't continuous - the 8080 has different register names, and those names came in with the 8086. Four 32-bit data registers are used for arithmetic, logical, and other operations. text global _start ; Entry point for the program _start: mov rax, 42 ; Moves the value 42 into the rax register push rax ; Pushes the value of rax onto the stack pop rbx ; Pops the top value from the stack into rbx ; Exit the program mov eax, 60 ; syscall number for exit xor edi, edi ; Exit code 0 syscall ; Invoke syscall to exit the • CPU uses contents of EAX register as source operand! 4 Recall from Last Lecture (cont. Per the docs The only supported use for this feature is to specify registers for input and output operands when calling Extended asm (see Extended Asm). mov eax, [variable] reads the value currently stored in the location addressed by variable into the eax register. xor ax,ax, but AFAIK in all Intel processors xor and sub are equally fast, so sub ax,ax is as fast as xor ax,ax. The result is usually returned in the EAX register. the instruction like mov eax,2 will affect whole rax). Each of those registers has three smaller registers that can be used to access the lower bits of the registers. EBX: also called Base, used as a Base-pointer for memory access. x86 register names on it are also consistent across 16, 32 and 64-bit x86 architectures with operand size indicated by mnemonic suffix. See an expert-written answer! Jun 3, 2014 · ; Load the 4 byte (DWORD) value from address EBP-0x4 relative to the stack segment ; into the EAX register MOV EAX, DWORD PTR SS:[EBP-0x4] ; Shift the value in the EAX register left 2 bits. These are the EBX, ECX, EDX, ESI, EDI, and EBP. Also note that the commented values are for eax having the value of 3 as mov eax, 3 was used in the question. For example the EAX register has AX, AH, and AL. Jan 8, 2017 · The upper half of EAX occupies bits numbered from 16 to 31. , AH and AL) EAX: Accumulator for operands, results For example it forces a register to be 0 on RISC architectures without a hardware zero register. It was added in 1985 during the transition to 32-bit processors. Isn't that the definition of little endian? – Dec 11, 2013 · This example which uses %% prefix for EAX register compiles fine on my x86 machine (Linux RedHat 5. Nov 3, 2014 · Though the "where" of the original question (at least as worded) appears to be based on the false premise that %eax is on the stack, and being a register, it isn't part of the stack on x86 (though you can emulate any hardware register set on a stack, and some architectures actually do this, but that isn't relevant), incidentally, registers are Aug 17, 2011 · bits 32 extern printf global main section . All other bases (base-2, 8, and 16) are working properly. xchg eax, ebx (1 byte, 3 uops on modern Intel CPUs but none of them are memory access. 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. Help me please !! Dec 2, 2011 · An example with a byte: 00101100 00111010 & 00101000 So you can use this operation to mask and flip bit regions in a register. Dec 7, 2017 · Since all system calls enter the kernel in the same way, the kernel needs some method of identifying the system call. Feb 11, 2019 · The CDQE instruction sign-extends a DWORD (32-bit value) in the EAX register to a QWORD (64-bit value) in the RAX register. e. For example add eax, [ebp] reads the contents of memory at the address pointed to by ebp and adds the value to register eax. In this example, the data stored at the memory address of myVar is loaded into the EAX register. The second 8 bits of the EAX register (that is bits 8 to 15) can be accessed as the AH register. See Tips for golfing in x86/x64 machine code. EAX: Known as the accumulator register, it's often used in arithmetic operations. Jul 31, 2021 · mov eax, dword [lst+8] mov eax, dword [rbx+8] mov eax, dword [rbx+rsi] In each case, the starting address plus 8 was accessed and the value of 105 placed in the eax register. 