Assembly what is al register

Unlike in high level languages where arrays can have many dimensions and are accessed by indices, arrays in x86 assembly language are simply a number of cells located contiguously in memory. An array can be declared by just listing the values, as in the first example below. 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 expression a given number of times.

For example, 4 DUP 2 is equivalent to 2, 2, 2, 2. Some examples:. Examples push eax — push eax on the stack push [var] — push the 4 bytes at address var onto the stack. Arithmetic and Logic Instructions add — Integer Addition The add instruction adds together its two operands, storing the result in its first operand.

Note, whereas both operands may be registers, at most one operand may be a memory location. Examples imul eax, [var] — multiply the contents of EAX by the bit contents of the memory location var. Store the result in EAX. The shifted operand can be shifted up to 31 places. The number of bits to shift is specified by the second operand, which can be either an 8-bit constant or the register CL.

In either case, shifts counts of greater then 31 are performed modulo Control Flow Instructions The x86 processor maintains an instruction pointer IP register that is a bit value indicating the location in memory where the current instruction starts.

Normally, it increments to point to the next instruction in memory begins after execution an instruction. The IP register cannot be manipulated directly, but is updated implicitly by provided control flow instructions. Labels can be inserted anywhere in x86 assembly code text by entering a label name followed by a colon. Elsewhere in the code, we can refer to the memory location that this instruction is located at in memory using the more convenient symbolic name begin.

This label is just a convenient way of expressing the location instead of its bit value. The contents of the machine status word include information about the last arithmetic operation performed. For example, one bit of this word indicates if the last result was zero. Another indicates if the last result was negative. Based on these condition codes, a number of conditional jumps can be performed. For example, the jz instruction performs a jump to the specified operand label if the result of the last arithmetic operation was zero.

All memory locations within a segment are relative to the starting address of the segment. A segment begins in an address evenly divisible by 16 or hexadecimal So, the rightmost hex digit in all such memory addresses is 0, which is not generally stored in the segment registers.

The segment registers stores the starting addresses of a segment. To get the exact location of data or instruction within a segment, an offset value or displacement is required.

To reference any memory location in a segment, the processor combines the segment address in the segment register with the offset value of the location. Look at the following simple program to understand the use of registers in assembly programming. Assembly - Registers Advertisements. Previous Page. Next Page. Live Demo. For example,. Try this in NetRun now!

Here's the full list of x86 registers. The 64 bit registers are shown in red. You can extract the value of each bit. For example:. Because "int" currently has 32 bits, if you start at one, and add a variable to itself 32 times, the one overflows and is lost completely.

In assembly, there's a handy instruction "jo" jump if overflow to check for overflow from the previous instruction. Notice the above program returns on overflow, which somebody else will need to check for. Responding correctly to overflow is actually quite difficult--see, e. Any thing to do with endian? One or zero? This can get tricky for vector registers, see stackoverflow. No -- AL is the 8 least significant bits of AX. AX is the 16 least significant bits of EAX. Jerry Coffin Jerry Coffin k 74 74 gold badges silver badges bronze badges.

See also the bottom of the x86 tag wiki for some gdb tips for asm. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Learn more about Collectives on Stack Overflow. The Overflow Blog. Podcast Explaining the semiconductor shortage, and how it might end. Does ES6 make JavaScript frameworks obsolete? Featured on Meta.