![]() Guide to x. 86 Assembly. University of Virginia Computer Science. CS2. 16: Program and Data Representation, Spring 2. July 2. 01. 6Contents: Registers . For example, there is a 1. Using the 1. 6- bit programming model can be quite. It has a segmented memory model, more restrictions on register. In this guide, we will limit our attention to more. The register names are. For example, EAX used to be called the. ECX was known as the counter since it was used to hold a loop. Whereas most of the registers have lost their special purposes in. ESP) and the base pointer. EBP). For example, the least. EAX can be treated as a 1. AX. The least significant byte of AX can be. The Microsoft x64 calling convention is followed on Windows and pre-boot UEFI (for long mode on x86-64). List of x86 calling conventions. This is a list of x86 calling conventions. In the Microsoft x64 calling convention, it's the caller's responsibility to allocate 32 bytes of 'shadow space' on the stack right before calling the function (regardless of the actual number of parameters used). In this article I will cover how the calling convention has changed for 64-bit Windows. Microsoft x64 Calling Convention http:// Microsoft Macro Assembler Directives http. Talk:X86 calling conventions WikiProject Computing / Software /. Microsoft abide by the Intel ABI. Should the x64 calling convention be labeled as a variant of ' AMD has an ABI specification that describes the calling convention to use on x86-64. All OSes follow it, except for Windows which has it's own x86-64 calling convention. Does anyone know the. Get the function prototypes from an unknown.dll. And indeed since no explicit calling convention is given. AL, while the most. AX can be used as a single 8- bit register. AH. These names refer to the same physical. ![]() When a two- byte quantity is placed into DX, the. DH, DL, and. EDX. These sub- registers are mainly hold- overs from older. However, they are sometimes. ASCII characters). For example, the names. EAX and eax refer to the same register. Figure 1. Data. declarations should be preceded by the . The Microsoft x64 calling convention is. List of x86 calling conventions. This is a list of x86 calling conventions. These are conventions primarily intended for C/C++ compilers (especially the 64-bit part below). DATA. directive. Following this directive, the directives DB, DW, and DD can be used to declare one, two, and four byte. For example, locations. An array can be declared. Two other. common methods used for declaring arrays of data are the DUP directive and the use of string literals. The. DUP directive tells the assembler to duplicate an. For example, 4 DUP(2) is equivalent to 2, 2, 2. In addition to supporting. One of the. registers can be optionally pre- multiplied by 2, 4, or 8. Here we illustrate some examples. This instruction has two operands: the. For example, in all of the above instructions, the size. When we were storing the value of a one byte register to memory. Consider the instruction mov . Should this instruction move the value 2 into the. EBX? The size directives BYTE PTR, WORD. PTR, and DWORD PTR serve this purpose. In this section, we will. This. section should not be considered an exhaustive list of x. While register- to- register moves are possible, direct memory- to- memory. In cases where memory transfers are desired, the source memory. Syntaxmov < reg> ,< reg> mov < reg> ,< mem> mov < mem> ,< reg> mov < reg> ,< const> mov < mem> ,< const> Examplesmov eax, ebx — copy the value in ebx into eaxmov byte ptr . Specifically, push first decrements ESP by 4, then places its. ESP. (the stack pointer) is decremented by push since the x. It first moves the 4 bytes. Note, the contents of the memory location are not. Note, whereas both operands may be registers, at most one. Again, the result operand. Furthermore, the third operand is restricted to. Store. the result in EAX. The quotient result of the. EAX, while the remainder is placed in EDX. Place the quotient in EAX and the. EDX. idiv DWORD PTR . Place the quotient in EAX and the remainder in EDX. Bitwise logical. and, or and exclusive or. These instructions perform the specified logical operation (logical. The shifted operand can be shifted up to 3. The. number of bits to shift is specified by the second operand, which can be. CL. In either case, shifts counts of. Elsewhere in the code, we can refer to the. The contents of the machine status. For example, one bit of this word indicates if the last. Another indicates if the last result was. Based on these condition codes, a number of conditional jumps. For example, the jz. Otherwise, control proceeds. For example, conditional branches. Unlike the simple jump instructions, the call instruction saves the location to return to. This instruction first pops a code location off the. It then performs an. The. calling convention is a protocol about how to call and return from. Furthermore, given a. Following this convention. C (and C++) code, and will also enable you to call C. It is based on the push, pop, call, and ret. Registers. are saved on the stack, and local variables used by subroutines are. The vast majority of high- level. The first set. of rules is employed by the caller of the subroutine, and the second set. It. should be emphasized that mistakes in the observance of these rules. The first. parameter resides at an offset of 8 bytes from the base pointer. Above. the parameters on the stack (and below the base pointer), the call instruction placed the return address, thus. The parameters should be pushed in inverted order. This instruction places the return address on top of the. This restores the stack to its. The caller can assume that no other. First parameter is in EAX, the second parameter is the. We have 1. 2 bytes (3. Thus. to get rid of the parameters, we can simply add 1. The base. pointer is used by convention as a point of reference for finding. When a subroutine is. Parameters and local variables. We push the old base pointer value at the beginning of. We then move the stack pointer into EBP to obtain our point of. Recall, the stack grows down, so to make space on the top of the. The amount by which the stack. For example, if 3 local integers (4 bytes each) were required. Next, save the values of the callee- saved registers that. To save registers, push them onto the. The callee- saved registers are EBX, EDI, and ESI (ESP and EBP. When the subroutine is returns, it must follow. Leave the return value in EAX. Restore the old values of any callee- saved registers (EDI and ESI). The register contents are restored by popping them. The obvious way to do this might be to. In. practice, a less error- prone way to deallocate the variables is to. Recall that the first thing we did on. This instruction will find and. The first half of the rules. The latter half of the. Example. Here is an example function definition that follows the callee rules. PUBLIC . This function uses EDI and ESI. Both parameters and local variables are located at constant. In particular, we notice that since parameters were placed. The. first parameter to the subroutine can always be found at memory location. In particular, the first local variable is always located at. This conventional use of the. The caller's register values are recovered from the stack.
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