1. EBX: The base register, frequently utilized for base pointers in memory accesses. Registers can be used in your software directly with instructions such as mov, add or cmp. However, I'm having trouble calling out the "generic" a register in code: Jan 31, 2017 · In C int fun(); can take any number of arguments, so it may even be a varargs function. This will put the upper 16-bits of EAX into DX and then restore EAX back to it's original value. The bit shifting and rotation instructions take a 32-bit register for the value to be shifted, and the fixed 8-bit register cl for the shift count. Is it "Can I move a register from EAX to EDX by using MOV EAX, EDX?", then yes, of course you can. Pass in a constant as the second argument which has bits flipped up you want to keep up. On the 80386, rather than combining 16-bit registers into 32-bit registers, Intel added 16 bits to each register. Within the instruction is the operand. Example instruction: mov eax, [esp] The second operand uses register direct mode. Loading a value into a 16- or 8-bit register name leaves all other bits unchanged. . Register Mode In this mode, the operand is in a register. The specific method used is up to the programmer. This egghunter encoding technique is used in situations where the attacker's raw egghunter is corrupted in memory. The x86-64 sysv abi convention demands that the register AL must contain the number of SSE registers used when invoking a varargs function. mov always requires both operands to be the same size, but for a case like mov [esp], eax the register source operand EAX implies the size for the memory destination Aug 13, 2020 · It's ambiguous and relies on some default, you shouldn't write code like that. You can work on the low 32-bits by accessing the EAX register. Feb 5, 2013 · Each asm construct is independent and values/registers set in one have no connection to the other. Apr 19, 2016 · A common function calling convention for x86 is cdecl, where a function's integral return value is saved in the EAX register. The inc mnemonic increments by a step of 1, regardless of register passed as operand, and regardless of the size of that register. When using the ? (Evaluate Expression) command, registers should be prefixed with an "at" sign ( @). Up to 4 registers are assigned to arguments in the order EAX, EDX, EBX, ECX. They are assigned "fake" memory addresses for convenience. Examples: cdq: cdqe: Convert double-word in eax into quadword in rax. EBX, ECX, and EDX work in the same way. el5 x86_64, kernel 2. x86 mnemonic: AND a, b operation: a = a & b Here's how to do it unless you didn't yet understood it: AND eax, 0xfffffffc Sep 20, 2019 · x86 has 8 registers, 32-bits wide: eax, ecx, edx, ebx, rsp, ebp, esi, edi. Finally, the updated value is stored back in memory at the location of myVar. The lower 32 bits, 16 bits, and 8 bits of each register are directly addressable in operands. Each one can store 32 bits(or 4 bytes) of data. Nov 18, 2009 · The 64-bit registers have names beginning with "r", so for example the 64-bit extension of eax is called rax. The 64-bit registers have names beginning with "r". Register Direct Mode In this mode, the operand is in effective-address-use-register direct mode. Apr 8, 2013 · Q: And suppose if I then poped the stack onto the EAX register does what was in the CL register from before go into the EAX register?. . Mar 4, 2016 · In x86, after you set ecx to zero and then do inc ecx, the register ecx will hold the value 1. Examples and $0x0f, %eax — clear all but the last 4 bits of EAX. Loading a value into a 32-bit register name sets the upper 32 bits of the register to zero. In hand-written assembly, you're more likely to see other registers used more often (e. Mar 21, 2013 · rol eax,16 ; rol / ror mov ax,0b1000000000000001 rol eax,16 ; rol / ror For hardcoded values (as in the question), you can also use and with or / xor / add . Personally, I would have loved to have seen register aliases for the upper halves as well. Data needs to be moved to a register first before the arithmetic is performed. The AX register is a 16-bit register that is situated in the lower half of the bigger EAX register. So I have question about eax ! Is register %eax is part of %rax? What is the meaning of the second line in assembly code? (movl $0, %eax) Can I change %eax to %rax at the second line of assembly code? I try to find answer but most of the explanation was hard to understand. Same for AX and EAX. A memory location can be an assembly language label, a constant, a register, or the sum of a register and a constant or two registers. Study with Quizlet and memorize flashcards containing terms like Which of the following are true? - The running time stack holds the return address of called procedures - In 32-bit mode, the register ESI register manages the stack. com ; Compile with: nasm -f elf helloworld. Write an assembly code snippet to add the first n odd numbers (1, 3, 5, 7, 9, 11, ), and store the sum in the EAX register. In your case with imul edx, you get EDX:EAX = EAX * EDX. For example, the following pseudocode loads EAX with 00H and causes CPUID to return a Maximum Return Value and the Vendor Identification String in the appropriate registers: MOV EAX, 00H CPUID. i r: print all register except floating point & vector register (xmm, ymm, zmm). For example, the instruction: mov EAX, [EBP] Moves the contents of the memory location pointed at by EBP to the EAX register. rodata The output is formatted in blocks to represent the manipulation of the eax register reproducing four bytes of the chosen egghunter at a time. For example, the following pseudocode loads EAX with 00H and causes CPUID to return a Maximum Return Value and the Vendor Identification String in the appropriate registers: MOV EAX, 00H. Let's start with a simple example of adding a 32-bit immediate value to the eax register. 