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DMCA report, please send email to ΑDC. Each assembly language is specific to a particular computer architecture. The use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. This is because each mnemonic along with the addressing modes and operands of an instruction translates rather directly into the numeric representations of that particular instruction, without much context or analysis. One-pass assemblers go through the source code once. Multi-pass assemblers create a table with all symbols and their values in the first passes, then use the table in later passes to generate code.
In both cases, the assembler must be able to determine the size of each instruction on the initial passes in order to calculate the addresses of subsequent symbols. The binary code for this instruction is 10110 followed by a 3-bit identifier for which register to use. This is much easier to read and to remember. In some assembly languages the same mnemonic such as MOV may be used for a family of related instructions for loading, copying and moving data, whether these are immediate values, values in registers, or memory locations pointed to by values in registers. Other assemblers may use separate opcode mnemonics such as L for “move memory to register”, ST for “move register to memory”, LR for “move register to register”, MVI for “move immediate operand to memory”, etc.
Assembly language examples for these follow. The syntax of MOV can also be more complex as the following examples show. In each case, the MOV mnemonic is translated directly into an opcode in the ranges 88-8E, A0-A3, B0-B8, C6 or C7 by an assembler, and the programmer does not have to know or remember which. Computers differ in the number and type of operations they support, in the different sizes and numbers of registers, and in the representations of data in storage. In these cases, the most popular one is usually that supplied by the manufacturer and used in its documentation.
There is a large degree of diversity in the way the authors of assemblers categorize statements and in the nomenclature that they use. Most instructions refer to a single value, or a pair of values. This is determined by the underlying processor architecture: the assembler merely reflects how this architecture works. For example, many CPU’s do not have an explicit NOP instruction, but do have instructions that can be used for the purpose. Some assemblers also support simple built-in macro-instructions that generate two or more machine instructions. Standard 694 for a uniform set of mnemonics to be used by all assemblers. The standard has since been withdrawn.