6. In your example, ah is 0x36 and al is 0x65. Thus, after movl $-1, %eax, the %rax register has value 0x00000000FFFFFFFF. Mar 30, 2010 · Likewise BX was BH and BL, etc. For example: shll %cl, %ebx means ebx = ebx << cl;. Example: If the value in the ECX register is 4(decimal) then the code should store 16 (decimal) in the EAX register. They are internally numbered 0 to 7 respectively. For example, you should use ? @ax rather than ? ax. For example, a simple assembly function can be: Examples and $0x0f, %eax — clear all but the last 4 bits of EAX. The same is true after movq $-1, %rax; addl $0, %eax! Feb 17, 2020 · EAX is always GCC's first choice for evaluating expressions. Syntax not <reg> not <mem> Example not %eax — flip all the bits of EAX neg — Negate May 4, 2017 · mov eax, DWORD PTR [eax] ; eax = *eax mov DWORD PTR [a], eax ; a = eax Of course, this clobbers the pointer and replaces it with the dereferenced value. Vladimir Keleshev • 2020-03-20. Note, only works for rax register. It’s a register. The a modifier enforces val to be placed in the eax register before the asm command is issued and Nd allows for one-byte constant values to be assembled as constants, freeing the edx register for other cases. incl %ecx means ecx = ecx + 1;. x6 4 R e g i st e rs x6 4 a sse mb l y co d e u se s si xt e e n 6 4 -b i t re g i st e rs. Aug 28, 2016 · Here’s an example how we can break up the EAX register: For better memorizing the registers are usually described as follows: EAX: also called Accumulator, often used for IO-stuff, math operations, interrupts. In C++ however it means it takes no arguments. data format: db "result = %8. eax is the 32-bit, "int" size register. In this post we are going to learn how to extract those information using inline assembly in c. asm ; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 helloworld. In particular, the notion of the processor registers does not match reality, there is no such thing as a EAX or RAX register. For the EAX, EBX, ECX, and EDX registers, subsections may be used. Anyway, ANDing a register with itself doesn't zero it. Pushing a pointer into the eax and ebx registers in GCC. It's just a bit field giving a selector called "mod" (which indicates whether r/m is treated as a plain register or a memory address), one register called "reg/opcode" (which is usually the destination register, and determines the column in the ModR/M table), and a final register called "r/m" (usually the source register, which selects the row Example instruction: mov eax, 26 The second operand uses immediate mode. Traditional names are outb, outw and outl respectively. However, for local variables ADDR translates to: lea eax, LocalVar ; load effective address of LocalVar into eax push eax Effective address is the physical address of the data in memory. i r f: print all FPU floating registers (st0-7 and a few other f*) Other register groups besides a (all) and f (float) can be found with: maint print reggroups Feb 1, 2024 · I/O access OUTx. Am I correct? The least-significant byte in register EAX (register AL) will always return 01H. Examples: cwde: cdq: Convert double-word in eax into quadword in edx:eax. ) Apr 8, 2022 · At the second line of assembly code register eax is used. For example, MOV EAX, [EBX] moves the value at the address stored in the EBX register into the EAX register. A few instructions such as movs and movz have two suffixes: the first is for the source operand, the second for the destination. This is different from add eax, ebp, which adds the contents of ebp to eax. Also, swapping registers that way is insane vs. 18, gcc 4. not — Bitwise logical not Logically negates the operand contents (that is, flips all bit values in the operand). The MOVZX instruction zero-extends the source to the destination. i++; i--; Global int variable i is in %eax, the full 32 bits of the “A” register. Example: The first 16 bits of the EAX register (that is bits 0 to 15) can be accessed as the AX register. That's why AT&T syntax has movzb and movzw instructions (typically used as movzbl -1(%rbp), %eax), for the two different source sizes of the Intel-syntax movzx mnemonic. A: Yes. To permit this, the wrapper function copies the system call number into a specific CPU register (%eax). 6. 18-238. So, what exactly do those letters stand Registers are essential for performing operations in assembly. If any argument cannot be assigned to a register (say it is too large) it, and all subsequent arguments, are assigned to the stack. 5. Yes, when you need to return a value, most compilers will expect it to be in EAX. You probably have mov %eax, %eax; mov %eax, %eax. For example, the 64-bit extension of eax is called rax. • Each lower-half can be addressed as a 16-bit register (e. That said, memory-mapped registers do exist, but in any modern processor they belong to other devices, never the CPU. But the following single line code But the following single line code XOR BYTE PTR [eax], al Tries to compute an xor operation of the value in the register AL (byte sized) and a value of the byte in memory at address 0AABBCCDDh (the content of EAX register). The return value from a function call is saved in the EAX register. The displacement is added and the memory location accessed while none of the source operand registers (rbx, rsi) are altered. Oct 9, 2021 · As I wrote earlier, the EAX register has AX, AH, and AL. MUL Op Unsigned integer multiplication of the operand by the AL, AX, or EAX register and stores in the register. Those different aliases just indicate the bit-size the instruction will target/affect (not 100% true/precise in 64b mode, where 32b eax target will automatically clear upper 32b, i. , ESI or EDI for strings). Oct 26, 2018 · BTW, "eax, ax, al registers" = it's the same one register, not three registers. xor is the most common way, eg. For example, you could have a 64-bit value that contains two 32-bit values. , AX) • Each 16-bit register can be addressed as two 8-bit registers (e. , A Synchronous event is one that is not aligned with the clock cycle, while an Asynchronous event is timed with the clock cycle. For example, mov eax, [ebx + esi] will load register EAX with the contents of the memory location whose address is given by the sum of the contents of registers EBX and ESI. For example the instruction: mov EAX, [EBP-4] destination. text global _start ;must be declared for using gcc _start: ;tell linker entry point mov eax,'3' sub eax, '0' mov ebx, '4' sub ebx, '0' add eax, ebx add eax, '0' mov [sum], eax mov ecx,msg mov edx, len mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call number (sys_write) int 0x80 ;call kernel mov ecx,sum mov edx, 1 mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call Note that it is OK to re-use EAX here (using EDI is also fine, but there's no need to trash another register), so I'd write your example like this to make it clear where memory accesses are happening: mov eax, [pointer] ; Get pointer value mov eax, [eax] ; Get actual value (dereference) mov [actualValue], eax ; Store actual value o The primary accumulator register is called EAX. printf("Byte %d: %x\n", j, p[j]); Global char variable i is in %al, the lower byte of the “A” register. So with the example given above, base-5 and a number of 1234 the EAX register reads movzx esi, al ; esi = eax & 0xff shr eax, 8 ; eax >>= 8; movzx ecx, al shr eax, 8 movzx ebx, al shr eax, 8 You will have first byte in eax, 2nd in ebx, 3rd in ecx and last byte (the one that was the lowest part of eax at the origin) in esi. Therefore, modifications to a 32-bit register are reflected in the corresponding 64-bit register, and vice versa. text mov eax, 123 mov [variable], eax sets the eax register to 123, and then stores the value of eax in the location addressed by the symbol variable. For example, if the destination register is %eax, it must be 4 bytes, if %ax it must be 2 bytes, and %al would be 1 byte. Then, what conclusion can I draw upon the utility of EAX register in system calls? I would like to know if it is possible to read the eax register of another process immediately after an assembly instruction has been executed. For example: MOV EAX, 1234h put the number 1234h in the register EAX and not the opposite. For example, movzbl moves a 1-byte source value to a 4-byte destination. It doesn't seem to be going into the eax register, but if I call WriteString it will appear, so I believe it is in EDX. Jul 14, 2016 · leal -0x17b7(%ebx), %eax ; EAX = EBX - 0x17b7 movl %eax, 0(%esp) ; Put EAX on stack (arg 0 to printf) ; EAX should point to some string calll printf In your code also, it didn't actually "use" the GOT (otherwise we would see a second memory de-reference); it used it as an anchor to the string, probably in the read-only data section ( . - Stack is called a LIFO data structure - The PUSH instruction cannot have an immediate operand - Local variables in procedures are created on the stack, Which of Nov 12, 2023 · You identify them by their names: EAX, for example. If you don't want to lose the pointer, then you will have to use a second "scratch" register; something like: Jan 5, 2014 · I'm not quite sure what your question is. (On processors that support the Intel 64 architecture, the high-order 32 bits of each of RAX and RDX are ignored. Sends a 8/16/32-bit value on a I/O location. Nov 27, 2014 · How do I move it into a register so I can do shifting operations on it? For example, mov eax, edx shr eax, 1 The problem that I'm having is that I can't figure out how to access String1. Mar 11, 2018 · Well, yes, the whole "variable" terminology is a bit misleading in assembly, as you have memory, addressable by bytes, and that's it, so you have either some value (in amount of bits you desire, but 8/16/32/64 are natively supported by modern x86 CPU), or you have memory address (pointing to some particular byte in the memory chip). Study with Quizlet and memorize flashcards containing terms like CISC (Complex Instruction Set Computers) as compared to RISC (Reduced Instruction Set Computers) includes:, In the EFLAGS register, if the OVERFLOW bit is a '1', then _____. The high 32 bits of the answer will be written to the EDX register and the low 32 bits to the EAX register; this is represented with the EDX:EAX notation. 6(Tikanga 2. Call the relevant interrupt (80h). Lower halves of the 32-bit registers can be used as four 16-bit data registers: AX, BX, CX and DX. The result is stored back in EAX. These four are the Kardashians of the register world—everyone knows them, and they are always in the spotlight. 8X",10,0 When operands are specified using this constraint, they get stored in General Purpose Registers(GPR). push eax and eax, 1 xchg ebx, eax ; swap contents, could also use `mov` here pop eax Note that both solutions do not change the value in eax, so you can still use the value in eax freely. Take the following example: asm ("movl %%eax, %0\n" :"=r"(myval)); Here the variable myval is kept in a register, the value in register eax is copied onto that register, and the value of myval is updated into the memory from this register. Assume the ECX register contains a number n. Thus, after movl $-1, %eax, the %rax register has value 0x0000'0000'FFFF'F Memory: 4 consecutive bytes of memory at address given by register Simplest example: (%eax) Oct 9, 2019 · For example, for me they both produce the same in gdb: Get the upper half of the EAX register. Syntax not <reg> not <mem> Example not %eax — flip all the bits of EAX neg — Negate eax, ebx, ecx and so on are actually registers, which can be seen as "hardware" variables, somewhat similar to higher level-language's variables. A 64 bit general purpose register, example:RAX can be accessed via EAX(lower 32 bit), AX(lower 16 bit), AH, AL(b bits). If clobber is used to indicate which registers should not be selected as input registers, if I explicitly instruct, is there no need to write clobber? CS CI 0 3 3 0 I n t ro Co mp u t e r S yst e ms Do e p p n e r x64 Cheat Sheet Fall 2019 1 . g. Oct 23, 2008 · The only way to get the upper half of EAX would be to do some shifting/rotating. Thus, after movl $-1, %eax, the %rax register has value 0x0000'0000'FFFF'FFFF. If you're curious about it read Jan 1, 2024 · For example, 'EAX' is the accumulator register as a 32-bit value. May 11, 2021 · So, if %rax is initially 0, and you execute the instruction movl $5, %eax, then %rax will also take on the value 5. In this example EAX being a 32 bit register, if we call AX it should return the first 16 bits, and if we call AH or AL it should return the next 8 bits after the 16 bits and AL should return the last 8 bits. For example, if EAX contained 1344 before the instruction, value from EBX is copied to logical memory addresses 1344-1347 (because this is 4 byte a - load into the eax register; b - load into the ebx register; c - load into the ecx register; d - load into the edx register; f - load into the floating point register; D - load into the edi register; S - load into the esi register; The outputs and inputs are referenced by numbers beginning with %0 inside asm statements. Sep 28, 2019 · MASM associates a size with data labels, so to keep it happy we have to apply the dword ptr size override to the memory operand to make it match the dword size of the register EAX. I thought I could do it by using a register for ax, eax or rax; b for bx, ebx or rbx; and so on. Jul 19, 2019 · cpuid is a processor specific instruction used to get processor's information and features. Usually, x86 tutorials don’t spend much time explaining the historical perspective of design and naming decisions. To give an example of the specific problem, I'm call DumpRegs to output the registers each pass through. The first 8 bits of the EAX register (that is bits 0 to 7) can be accessed as the AL register. )" • Example: allocate memory for two integers! • subl $4, %esp! The processor model as documented in the Intel/AMD processor manual is a pretty imperfect model for the real execution engine of a modern core. 7. data msg db 'Hello World!', 0Ah ; assign msg variable with your message string SECTION . Also it is nasm syntax I am not familiar with masm so you may need some tweaks. Source: A tutorial on Register Allocation by Puzzle Solving (on compilers. 2). – Examples: • addl $3, %eax //add constant 3 to register eax • movl %eax, %ebx //move contents of register eax to register ebx • movl 8(%ebp), %eax //move Nov 15, 2018 · The other 4 are EAX, EBX, ECX, and EDX. /helloworld SECTION . Jan 19, 2014 · However, to allow that in HW, you need to be able to keep the results in some temporary register - for that they came up with register renaming - instead of calling the result of each operation rax, each instruction will assign a new physical register with some unique ID, and update some global table (often called RAT - register alias table May 16, 2017 · If you want to work with only an 8-bit quantity, then you'd work with the AL register. Nov 17, 2015 · For example. eax and other 32-bit appeared recently in 386, not before. – Jul 5, 2020 · The cdq instruction causes the edx register to be filled with a sign bit from eax register (sign bit is the most significant bit of the register value, so in this case it's the 31st bit). Similarly, in the 64-bit version, the 'E' is replaced with an 'R' ( register ), so the 64-bit version of 'EAX' is called 'RAX'. The same is true after movq $-1, %rax; addl $0, %eax! 000319df 8b4508 movl 0x08(%ebp), %eax 000319e2 8b4004 movl 0x04(%eax), %eax 000319e5 85c0 testl %eax, %eax 000319e7 7407 je 0x000319f0 I am trying to understand that point of testl between %eax and %eax? I think the specifics of what this code isn't important, I am just trying to understand the test with itself - wouldn't the value always be true? Nov 16, 2011 · section . Use it. o EBX is often used to hold the starting address of an array. So if we know that in eax we have e. The general-purpose registers, base registers, and index registers can all be used as the base in addressing modes, and all of those registers except for the stack pointer can be used as the index in For example, the addressing modes of Pentium Processor looks like [EBX] [EDI x 2] +FF. Copy ONLY your code snippet and paste it in the box below. Worth noting the CISC architecture here, with special purpose registers, in both the 8086 and 8080 variants, though - RISC came along later, with its 'all registers are much the same' approach (helped along by having 32 bits per instruction instead of trying to encode most instructions helloworld. Modifying a 32-bit register name sets the upper 32 bits of the register to zero. ECX: The count register, typically used for loop counters. This includes registers, like esi, whose lower 8 bits were not previously addressable. Common Instructions Now, let's dive into some of the most commonly used instructions in the x86 instruction set. -- register direct mode-- The effective address is in a register. eax now holds the value of your x variable. i r a: print all register, include floating point & vector register (xmm, ymm, zmm). xor %edx, %edx — set the contents of EDX to zero. lea eax, DWORD PTR [eax+eax*4] I know when add integers in assembly, it stores result in the destination. In order to make this work, you need a single asm. Opcode indicates size of register. But why do you claim "nternally, a value in the EAX register is stored in big-endian format"? In your example, the AL register contains 0xbc, which means the least significant bits are stored in the lower 8 bits of EAX. That means ax can be a 16, 32 or 64-bit register depending on the instruction suffix. For non-hardcoded values you would need to shift the value in some other register or in memory to use this method: Sep 29, 2020 · They're move instructions, moving data from one place to another - in these cases, from memory into a register: register_eax = *(unsigned long *)register_eax; Your other example is something like: register_eax = *(unsigned long *)(register_ebp + 8); Aug 21, 2019 · I want to know the value of eax register after this line of code in assembly . 1, then execution of the cdq instruction will cause edx to be reset to zero. mov eax, 100 mov ebx, 200 mov ecx, 300 add eax, ebx add eax, ecx I think you should keep adding add the numbers to the eax register right after the Jan 3, 2011 · Like push 1 (2 bytes) / pop eax (1 byte) for 3 bytes total, vs. There are six registers that store the arguments of the system call used. asm; Hello World Program - asmtutor. The content of EBX register is copied to EAX register. Its bits are numbered from 0 to 15. The new registers are named r8 through r15. More precisely, what was in CL would go into AL. You might also need mov al, [var1] to get the value at that address, but I am not sure about that. The contents of register esp is the effective address. o EDX is a general purpose register. The same is true after movq $-1, %rax; addl $0, %eax! The relationship between different register bit widths is a little weird. ECX: also called Counter, often used for loop-counting Feb 15, 2014 · When making a system call sys_read (int80h with the value 3 in eax) in the assembler, what will be stored in the register eax after the call is made? Is it the number of characters in the string that was read? Mar 25, 2024 · 1. edu) Apr 23, 2014 · In this syntax, brackets around a register means a memory location is used (as source or destination, according to the instruction) with starting address specified at the register (EAX in your case). Modifying a 32-bit register sets the upper 32 bits of the full 64-bit register to zero. Note, only works for ax to eax register. The question I have involves the eax register when processing the base-10 for output. As complete 32-bit data registers: EAX, EBX, ECX, EDX. The instruction add eax, [ebp+8] performs a similar operation but with the contents of memory at address ebp+8. 0x08048400 <+19>: mov DWORD PTR [ebp-0x4],eax The relationship between register variants with different sizes is a little weird. However, they are not entirely independent, so one should be careful. Put the system call number in the EAX register. text global _start ;must be declared for using gcc _start: ;tell linker entry point ;create the file mov eax, 8 mov ebx, file_name mov ecx, 0777 ;read, write and execute by all int 0x80 ;call kernel mov [fd_out], eax ; write into the file mov edx,len ;number of bytes mov ecx, msg ;message to write mov ebx, [fd_out] ;file descriptor mov The relationship between different register bit widths is a little weird. The reason is that the eax register is used in mov $3, %eax in the src2 function, and eax is also selected as the input register. so if I call the function multiply( 3 , 8 ), the value of eax register after that line should be 120. For example, MOV EAX, EBX. It is copied to register eax. AX is used to access the lower 16 bits of EAX. The upper half of the EAX register has no separate name to go by! Swapping the upper and lower 16 bits of EAX requires rotating the contents 16 times for the address stored in register EAX. 0x080483fd <+16>: add eax,0x1 The value of eax is increased so it now holds the value x + 1. Arithmetic opcodes can not operate with two memory operands. Here is a NASM example: PUSH EAX EBX ECX This becomes important when a procedure needs to save several registers and at the same time return a value to its caller in the EAX register. Oct 10, 2010 · No, EAX (for example) isn't reserved exclusively for that purpose. i++; i--; No carry: CF=0 (unsigned diff is correct) ?? Negative: SF=1 (first bit is 1) ?? • Each lower-half can be addressed as a 16-bit register (e. 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. Secondary accumulator registers are: EBX, ECX, EDX. Now, let's take a look at some examples: A simple encoding: ADD eax, 0x4351ff23. When Jun 5, 2017 · To get the result of (1234h - 123h) into the register EAX you should code: MOV EAX, val_A MOV EBX, val_B SUB EAX, EBX Just to be clear again, SUB instruction does work like MOV and that why the result moves into the first register. The contents of memory at the effective address are copied into register eax. Here's a simple example of using registers to add two numbers: In this example, we use EAX and EBX to store values and perform addition. The leading e stands for extended and means that your register is The relationship between different register bit widths is a little weird. CPUID. Jun 29, 2022 · The above example outputs are 2 and 4 respectively. For example: ROR EAX, 16; MOV DX AX; ROR EAX, 16. For yet more flexibility, we will provide a based indexed with displacement addressing mode that combines the features of the indexed mode and the based indexed mode. g AH and HL) EAX: Accumulator for operands, results example: movl 4(%eax), %ebx takes value inside register %eax, adds 4 to it, and then fetches the contents of memory at that address, putting The instruction's output is dependent on the contents of the EAX register upon execution. In my case I have the following assembly code: mov @JimMischel I do realize this post is 3 years old by now. Destination can be register or memory. Table 3-8 shows information returned, depending on the initial value loaded into the EAX register. text global _start ;must be declared for using gcc _start: ;tell linker entry point ;create the file mov eax, 8 mov ebx, file_name mov ecx, 0777 ;read, write and execute by all int 0x80 ;call kernel mov [fd_out], eax ; write into the file mov edx,len ;number of bytes mov ecx, msg ;message to write mov ebx, [fd_out] ;file descriptor mov Nov 30, 2015 · Here is an example: EAX If you see the EAX register just after a function call, chances are that EAX contains the return value of the function. 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. SUB Dest, Source Subtracts the source from the destination and stores the result in the destination. Question: Assume the ECX register contains a number n. o -o helloworld ; Run with: . The relationship between different register bit widths is a little weird. • mov [esi+eax], cl ; Move the contents of CL into the byte at address ESI+EAX • mov edx, [esi+4*ebx] ; Move the 4 bytes of data at address ESI+4*EBX into EDX Some examples of incorrect address calculations include: • mov eax, [ebx-ecx] ; Can only add register values • mov [eax+esi+edi], ebx ; At most 2 registers in address computation 3. cs. AL is used to access the lower 8 bits of EAX and AH is used to access the higher 8 bits. Mar 20, 2020 · EAX x86 Register Meaning and History. section . Mov and lea Jul 7, 2017 · Note that the 32-bit and the 64-bit registers are not separate registers since they overlap: the 64-bit rax, for example, has eax as its bottom 32-bits, and so on for rbx and ebx, r8 and r8d and so on. Note, only works for eax to edx:eax registers. So why is this important for writing shellcode? Remember back to why null bytes are a bad thing. text main: push ebx mov ax, 0x1234 mov bx, 0x10 mul bx and eax, 0x0000ffff ; clear upper bits of eax shl edx, 16 ; shift DX into position or eax, edx ; and combine push eax push format call printf add esp, 8 mov eax, 0 pop ebx ret section . Source can be register, memory, or immediate. mov eax, 456 mov [variable], eax Jul 31, 2021 · Examples: cwd: cwde: Convert word in ax into double-word in eax. 3. Examples: cdqe Dec 26, 2015 · The ah and al registers are just aliases for the lower two bytes of eax, and so you can just monitor the eax entry in the register window. For example, MOV EAX 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 copied to low-order 32 bits of the MSR. o ECX is often used as a counter or index register for an array or a loop. ucla. text global _start _start: mov edx, 13 ; number of bytes to write - one for each The Protected Mode does not use segment registers. Example: If the value in the ECX register is 4(decimal) then the code should store 16 (decimal) in the EAX register. It's fine for the explicit source operand to be one of the implicit operands, even EAX to square into EDX:EAX. Oct 25, 2012 · Conditional jump instructions like JE are designed to look at the ZF for jumping/notjumping so using TEST and JE together is equivalent to perform a conditional jump based on the value of a specific register: example: TEST EAX,EAX JE some_address With the advent of x64 and 64-bit architecture, register size doubled once again and the EAX register was renamed RAX. In addition, there's no need to actually move a value into eax -- that is what the "a" input constraint does. The most significant bit (bit 31) of each register indicates whether the register contains valid information (set to 0) or is reserved (set to 1). We can still reference EAX and its sub-registers for added control. The register calling convention may be selected by command line switch. bpcgwy qjnfs dpjxw clxwt kqqpb cnjzo hyfa pfaqw gyoduq csbls